Play whole file in spectrum analyzer demo

This change required a significant refactoring of the application,
including:

* Re-write WavFile class so that it inherits from QFile rather
  than taking a QIODevice as an argument to its method calls.

* Modified Engine class so that its internal QByteArray buffer
  (m_buffer) need not correspond to the entire clip.  This was done
  by introducing the m_bufferPosition variable, which indicates the
  offset from the start of the clip to the start of the region of
  the clip which is currently held in memory.  For tone generation
  and record/playback modes, the buffer does still map directly to
  the whole clip, so m_bufferPosition is always zero.  For file
  playback, the WavFile instance is the QIODevice which is passed
  to QAudioOutput; m_buffer is just the part of the file which is
  currently in memory for spectrum analysis, level calculation and
  waveform rendering.

* For file playback, introduced a second WavFile instance as a
  member of the Engine class.  This is because QFile::seek() is
  called in order to read the part of the file currently required
  for analysis.  If the QAudioOutput implementation passes its
  QIODevice across a thread boundary, this seeking causes playback
  to jump around within the file rather than progressing smoothly
  forward.

* Modified the audioLength utility function so that its return
  value is always a multiple of the sample size.

In the process of making the above changes, a few other minor
modifications were made:

* Modify all internal APIs concerned with buffer offsets and lengths
  to deal in bytes.  Previously, some calls passed values in
  microseconds and others in bytes, which was confusing.

* Remove write functionality from WavFile class, since it is not
  used in this application.

Task-number: QTBUG-12936

11 files changed, 369 insertions, 467 deletions

diff --git a/demos/spectrum/app/engine.cpp b/demos/spectrum/app/engine.cpp
index 0495fe3..cd847fe 100644
--- a/demos/spectrum/app/engine.cpp
+++ b/demos/spectrum/app/engine.cpp
@@ -85,6 +85,7 @@ Engine::Engine(QObject *parent)
     ,   m_state(QAudio::StoppedState)
     ,   m_generateTone(false)
     ,   m_file(0)
+    ,   m_analysisFile(0)
     ,   m_availableAudioInputDevices
             (QAudioDeviceInfo::availableDevices(QAudio::AudioInput))
     ,   m_audioInputDevice(QAudioDeviceInfo::defaultInputDevice())
@@ -96,15 +97,19 @@ Engine::Engine(QObject *parent)
     ,   m_audioOutputDevice(QAudioDeviceInfo::defaultOutputDevice())
     ,   m_audioOutput(0)
     ,   m_playPosition(0)
+    ,   m_bufferPosition(0)
+    ,   m_bufferLength(0)
     ,   m_dataLength(0)
+    ,   m_levelBufferLength(0)
     ,   m_rmsLevel(0.0)
     ,   m_peakLevel(0.0)
-    ,   m_spectrumLengthBytes(0)
+    ,   m_spectrumBufferLength(0)
     ,   m_spectrumAnalyser()
     ,   m_spectrumPosition(0)
     ,   m_count(0)
 {
     qRegisterMetaType<FrequencySpectrum>("FrequencySpectrum");
+    qRegisterMetaType<WindowFunction>("WindowFunction");
     CHECKED_CONNECT(&m_spectrumAnalyser,
                     SIGNAL(spectrumChanged(FrequencySpectrum)),
                     this,
@@ -132,34 +137,33 @@ Engine::~Engine()
 
 bool Engine::loadFile(const QString &fileName)
 {
+    reset();
     bool result = false;
-    m_generateTone = false;
-
-    Q_ASSERT(!fileName.isEmpty());
+    Q_ASSERT(!m_generateTone);
     Q_ASSERT(!m_file);
-    m_file = new QFile(fileName, this);
-    m_file->setFileName(fileName);
-    Q_ASSERT(m_file->exists());
-    if (m_file->open(QFile::ReadOnly)) {
-        m_wavFile.readHeader(*m_file);
-        if (isPCMS16LE(m_wavFile.format())) {
+    Q_ASSERT(!fileName.isEmpty());
+    m_file = new WavFile(this);
+    if (m_file->open(fileName)) {
+        if (isPCMS16LE(m_file->fileFormat())) {
             result = initialize();
         } else {
             emit errorMessage(tr("Audio format not supported"),
-                              formatToString(m_wavFile.format()));
+                              formatToString(m_file->fileFormat()));
         }
     } else {
         emit errorMessage(tr("Could not open file"), fileName);
     }
-
-    delete m_file;
-    m_file = 0;
-
+    if (result) {
+        m_analysisFile = new WavFile(this);
+        m_analysisFile->open(fileName);
+    }
     return result;
 }
 
 bool Engine::generateTone(const Tone &tone)
 {
+    reset();
+    Q_ASSERT(!m_generateTone);
     Q_ASSERT(!m_file);
     m_generateTone = true;
     m_tone = tone;
@@ -172,6 +176,7 @@ bool Engine::generateTone(const Tone &tone)
 
 bool Engine::generateSweptTone(qreal amplitude)
 {
+    Q_ASSERT(!m_generateTone);
     Q_ASSERT(!m_file);
     m_generateTone = true;
     m_tone.startFreq = 1;
@@ -185,41 +190,18 @@ bool Engine::generateSweptTone(qreal amplitude)
 
 bool Engine::initializeRecord()
 {
+    reset();
     ENGINE_DEBUG << "Engine::initializeRecord";
+    Q_ASSERT(!m_generateTone);
     Q_ASSERT(!m_file);
     m_generateTone = false;
     m_tone = SweptTone();
     return initialize();
 }
 
-qint64 Engine::bufferDuration() const
-{
-    return BufferDurationUs;
-}
-
-qint64 Engine::dataDuration() const
-{
-    qint64 result = 0;
-    if (QAudioFormat() != m_format)
-        result = audioDuration(m_format, m_dataLength);
-    return result;
-}
-
-qint64 Engine::audioBufferLength() const
+qint64 Engine::bufferLength() const
 {
-    qint64 length = 0;
-    if (QAudio::ActiveState == m_state || QAudio::IdleState == m_state) {
-        Q_ASSERT(QAudioFormat() != m_format);
-        switch (m_mode) {
-        case QAudio::AudioInput:
-            length = m_audioInput->bufferSize();
-            break;
-        case QAudio::AudioOutput:
-            length = m_audioOutput->bufferSize();
-            break;
-        }
-    }
-    return length;
+    return m_file ? m_file->size() : m_bufferLength;
 }
 
 void Engine::setWindowFunction(WindowFunction type)
@@ -252,7 +234,7 @@ void Engine::startRecording()
                             this, SLOT(audioNotify()));
             m_count = 0;
             m_dataLength = 0;
-            emit dataDurationChanged(0);
+            emit dataLengthChanged(0);
             m_audioInputIODevice = m_audioInput->start();
             CHECKED_CONNECT(m_audioInputIODevice, SIGNAL(readyRead()),
                             this, SLOT(audioDataReady()));
@@ -275,7 +257,6 @@ void Engine::startPlayback()
         } else {
             m_spectrumAnalyser.cancelCalculation();
             spectrumChanged(0, 0, FrequencySpectrum());
-
             setPlayPosition(0, true);
             stopRecording();
             m_mode = QAudio::AudioOutput;
@@ -284,10 +265,17 @@ void Engine::startPlayback()
             CHECKED_CONNECT(m_audioOutput, SIGNAL(notify()),
                             this, SLOT(audioNotify()));
             m_count = 0;
-            m_audioOutputIODevice.close();
-            m_audioOutputIODevice.setBuffer(&m_buffer);
-            m_audioOutputIODevice.open(QIODevice::ReadOnly);
-            m_audioOutput->start(&m_audioOutputIODevice);
+            if (m_file) {
+                m_file->seek(0);
+                m_bufferPosition = 0;
+                m_dataLength = 0;
+                m_audioOutput->start(m_file);
+            } else {
+                m_audioOutputIODevice.close();
+                m_audioOutputIODevice.setBuffer(&m_buffer);
+                m_audioOutputIODevice.open(QIODevice::ReadOnly);
+                m_audioOutput->start(&m_audioOutputIODevice);
+            }
         }
     }
 }
@@ -332,40 +320,55 @@ void Engine::audioNotify()
 {
     switch (m_mode) {
     case QAudio::AudioInput: {
-            const qint64 recordPosition =
-                    qMin(BufferDurationUs, m_audioInput->processedUSecs());
+            const qint64 recordPosition = qMin(m_bufferLength, audioLength(m_format, m_audioInput->processedUSecs()));
             setRecordPosition(recordPosition);
-
-            // Calculate level of most recently captured data
-            qint64 levelLength = audioLength(m_format, LevelWindowUs);
-            levelLength = qMin(m_dataLength, levelLength);
-            const qint64 levelPosition = m_dataLength - levelLength;
-            calculateLevel(levelPosition, levelLength);
-
-            // Calculate spectrum of most recently captured data
-            if (m_dataLength >= m_spectrumLengthBytes) {
-                const qint64 spectrumPosition = m_dataLength - m_spectrumLengthBytes;
+            const qint64 levelPosition = m_dataLength - m_levelBufferLength;
+            if (levelPosition >= 0)
+                calculateLevel(levelPosition, m_levelBufferLength);
+            if (m_dataLength >= m_spectrumBufferLength) {
+                const qint64 spectrumPosition = m_dataLength - m_spectrumBufferLength;
                 calculateSpectrum(spectrumPosition);
             }
+            emit bufferChanged(0, m_dataLength, m_buffer);
         }
         break;
     case QAudio::AudioOutput: {
-            const qint64 playPosition =
-                    qMin(dataDuration(), m_audioOutput->processedUSecs());
-            setPlayPosition(playPosition);
-
-            qint64 analysisPosition = audioLength(m_format, playPosition);
-
-            // Calculate level of data starting at current playback position
-            const qint64 levelLength = audioLength(m_format, LevelWindowUs);
-            if (analysisPosition + levelLength < m_dataLength)
-                calculateLevel(analysisPosition, levelLength);
-
-            if (analysisPosition + m_spectrumLengthBytes < m_dataLength)
-                calculateSpectrum(analysisPosition);
-
-            if (dataDuration() == playPosition)
-                stopPlayback();
+            const qint64 playPosition = audioLength(m_format, m_audioOutput->processedUSecs());
+            setPlayPosition(qMin(bufferLength(), playPosition));
+            const qint64 levelPosition = playPosition - m_levelBufferLength;
+            const qint64 spectrumPosition = playPosition - m_spectrumBufferLength;
+            if (m_file) {
+                if (levelPosition > m_bufferPosition ||
+                    spectrumPosition > m_bufferPosition ||
+                    qMax(m_levelBufferLength, m_spectrumBufferLength) > m_dataLength) {
+                    m_bufferPosition = 0;
+                    m_dataLength = 0;
+                    // Data needs to be read into m_buffer in order to be analysed
+                    const qint64 readPos = qMax(qint64(0), qMin(levelPosition, spectrumPosition));
+                    const qint64 readEnd = qMin(m_analysisFile->size(), qMax(levelPosition + m_levelBufferLength, spectrumPosition + m_spectrumBufferLength));
+                    const qint64 readLen = readEnd - readPos + audioLength(m_format, WaveformWindowDuration);
+                    qDebug() << "Engine::audioNotify [1]"
+                             << "analysisFileSize" << m_analysisFile->size()
+                             << "readPos" << readPos
+                             << "readLen" << readLen;
+                    if (m_analysisFile->seek(readPos + m_analysisFile->headerLength())) {
+                        m_buffer.resize(readLen);
+                        m_bufferPosition = readPos;
+                        m_dataLength = m_analysisFile->read(m_buffer.data(), readLen);
+                        qDebug() << "Engine::audioNotify [2]" << "bufferPosition" << m_bufferPosition << "dataLength" << m_dataLength;
+                    } else {
+                        qDebug() << "Engine::audioNotify [2]" << "file seek error";
+                    }
+                    emit bufferChanged(m_bufferPosition, m_dataLength, m_buffer);
+                }
+            } else {
+                if (playPosition >= m_dataLength)
+                    stopPlayback();
+            }
+            if (levelPosition >= 0 && levelPosition + m_levelBufferLength < m_bufferPosition + m_dataLength)
+                calculateLevel(levelPosition, m_levelBufferLength);
+            if (spectrumPosition >= 0 && spectrumPosition + m_spectrumBufferLength < m_bufferPosition + m_dataLength)
+                calculateSpectrum(spectrumPosition);
         }
         break;
     }
@@ -376,27 +379,32 @@ void Engine::audioStateChanged(QAudio::State state)
     ENGINE_DEBUG << "Engine::audioStateChanged from" << m_state
                  << "to" << state;
 
-    if (QAudio::StoppedState == state) {
-        // Check error
-        QAudio::Error error = QAudio::NoError;
-        switch (m_mode) {
-        case QAudio::AudioInput:
-            error = m_audioInput->error();
-            break;
-        case QAudio::AudioOutput:
-            error = m_audioOutput->error();
-            break;
-        }
-        if (QAudio::NoError != error) {
-            reset();
-            return;
+    if (QAudio::IdleState == state && m_file && m_file->pos() == m_file->size()) {
+        stopPlayback();
+    } else {
+        if (QAudio::StoppedState == state) {
+            // Check error
+            QAudio::Error error = QAudio::NoError;
+            switch (m_mode) {
+            case QAudio::AudioInput:
+                error = m_audioInput->error();
+                break;
+            case QAudio::AudioOutput:
+                error = m_audioOutput->error();
+                break;
+            }
+            if (QAudio::NoError != error) {
+                reset();
+                return;
+            }
         }
+        setState(state);
     }
-    setState(state);
 }
 
 void Engine::audioDataReady()
 {
+    Q_ASSERT(0 == m_bufferPosition);
     const qint64 bytesReady = m_audioInput->bytesReady();
     const qint64 bytesSpace = m_buffer.size() - m_dataLength;
     const qint64 bytesToRead = qMin(bytesReady, bytesSpace);
@@ -407,9 +415,7 @@ void Engine::audioDataReady()
 
     if (bytesRead) {
         m_dataLength += bytesRead;
-
-        const qint64 duration = audioDuration(m_format, m_dataLength);
-        emit dataDurationChanged(duration);
+        emit dataLengthChanged(dataLength());
     }
 
     if (m_buffer.size() == m_dataLength)
@@ -419,9 +425,7 @@ void Engine::audioDataReady()
 void Engine::spectrumChanged(const FrequencySpectrum &spectrum)
 {
     ENGINE_DEBUG << "Engine::spectrumChanged" << "pos" << m_spectrumPosition;
-    const qint64 positionUs = audioDuration(m_format, m_spectrumPosition);
-    const qint64 lengthUs = audioDuration(m_format, m_spectrumLengthBytes);
-    emit spectrumChanged(positionUs, lengthUs, spectrum);
+    emit spectrumChanged(m_spectrumPosition, m_spectrumBufferLength, spectrum);
 }
 
 
@@ -429,12 +433,8 @@ void Engine::spectrumChanged(const FrequencySpectrum &spectrum)
 // Private functions
 //-----------------------------------------------------------------------------
 
-void Engine::reset()
+void Engine::resetAudioDevices()
 {
-    stopRecording();
-    stopPlayback();
-    setState(QAudio::AudioInput, QAudio::StoppedState);
-    setFormat(QAudioFormat());
     delete m_audioInput;
     m_audioInput = 0;
     m_audioInputIODevice = 0;
@@ -442,13 +442,29 @@ void Engine::reset()
     delete m_audioOutput;
     m_audioOutput = 0;
     setPlayPosition(0);
-    m_buffer.clear();
-    m_dataLength = 0;
     m_spectrumPosition = 0;
-    emit dataDurationChanged(0);
     setLevel(0.0, 0.0, 0);
 }
 
+void Engine::reset()
+{
+    stopRecording();
+    stopPlayback();
+    setState(QAudio::AudioInput, QAudio::StoppedState);
+    setFormat(QAudioFormat());
+    m_generateTone = false;
+    delete m_file;
+    m_file = 0;
+    delete m_analysisFile;
+    m_analysisFile = 0;
+    m_buffer.clear();
+    m_bufferPosition = 0;
+    m_bufferLength = 0;
+    m_dataLength = 0;
+    emit dataLengthChanged(0);
+    resetAudioDevices();
+}
+
 bool Engine::initialize()
 {
     bool result = false;
@@ -457,45 +473,39 @@ bool Engine::initialize()
 
     if (selectFormat()) {
         if (m_format != format) {
-            format = m_format;
-            reset();
-            m_format = format;
-
-            const qint64 bufferLength = audioLength(m_format, BufferDurationUs);
-            m_buffer.resize(bufferLength);
-            m_buffer.fill(0);
-            emit bufferDurationChanged(BufferDurationUs);
-
-            if (m_generateTone) {
-                if (0 == m_tone.endFreq) {
-                    const qreal nyquist = nyquistFrequency(m_format);
-                    m_tone.endFreq = qMin(qreal(SpectrumHighFreq), nyquist);
-                }
-
-                // Call function defined in utils.h, at global scope
-                ::generateTone(m_tone, m_format, m_buffer);
-                m_dataLength = m_buffer.size();
-                emit dataDurationChanged(bufferDuration());
-                setRecordPosition(bufferDuration());
+            resetAudioDevices();
+            if (m_file) {
+                emit bufferLengthChanged(bufferLength());
+                emit dataLengthChanged(dataLength());
+                emit bufferChanged(0, 0, m_buffer);
+                setRecordPosition(bufferLength());
                 result = true;
-            } else if (m_file) {
-                const qint64 length = m_wavFile.readData(*m_file, m_buffer, m_format);
-                if (length) {
-                    m_dataLength = length;
-                    emit dataDurationChanged(dataDuration());
-                    setRecordPosition(dataDuration());
+            } else {
+                m_bufferLength = audioLength(m_format, BufferDurationUs);
+                m_buffer.resize(m_bufferLength);
+                m_buffer.fill(0);
+                emit bufferLengthChanged(bufferLength());
+                if (m_generateTone) {
+                    if (0 == m_tone.endFreq) {
+                        const qreal nyquist = nyquistFrequency(m_format);
+                        m_tone.endFreq = qMin(qreal(SpectrumHighFreq), nyquist);
+                    }
+                    // Call function defined in utils.h, at global scope
+                    ::generateTone(m_tone, m_format, m_buffer);
+                    m_dataLength = m_bufferLength;
+                    emit dataLengthChanged(dataLength());
+                    emit bufferChanged(0, m_dataLength, m_buffer);
+                    setRecordPosition(m_bufferLength);
+                    result = true;
+                } else {
+                    emit bufferChanged(0, 0, m_buffer);
+                    m_audioInput = new QAudioInput(m_audioInputDevice, m_format, this);
+                    m_audioInput->setNotifyInterval(NotifyIntervalMs);
                     result = true;
                 }
-            } else {
-                m_audioInput = new QAudioInput(m_audioInputDevice, m_format, this);
-                m_audioInput->setNotifyInterval(NotifyIntervalMs);
-                result = true;
             }
-
             m_audioOutput = new QAudioOutput(m_audioOutputDevice, m_format, this);
             m_audioOutput->setNotifyInterval(NotifyIntervalMs);
-            m_spectrumLengthBytes = SpectrumLengthSamples *
-                                    (m_format.sampleSize() / 8) * m_format.channels();
         }
     } else {
         if (m_file)
@@ -507,6 +517,8 @@ bool Engine::initialize()
             emit errorMessage(tr("No common input / output format found"), "");
     }
 
+    ENGINE_DEBUG << "Engine::initialize" << "m_bufferLength" << m_bufferLength;
+    ENGINE_DEBUG << "Engine::initialize" << "m_dataLength" << m_dataLength;
     ENGINE_DEBUG << "Engine::initialize" << "format" << m_format;
 
     return result;
@@ -521,18 +533,10 @@ bool Engine::selectFormat()
         if (m_file)
             // Header is read from the WAV file; just need to check whether
             // it is supported by the audio output device
-            format = m_wavFile.format();
+            format = m_file->fileFormat();
         if (m_audioOutputDevice.isFormatSupported(format)) {
             setFormat(format);
             foundSupportedFormat = true;
-        } else {
-            // Try flipping mono <-> stereo
-            const int channels = (format.channels() == 1) ? 2 : 1;
-            format.setChannels(channels);
-            if (m_audioOutputDevice.isFormatSupported(format)) {
-                setFormat(format);
-                foundSupportedFormat = true;
-            }
         }
     } else {
 
@@ -656,12 +660,12 @@ void Engine::calculateLevel(qint64 position, qint64 length)
     Q_UNUSED(position)
     Q_UNUSED(length)
 #else
-    Q_ASSERT(position + length <= m_dataLength);
+    Q_ASSERT(position + length <= m_bufferPosition + m_dataLength);
 
     qreal peakLevel = 0.0;
 
     qreal sum = 0.0;
-    const char *ptr = m_buffer.constData() + position;
+    const char *ptr = m_buffer.constData() + position - m_bufferPosition;
     const char *const end = ptr + length;
     while (ptr < end) {
         const qint16 value = *reinterpret_cast<const qint16*>(ptr);
@@ -687,18 +691,18 @@ void Engine::calculateSpectrum(qint64 position)
 #ifdef DISABLE_SPECTRUM
     Q_UNUSED(position)
 #else
-    Q_ASSERT(position + m_spectrumLengthBytes <= m_dataLength);
-    Q_ASSERT(0 == m_spectrumLengthBytes % 2); // constraint of FFT algorithm
+    Q_ASSERT(position + m_spectrumBufferLength <= m_bufferPosition + m_dataLength);
+    Q_ASSERT(0 == m_spectrumBufferLength % 2); // constraint of FFT algorithm
 
     // QThread::currentThread is marked 'for internal use only', but
     // we're only using it for debug output here, so it's probably OK :)
     ENGINE_DEBUG << "Engine::calculateSpectrum" << QThread::currentThread()
-                 << "count" << m_count << "pos" << position << "len" << m_spectrumLengthBytes
+                 << "count" << m_count << "pos" << position << "len" << m_spectrumBufferLength
                  << "spectrumAnalyser.isReady" << m_spectrumAnalyser.isReady();
 
     if(m_spectrumAnalyser.isReady()) {
-        m_spectrumBuffer = QByteArray::fromRawData(m_buffer.constData() + position,
-                                                   m_spectrumLengthBytes);
+        m_spectrumBuffer = QByteArray::fromRawData(m_buffer.constData() + position - m_bufferPosition,
+                                                   m_spectrumBufferLength);
         m_spectrumPosition = position;
         m_spectrumAnalyser.calculate(m_spectrumBuffer, m_format);
     }
@@ -709,6 +713,9 @@ void Engine::setFormat(const QAudioFormat &format)
 {
     const bool changed = (format != m_format);
     m_format = format;
+    m_levelBufferLength = audioLength(m_format, LevelWindowUs);
+    m_spectrumBufferLength = SpectrumLengthSamples *
+                            (m_format.sampleSize() / 8) * m_format.channels();
     if (changed)
         emit formatChanged(m_format);
 }
diff --git a/demos/spectrum/app/engine.h b/demos/spectrum/app/engine.h
index e14ac83..c97083e 100644
--- a/demos/spectrum/app/engine.h
+++ b/demos/spectrum/app/engine.h
@@ -91,12 +91,6 @@ public:
     QAudio::State state() const     { return m_state; }
 
     /**
-     * \return Reference to internal audio buffer
-     * \note This reference is valid for the lifetime of the Engine
-     */
-    const QByteArray& buffer() const    { return m_buffer; }
-
-    /**
      * \return Current audio format
      * \note May be QAudioFormat() if engine is not initialized
      */
@@ -129,7 +123,7 @@ public:
 
     /**
      * Position of the audio input device.
-     * \return Position in microseconds.
+     * \return Position in bytes.
      */
     qint64 recordPosition() const   { return m_recordPosition; }
 
@@ -147,27 +141,21 @@ public:
 
     /**
      * Position of the audio output device.
-     * \return Position in microseconds.
+     * \return Position in bytes.
      */
     qint64 playPosition() const     { return m_playPosition; }
 
     /**
      * Length of the internal engine buffer.
-     * \return Buffer length in microseconds.
+     * \return Buffer length in bytes.
      */
-    qint64 bufferDuration() const;
+    qint64 bufferLength() const;
 
     /**
      * Amount of data held in the buffer.
-     * \return Data duration in microseconds.
+     * \return Data length in bytes.
      */
-    qint64 dataDuration() const;
-
-    /**
-     * Returns the size of the underlying audio buffer in bytes.
-     * This should be an approximation of the capture latency.
-     */
-    qint64 audioBufferLength() const;
+    qint64 dataLength() const       { return m_dataLength; }
 
     /**
      * Set window function applied to audio data before spectral analysis.
@@ -203,23 +191,23 @@ signals:
      * Length of buffer has changed.
      * \param duration Duration in microseconds
      */
-    void bufferDurationChanged(qint64 duration);
+    void bufferLengthChanged(qint64 duration);
 
     /**
      * Amount of data in buffer has changed.
-     * \param duration Duration of data in microseconds
+     * \param Length of data in bytes
      */
-    void dataDurationChanged(qint64 duration);
+    void dataLengthChanged(qint64 duration);
 
     /**
      * Position of the audio input device has changed.
-     * \param position Position in microseconds
+     * \param position Position in bytes
      */
     void recordPositionChanged(qint64 position);
 
     /**
      * Position of the audio output device has changed.
-     * \param position Position in microseconds
+     * \param position Position in bytes
      */
     void playPositionChanged(qint64 position);
 
@@ -233,12 +221,19 @@ signals:
 
     /**
      * Spectrum has changed.
-     * \param position Position of start of window in microseconds
-     * \param length   Length of window in microseconds
+     * \param position Position of start of window in bytes
+     * \param length   Length of window in bytes
      * \param spectrum Resulting frequency spectrum
      */
     void spectrumChanged(qint64 position, qint64 length, const FrequencySpectrum &spectrum);
 
+    /**
+     * Buffer containing audio data has changed.
+     * \param position Position of start of buffer in bytes
+     * \param buffer   Buffer
+     */
+    void bufferChanged(qint64 position, qint64 length, const QByteArray &buffer);
+
 private slots:
     void audioNotify();
     void audioStateChanged(QAudio::State state);
@@ -246,6 +241,7 @@ private slots:
     void spectrumChanged(const FrequencySpectrum &spectrum);
 
 private:
+    void resetAudioDevices();
     bool initialize();
     bool selectFormat();
     void stopRecording();
@@ -275,8 +271,10 @@ private:
     bool                m_generateTone;
     SweptTone           m_tone;
 
-    QFile*              m_file;
-    WavFile             m_wavFile;
+    WavFile*            m_file;
+    // We need a second file handle via which to read data into m_buffer
+    // for analysis
+    WavFile*            m_analysisFile;
 
     QAudioFormat        m_format;
 
@@ -293,12 +291,15 @@ private:
     QBuffer             m_audioOutputIODevice;
 
     QByteArray          m_buffer;
+    qint64              m_bufferPosition;
+    qint64              m_bufferLength;
     qint64              m_dataLength;
 
+    int                 m_levelBufferLength;
     qreal               m_rmsLevel;
     qreal               m_peakLevel;
 
-    int                 m_spectrumLengthBytes;
+    int                 m_spectrumBufferLength;
     QByteArray          m_spectrumBuffer;
     SpectrumAnalyser    m_spectrumAnalyser;
     qint64              m_spectrumPosition;
diff --git a/demos/spectrum/app/mainwidget.cpp b/demos/spectrum/app/mainwidget.cpp
index fba28c6..4b53bbe 100644
--- a/demos/spectrum/app/mainwidget.cpp
+++ b/demos/spectrum/app/mainwidget.cpp
@@ -65,7 +65,7 @@ MainWidget::MainWidget(QWidget *parent)
     ,   m_mode(NoMode)
     ,   m_engine(new Engine(this))
 #ifndef DISABLE_WAVEFORM
-    ,   m_waveform(new Waveform(m_engine->buffer(), this))
+    ,   m_waveform(new Waveform(this))
 #endif
     ,   m_progressBar(new ProgressBar(this))
     ,   m_spectrograph(new Spectrograph(this))
@@ -166,19 +166,18 @@ void MainWidget::timerEvent(QTimerEvent *event)
     m_infoMessage->setText("");
 }
 
-void MainWidget::positionChanged(qint64 positionUs)
+void MainWidget::audioPositionChanged(qint64 position)
 {
 #ifndef DISABLE_WAVEFORM
-    qint64 positionBytes = audioLength(m_engine->format(), positionUs);
-    m_waveform->positionChanged(positionBytes);
+    m_waveform->audioPositionChanged(position);
 #else
-    Q_UNUSED(positionUs)
+    Q_UNUSED(position)
 #endif
 }
 
-void MainWidget::bufferDurationChanged(qint64 durationUs)
+void MainWidget::bufferLengthChanged(qint64 length)
 {
-    m_progressBar->bufferDurationChanged(durationUs);
+    m_progressBar->bufferLengthChanged(length);
 }
 
 
@@ -186,18 +185,6 @@ void MainWidget::bufferDurationChanged(qint64 durationUs)
 // Private slots
 //-----------------------------------------------------------------------------
 
-void MainWidget::dataDurationChanged(qint64 duration)
-{
-#ifndef DISABLE_WAVEFORM
-    const qint64 dataLength = audioLength(m_engine->format(), duration);
-    m_waveform->dataLengthChanged(dataLength);
-#else
-    Q_UNUSED(duration)
-#endif
-
-    updateButtonStates();
-}
-
 void MainWidget::showFileDialog()
 {
     const QString dir;
@@ -363,13 +350,13 @@ void MainWidget::connectUi()
     CHECKED_CONNECT(m_engine, SIGNAL(formatChanged(const QAudioFormat &)),
             this, SLOT(formatChanged(const QAudioFormat &)));
 
-    m_progressBar->bufferDurationChanged(m_engine->bufferDuration());
+    m_progressBar->bufferLengthChanged(m_engine->bufferLength());
 
-    CHECKED_CONNECT(m_engine, SIGNAL(bufferDurationChanged(qint64)),
-            this, SLOT(bufferDurationChanged(qint64)));
+    CHECKED_CONNECT(m_engine, SIGNAL(bufferLengthChanged(qint64)),
+            this, SLOT(bufferLengthChanged(qint64)));
 
-    CHECKED_CONNECT(m_engine, SIGNAL(dataDurationChanged(qint64)),
-            this, SLOT(dataDurationChanged(qint64)));
+    CHECKED_CONNECT(m_engine, SIGNAL(dataLengthChanged(qint64)),
+            this, SLOT(updateButtonStates()));
 
     CHECKED_CONNECT(m_engine, SIGNAL(recordPositionChanged(qint64)),
             m_progressBar, SLOT(recordPositionChanged(qint64)));
@@ -378,10 +365,10 @@ void MainWidget::connectUi()
             m_progressBar, SLOT(playPositionChanged(qint64)));
 
     CHECKED_CONNECT(m_engine, SIGNAL(recordPositionChanged(qint64)),
-            this, SLOT(positionChanged(qint64)));
+            this, SLOT(audioPositionChanged(qint64)));
 
     CHECKED_CONNECT(m_engine, SIGNAL(playPositionChanged(qint64)),
-            this, SLOT(positionChanged(qint64)));
+            this, SLOT(audioPositionChanged(qint64)));
 
     CHECKED_CONNECT(m_engine, SIGNAL(levelChanged(qreal, qreal, int)),
             m_levelMeter, SLOT(levelChanged(qreal, qreal, int)));
@@ -397,6 +384,11 @@ void MainWidget::connectUi()
 
     CHECKED_CONNECT(m_spectrograph, SIGNAL(infoMessage(QString, int)),
             this, SLOT(infoMessage(QString, int)));
+
+#ifndef DISABLE_WAVEFORM
+    CHECKED_CONNECT(m_engine, SIGNAL(bufferChanged(qint64, qint64, const QByteArray &)),
+            m_waveform, SLOT(bufferChanged(qint64, qint64, const QByteArray &)));
+#endif
 }
 
 void MainWidget::createMenus()
@@ -428,7 +420,7 @@ void MainWidget::updateButtonStates()
                                QAudio::IdleState == m_engine->state());
     m_pauseButton->setEnabled(pauseEnabled);
 
-    const bool playEnabled = (m_engine->dataDuration() &&
+    const bool playEnabled = (/*m_engine->dataLength() &&*/
                               (QAudio::AudioOutput != m_engine->mode() ||
                                (QAudio::ActiveState != m_engine->state() &&
                                 QAudio::IdleState != m_engine->state())));
diff --git a/demos/spectrum/app/mainwidget.h b/demos/spectrum/app/mainwidget.h
index cc1f9ed..13131c0 100644
--- a/demos/spectrum/app/mainwidget.h
+++ b/demos/spectrum/app/mainwidget.h
@@ -80,22 +80,21 @@ public slots:
                          const FrequencySpectrum &spectrum);
     void infoMessage(const QString &message, int timeoutMs);
     void errorMessage(const QString &heading, const QString &detail);
-    void positionChanged(qint64 position);
-    void bufferDurationChanged(qint64 duration);
+    void audioPositionChanged(qint64 position);
+    void bufferLengthChanged(qint64 length);
 
 private slots:
     void showFileDialog();
     void showSettingsDialog();
     void showToneGeneratorDialog();
     void initializeRecord();
-    void dataDurationChanged(qint64 duration);
     void updateModeMenu();
+    void updateButtonStates();
 
 private:
     void createUi();
     void createMenus();
     void connectUi();
-    void updateButtonStates();
     void reset();
 
     enum Mode {
@@ -112,7 +111,9 @@ private:
 
     Engine*                 m_engine;
 
+#ifndef DISABLE_WAVEFORM
     Waveform*               m_waveform;
+#endif
     ProgressBar*            m_progressBar;
     Spectrograph*           m_spectrograph;
     LevelMeter*             m_levelMeter;
diff --git a/demos/spectrum/app/progressbar.cpp b/demos/spectrum/app/progressbar.cpp
index 6bfc690..0ac76f1 100644
--- a/demos/spectrum/app/progressbar.cpp
+++ b/demos/spectrum/app/progressbar.cpp
@@ -44,7 +44,7 @@
 
 ProgressBar::ProgressBar(QWidget *parent)
     :   QWidget(parent)
-    ,   m_bufferDuration(0)
+    ,   m_bufferLength(0)
     ,   m_recordPosition(0)
     ,   m_playPosition(0)
     ,   m_windowPosition(0)
@@ -64,7 +64,7 @@ ProgressBar::~ProgressBar()
 
 void ProgressBar::reset()
 {
-    m_bufferDuration = 0;
+    m_bufferLength = 0;
     m_recordPosition = 0;
     m_playPosition = 0;
     m_windowPosition = 0;
@@ -86,26 +86,26 @@ void ProgressBar::paintEvent(QPaintEvent * /*event*/)
     painter.fillRect(rect(), Qt::black);
 #endif
 
-    if (m_bufferDuration) {
+    if (m_bufferLength) {
         QRect bar = rect();
-        const qreal play = qreal(m_playPosition) / m_bufferDuration;
+        const qreal play = qreal(m_playPosition) / m_bufferLength;
         bar.setLeft(rect().left() + play * rect().width());
-        const qreal record = qreal(m_recordPosition) / m_bufferDuration;
+        const qreal record = qreal(m_recordPosition) / m_bufferLength;
         bar.setRight(rect().left() + record * rect().width());
         painter.fillRect(bar, bufferColor);
 
         QRect window = rect();
-        const qreal windowLeft = qreal(m_windowPosition) / m_bufferDuration;
+        const qreal windowLeft = qreal(m_windowPosition) / m_bufferLength;
         window.setLeft(rect().left() + windowLeft * rect().width());
-        const qreal windowWidth = qreal(m_windowLength) / m_bufferDuration;
+        const qreal windowWidth = qreal(m_windowLength) / m_bufferLength;
         window.setWidth(windowWidth * rect().width());
         painter.fillRect(window, windowColor);
     }
 }
 
-void ProgressBar::bufferDurationChanged(qint64 bufferSize)
+void ProgressBar::bufferLengthChanged(qint64 bufferSize)
 {
-    m_bufferDuration = bufferSize;
+    m_bufferLength = bufferSize;
     m_recordPosition = 0;
     m_playPosition = 0;
     m_windowPosition = 0;
@@ -116,7 +116,7 @@ void ProgressBar::bufferDurationChanged(qint64 bufferSize)
 void ProgressBar::recordPositionChanged(qint64 recordPosition)
 {
     Q_ASSERT(recordPosition >= 0);
-    Q_ASSERT(recordPosition <= m_bufferDuration);
+    Q_ASSERT(recordPosition <= m_bufferLength);
     m_recordPosition = recordPosition;
     repaint();
 }
@@ -124,7 +124,7 @@ void ProgressBar::recordPositionChanged(qint64 recordPosition)
 void ProgressBar::playPositionChanged(qint64 playPosition)
 {
     Q_ASSERT(playPosition >= 0);
-    Q_ASSERT(playPosition <= m_bufferDuration);
+    Q_ASSERT(playPosition <= m_bufferLength);
     m_playPosition = playPosition;
     repaint();
 }
@@ -132,8 +132,8 @@ void ProgressBar::playPositionChanged(qint64 playPosition)
 void ProgressBar::windowChanged(qint64 position, qint64 length)
 {
     Q_ASSERT(position >= 0);
-    Q_ASSERT(position <= m_bufferDuration);
-    Q_ASSERT(position + length <= m_bufferDuration);
+    Q_ASSERT(position <= m_bufferLength);
+    Q_ASSERT(position + length <= m_bufferLength);
     m_windowPosition = position;
     m_windowLength = length;
     repaint();
diff --git a/demos/spectrum/app/progressbar.h b/demos/spectrum/app/progressbar.h
index 8514adb..e715cf5 100644
--- a/demos/spectrum/app/progressbar.h
+++ b/demos/spectrum/app/progressbar.h
@@ -57,13 +57,13 @@ public:
     void paintEvent(QPaintEvent *event);
 
 public slots:
-    void bufferDurationChanged(qint64 bufferSize);
+    void bufferLengthChanged(qint64 length);
     void recordPositionChanged(qint64 recordPosition);
     void playPositionChanged(qint64 playPosition);
     void windowChanged(qint64 position, qint64 length);
 
 private:
-    qint64 m_bufferDuration;
+    qint64 m_bufferLength;
     qint64 m_recordPosition;
     qint64 m_playPosition;
     qint64 m_windowPosition;
diff --git a/demos/spectrum/app/utils.cpp b/demos/spectrum/app/utils.cpp
index 4ead6c2..49a7626 100644
--- a/demos/spectrum/app/utils.cpp
+++ b/demos/spectrum/app/utils.cpp
@@ -49,8 +49,10 @@ qint64 audioDuration(const QAudioFormat &format, qint64 bytes)
 
 qint64 audioLength(const QAudioFormat &format, qint64 microSeconds)
 {
-   return (format.frequency() * format.channels() * (format.sampleSize() / 8))
+   qint64 result = (format.frequency() * format.channels() * (format.sampleSize() / 8))
        * microSeconds / 1000000;
+   result -= result % (format.channelCount() * format.sampleSize());
+   return result;
 }
 
 qreal nyquistFrequency(const QAudioFormat &format)
diff --git a/demos/spectrum/app/waveform.cpp b/demos/spectrum/app/waveform.cpp
index 1f7d315..bd854c0 100644
--- a/demos/spectrum/app/waveform.cpp
+++ b/demos/spectrum/app/waveform.cpp
@@ -44,12 +44,18 @@
 #include <QResizeEvent>
 #include <QDebug>
 
-
-Waveform::Waveform(const QByteArray &buffer, QWidget *parent)
+//#define PAINT_EVENT_TRACE
+#ifdef PAINT_EVENT_TRACE
+#   define WAVEFORM_PAINT_DEBUG qDebug()
+#else
+#   define WAVEFORM_PAINT_DEBUG nullDebug()
+#endif
+
+Waveform::Waveform(QWidget *parent)
     :   QWidget(parent)
-    ,   m_buffer(buffer)
-    ,   m_dataLength(0)
-    ,   m_position(0)
+    ,   m_bufferPosition(0)
+    ,   m_bufferLength(0)
+    ,   m_audioPosition(0)
     ,   m_active(false)
     ,   m_tileLength(0)
     ,   m_tileArrayStart(0)
@@ -72,19 +78,19 @@ void Waveform::paintEvent(QPaintEvent * /*event*/)
     painter.fillRect(rect(), Qt::black);
 
     if (m_active) {
-        WAVEFORM_DEBUG << "Waveform::paintEvent"
-                       << "windowPosition" << m_windowPosition
-                       << "windowLength" << m_windowLength;
+        WAVEFORM_PAINT_DEBUG << "Waveform::paintEvent"
+                             << "windowPosition" << m_windowPosition
+                             << "windowLength" << m_windowLength;
         qint64 pos = m_windowPosition;
         const qint64 windowEnd = m_windowPosition + m_windowLength;
         int destLeft = 0;
         int destRight = 0;
         while (pos < windowEnd) {
             const TilePoint point = tilePoint(pos);
-            WAVEFORM_DEBUG << "Waveform::paintEvent" << "pos" << pos
-                           << "tileIndex" << point.index
-                           << "positionOffset" << point.positionOffset
-                           << "pixelOffset" << point.pixelOffset;
+            WAVEFORM_PAINT_DEBUG << "Waveform::paintEvent" << "pos" << pos
+                                 << "tileIndex" << point.index
+                                 << "positionOffset" << point.positionOffset
+                                 << "pixelOffset" << point.pixelOffset;
 
             if (point.index != NullIndex) {
                 const Tile &tile = m_tiles[point.index];
@@ -104,9 +110,9 @@ void Waveform::paintEvent(QPaintEvent * /*event*/)
                     sourceRect.setLeft(point.pixelOffset);
                     sourceRect.setRight(sourceRight);
 
-                    WAVEFORM_DEBUG << "Waveform::paintEvent" << "tileIndex" << point.index
-                                   << "source" << point.pixelOffset << sourceRight
-                                   << "dest" << destLeft << destRight;
+                    WAVEFORM_PAINT_DEBUG << "Waveform::paintEvent" << "tileIndex" << point.index
+                                         << "source" << point.pixelOffset << sourceRight
+                                         << "dest" << destLeft << destRight;
 
                     painter.drawPixmap(destRect, *tile.pixmap, sourceRect);
 
@@ -114,25 +120,25 @@ void Waveform::paintEvent(QPaintEvent * /*event*/)
 
                     if (point.index < m_tiles.count()) {
                         pos = tilePosition(point.index + 1);
-                        WAVEFORM_DEBUG << "Waveform::paintEvent" << "pos ->" << pos;
+                        WAVEFORM_PAINT_DEBUG << "Waveform::paintEvent" << "pos ->" << pos;
                     } else {
                         // Reached end of tile array
-                        WAVEFORM_DEBUG << "Waveform::paintEvent" << "reached end of tile array";
+                        WAVEFORM_PAINT_DEBUG << "Waveform::paintEvent" << "reached end of tile array";
                         break;
                     }
                 } else {
                     // Passed last tile which is painted
-                    WAVEFORM_DEBUG << "Waveform::paintEvent" << "tile" << point.index << "not painted";
+                    WAVEFORM_PAINT_DEBUG << "Waveform::paintEvent" << "tile" << point.index << "not painted";
                     break;
                 }
             } else {
                 // pos is past end of tile array
-                WAVEFORM_DEBUG << "Waveform::paintEvent" << "pos" << pos << "past end of tile array";
+                WAVEFORM_PAINT_DEBUG << "Waveform::paintEvent" << "pos" << pos << "past end of tile array";
                 break;
             }
         }
 
-        WAVEFORM_DEBUG << "Waveform::paintEvent" << "final pos" << pos << "final x" << destRight;
+        WAVEFORM_PAINT_DEBUG << "Waveform::paintEvent" << "final pos" << pos << "final x" << destRight;
     }
 }
 
@@ -146,7 +152,6 @@ void Waveform::initialize(const QAudioFormat &format, qint64 audioBufferSize, qi
 {
     WAVEFORM_DEBUG << "Waveform::initialize"
                    << "audioBufferSize" << audioBufferSize
-                   << "m_buffer.size()" << m_buffer.size()
                    << "windowDurationUs" << windowDurationUs;
 
     reset();
@@ -186,8 +191,9 @@ void Waveform::reset()
 {
     WAVEFORM_DEBUG << "Waveform::reset";
 
-    m_dataLength = 0;
-    m_position = 0;
+    m_bufferPosition = 0;
+    m_buffer = QByteArray();
+    m_audioPosition = 0;
     m_format = QAudioFormat();
     m_active = false;
     deletePixmaps();
@@ -198,30 +204,31 @@ void Waveform::reset()
     m_windowLength = 0;
 }
 
-void Waveform::dataLengthChanged(qint64 length)
+void Waveform::bufferChanged(qint64 position, qint64 length, const QByteArray &buffer)
 {
-    WAVEFORM_DEBUG << "Waveform::dataLengthChanged" << length;
-    const qint64 oldLength = m_dataLength;
-    m_dataLength = length;
-
-    if (m_active) {
-        if (m_dataLength < oldLength)
-            positionChanged(m_dataLength);
-        else
-            paintTiles();
-    }
+    WAVEFORM_DEBUG << "Waveform::bufferChanged"
+                   << "audioPosition" << m_audioPosition
+                   << "bufferPosition" << position
+                   << "bufferLength" << length;
+    m_bufferPosition = position;
+    m_bufferLength = length;
+    m_buffer = buffer;
+    paintTiles();
 }
 
-void Waveform::positionChanged(qint64 position)
+void Waveform::audioPositionChanged(qint64 position)
 {
-    WAVEFORM_DEBUG << "Waveform::positionChanged" << position;
-
-    if (position + m_windowLength > m_dataLength)
-        position = m_dataLength - m_windowLength;
-
-    m_position = position;
-
-    setWindowPosition(position);
+    WAVEFORM_DEBUG << "Waveform::audioPositionChanged"
+                   << "audioPosition" << position
+                   << "bufferPosition" << m_bufferPosition
+                   << "bufferLength" << m_bufferLength;
+
+    if (position >= m_bufferPosition) {
+        if (position + m_windowLength > m_bufferPosition + m_bufferLength)
+            position = qMax(qint64(0), m_bufferPosition + m_bufferLength - m_windowLength);
+        m_audioPosition = position;
+        setWindowPosition(position);
+    }
 }
 
 void Waveform::deletePixmaps()
@@ -255,8 +262,6 @@ void Waveform::createPixmaps(const QSize &widgetSize)
         m_tiles[i].pixmap = m_pixmaps[i];
         m_tiles[i].painted = false;
     }
-
-    paintTiles();
 }
 
 void Waveform::setWindowPosition(qint64 position)
@@ -327,8 +332,9 @@ bool Waveform::paintTiles()
     for (int i=0; i<m_tiles.count(); ++i) {
         const Tile &tile = m_tiles[i];
         if (!tile.painted) {
-            const qint64 tileEnd = m_tileArrayStart + (i + 1) * m_tileLength;
-            if (m_dataLength >= tileEnd) {
+            const qint64 tileStart = m_tileArrayStart + i * m_tileLength;
+            const qint64 tileEnd = tileStart + m_tileLength;
+            if (m_bufferPosition <= tileStart && m_bufferPosition + m_bufferLength >= tileEnd) {
                 paintTile(i);
                 updateRequired = true;
             }
@@ -343,16 +349,23 @@ bool Waveform::paintTiles()
 
 void Waveform::paintTile(int index)
 {
-    WAVEFORM_DEBUG << "Waveform::paintTile" << "index" << index;
-
     const qint64 tileStart = m_tileArrayStart + index * m_tileLength;
-    Q_ASSERT(m_dataLength >= tileStart + m_tileLength);
+
+    WAVEFORM_DEBUG << "Waveform::paintTile"
+                   << "index" << index
+                   << "bufferPosition" << m_bufferPosition
+                   << "bufferLength" << m_bufferLength
+                   << "start" << tileStart
+                   << "end" << tileStart + m_tileLength;
+
+    Q_ASSERT(m_bufferPosition <= tileStart);
+    Q_ASSERT(m_bufferPosition + m_bufferLength >= tileStart + m_tileLength);
 
     Tile &tile = m_tiles[index];
     Q_ASSERT(!tile.painted);
 
     const qint16* base = reinterpret_cast<const qint16*>(m_buffer.constData());
-    const qint16* buffer = base + (tileStart / 2);
+    const qint16* buffer = base + ((tileStart - m_bufferPosition) / 2);
     const int numSamples = m_tileLength / (2 * m_format.channels());
 
     QPainter painter(tile.pixmap);
@@ -376,6 +389,11 @@ void Waveform::paintTile(int index)
 
     for (int i=0; i<numSamples; ++i) {
         const qint16* ptr = buffer + i * m_format.channels();
+
+        const int offset = reinterpret_cast<const char*>(ptr) - m_buffer.constData();
+        Q_ASSERT(offset >= 0);
+        Q_ASSERT(offset < m_bufferLength);
+
         const qint16 pcmValue = *ptr;
         const qreal realValue = pcmToReal(pcmValue);
 
diff --git a/demos/spectrum/app/waveform.h b/demos/spectrum/app/waveform.h
index 57c9eec..1c54c86 100644
--- a/demos/spectrum/app/waveform.h
+++ b/demos/spectrum/app/waveform.h
@@ -60,7 +60,7 @@ QT_FORWARD_DECLARE_CLASS(QByteArray)
 class Waveform : public QWidget {
     Q_OBJECT
 public:
-    Waveform(const QByteArray &buffer, QWidget *parent = 0);
+    Waveform(QWidget *parent = 0);
     ~Waveform();
 
     // QWidget
@@ -73,8 +73,8 @@ public:
     void setAutoUpdatePosition(bool enabled);
 
 public slots:
-    void dataLengthChanged(qint64 length);
-    void positionChanged(qint64 position);
+    void bufferChanged(qint64 position, qint64 length, const QByteArray &buffer);
+    void audioPositionChanged(qint64 position);
 
 private:
     static const int NullIndex = -1;
@@ -167,9 +167,11 @@ private:
     void resetTiles(qint64 newStartPos);
 
 private:
-    const QByteArray&       m_buffer;
-    qint64                  m_dataLength;
-    qint64                  m_position;
+    qint64                  m_bufferPosition;
+    qint64                  m_bufferLength;
+    QByteArray              m_buffer;
+
+    qint64                  m_audioPosition;
     QAudioFormat            m_format;
 
     bool                    m_active;
diff --git a/demos/spectrum/app/wavfile.cpp b/demos/spectrum/app/wavfile.cpp
index 74d5918..44c3ac5 100644
--- a/demos/spectrum/app/wavfile.cpp
+++ b/demos/spectrum/app/wavfile.cpp
@@ -78,179 +78,74 @@ struct CombinedHeader
     WAVEHeader  wave;
 };
 
-
-
-WavFile::WavFile(const QAudioFormat &format, qint64 dataLength)
-    : m_format(format)
-    , m_dataLength(dataLength)
-    , m_dataPosition(0)
-{
-}
-
-bool WavFile::readHeader(QIODevice &device)
-{
-    if (!device.isSequential()) {
-        if (!device.seek(0))
-            return false;
-        // XXX: else, assume that current position is the start of the header
-    }
-
-    CombinedHeader header;
-    if (device.read(reinterpret_cast<char *>(&header), sizeof(CombinedHeader)) != sizeof(CombinedHeader))
-        return false;
-
-    if ((memcmp(&header.riff.descriptor.id, "RIFF", 4) == 0
-        || memcmp(&header.riff.descriptor.id, "RIFX", 4) == 0)
-        && memcmp(&header.riff.type, "WAVE", 4) == 0
-        && memcmp(&header.wave.descriptor.id, "fmt ", 4) == 0
-        && (header.wave.audioFormat == 1 || header.wave.audioFormat == 0)) {
-
-        // Read off remaining header information
-        DATAHeader dataHeader;
-
-        if (qFromLittleEndian<quint32>(header.wave.descriptor.size) > sizeof(WAVEHeader)) {
-            // Extended data available
-            quint16 extraFormatBytes;
-            if (device.peek((char*)&extraFormatBytes, sizeof(quint16)) != sizeof(quint16))
-                return false;
-            const qint64 throwAwayBytes = sizeof(quint16) + qFromLittleEndian<quint16>(extraFormatBytes);
-            if (device.read(throwAwayBytes).size() != throwAwayBytes)
-                return false;
-        }
-
-        if (device.read((char*)&dataHeader, sizeof(DATAHeader)) != sizeof(DATAHeader))
-            return false;
-
-        // Establish format
-        if (memcmp(&header.riff.descriptor.id, "RIFF", 4) == 0)
-            m_format.setByteOrder(QAudioFormat::LittleEndian);
-        else
-            m_format.setByteOrder(QAudioFormat::BigEndian);
-
-        int bps = qFromLittleEndian<quint16>(header.wave.bitsPerSample);
-        m_format.setChannels(qFromLittleEndian<quint16>(header.wave.numChannels));
-        m_format.setCodec("audio/pcm");
-        m_format.setFrequency(qFromLittleEndian<quint32>(header.wave.sampleRate));
-        m_format.setSampleSize(qFromLittleEndian<quint16>(header.wave.bitsPerSample));
-        m_format.setSampleType(bps == 8 ? QAudioFormat::UnSignedInt : QAudioFormat::SignedInt);
-
-        m_dataLength = qFromLittleEndian<quint32>(dataHeader.descriptor.size);
-        m_dataPosition = 0;
-    }
-
-    return true;
-}
-
-bool WavFile::writeHeader(QIODevice &device)
+WavFile::WavFile(QObject *parent)
+    : QFile(parent)
+    , m_headerLength(0)
 {
-    CombinedHeader header;
-    DATAHeader dataHeader;
-
-    memset(&header, 0, sizeof(CombinedHeader));
-
-    // RIFF header
-    if (m_format.byteOrder() == QAudioFormat::LittleEndian)
-        strncpy(&header.riff.descriptor.id[0], "RIFF", 4);
-    else
-        strncpy(&header.riff.descriptor.id[0], "RIFX", 4);
-    qToLittleEndian<quint32>(quint32(m_dataLength + sizeof(CombinedHeader) + sizeof(DATAHeader) - sizeof(chunk)),
-                             reinterpret_cast<unsigned char*>(&header.riff.descriptor.size));
-    strncpy(&header.riff.type[0], "WAVE", 4);
-
-    // WAVE header
-    strncpy(&header.wave.descriptor.id[0], "fmt ", 4);
-    qToLittleEndian<quint32>(quint32(16),
-                             reinterpret_cast<unsigned char*>(&header.wave.descriptor.size));
-    qToLittleEndian<quint16>(quint16(1),
-                             reinterpret_cast<unsigned char*>(&header.wave.audioFormat));
-    qToLittleEndian<quint16>(quint16(m_format.channels()),
-                             reinterpret_cast<unsigned char*>(&header.wave.numChannels));
-    qToLittleEndian<quint32>(quint32(m_format.frequency()),
-                             reinterpret_cast<unsigned char*>(&header.wave.sampleRate));
-    qToLittleEndian<quint32>(quint32(m_format.frequency() * m_format.channels() * m_format.sampleSize() / 8),
-                             reinterpret_cast<unsigned char*>(&header.wave.byteRate));
-    qToLittleEndian<quint16>(quint16(m_format.channels() * m_format.sampleSize() / 8),
-                             reinterpret_cast<unsigned char*>(&header.wave.blockAlign));
-    qToLittleEndian<quint16>(quint16(m_format.sampleSize()),
-                             reinterpret_cast<unsigned char*>(&header.wave.bitsPerSample));
-
-    // DATA header
-    strncpy(dataHeader.descriptor.id, "data", 4);
-    qToLittleEndian<quint32>(quint32(m_dataLength),
-                             reinterpret_cast<unsigned char*>(&dataHeader.descriptor.size));
 
-    return device.write(reinterpret_cast<const char *>(&header), sizeof(CombinedHeader)) == sizeof(CombinedHeader)
-           && device.write(reinterpret_cast<const char*>(&dataHeader), sizeof(DATAHeader)) == sizeof(DATAHeader);
 }
 
-const QAudioFormat& WavFile::format() const
+bool WavFile::open(const QString &fileName)
 {
-    return m_format;
+    close();
+    setFileName(fileName);
+    return QFile::open(QIODevice::ReadOnly) && readHeader();
 }
 
-qint64 WavFile::dataLength() const
+const QAudioFormat &WavFile::fileFormat() const
 {
-    return m_dataLength;
+    return m_fileFormat;
 }
 
-qint64 WavFile::headerLength()
+qint64 WavFile::headerLength() const
 {
-    return sizeof(CombinedHeader);
-}
-
-bool WavFile::writeDataLength(QIODevice &device, qint64 dataLength)
-{
-    bool result = false;
-    if (!device.isSequential()) {
-        device.seek(40);
-        unsigned char dataLengthLE[4];
-        qToLittleEndian<quint32>(quint32(dataLength), dataLengthLE);
-        result = (device.write(reinterpret_cast<const char *>(dataLengthLE), 4) == 4);
-    }
-    return result;
+return m_headerLength;
 }
 
-qint64 WavFile::readData(QIODevice &device, QByteArray &buffer,
-                         QAudioFormat outputFormat)
+bool WavFile::readHeader()
 {
-    // Sanity checks
-    if (!outputFormat.isValid())
-        outputFormat = m_format;
-
-    if (!isPCMS16LE(outputFormat) || !isPCMS16LE(m_format))
-        return 0;
-
-    if (m_dataPosition == m_dataLength)
-        return 0;
-
-    // Process
-    qint64 result = 0;
-
-    const int frameSize = 2 * m_format.channels(); // 16 bit samples
-    QVector<char> inputSample(frameSize);
-
-    qint16 *output = reinterpret_cast<qint16*>(buffer.data());
+    seek(0);
+    CombinedHeader header;
+    bool result = read(reinterpret_cast<char *>(&header), sizeof(CombinedHeader)) == sizeof(CombinedHeader);
+    if (result) {
+        if ((memcmp(&header.riff.descriptor.id, "RIFF", 4) == 0
+            || memcmp(&header.riff.descriptor.id, "RIFX", 4) == 0)
+            && memcmp(&header.riff.type, "WAVE", 4) == 0
+            && memcmp(&header.wave.descriptor.id, "fmt ", 4) == 0
+            && (header.wave.audioFormat == 1 || header.wave.audioFormat == 0)) {
+
+            // Read off remaining header information
+            DATAHeader dataHeader;
+
+            if (qFromLittleEndian<quint32>(header.wave.descriptor.size) > sizeof(WAVEHeader)) {
+                // Extended data available
+                quint16 extraFormatBytes;
+                if (peek((char*)&extraFormatBytes, sizeof(quint16)) != sizeof(quint16))
+                    return false;
+                const qint64 throwAwayBytes = sizeof(quint16) + qFromLittleEndian<quint16>(extraFormatBytes);
+                if (read(throwAwayBytes).size() != throwAwayBytes)
+                    return false;
+            }
 
-    while (result < buffer.size()) {
-        if (m_dataPosition == m_dataLength)
-            break;
+            if (read((char*)&dataHeader, sizeof(DATAHeader)) != sizeof(DATAHeader))
+                return false;
 
-        // XXX only working with particular alignments
-        if (device.read(inputSample.data(), inputSample.count())) {
-            int inputIdx = 0;
-            for (int outputIdx = 0; outputIdx < outputFormat.channels(); ++outputIdx) {
-                const qint16* input = reinterpret_cast<const qint16*>(inputSample.data() + 2 * inputIdx);
-                *output++ = qFromLittleEndian<qint16>(*input);
-                result += 2;
-                if (inputIdx < m_format.channels())
-                    ++inputIdx;
-            }
-            m_dataPosition += frameSize;
+            // Establish format
+            if (memcmp(&header.riff.descriptor.id, "RIFF", 4) == 0)
+                m_fileFormat.setByteOrder(QAudioFormat::LittleEndian);
+            else
+                m_fileFormat.setByteOrder(QAudioFormat::BigEndian);
+
+            int bps = qFromLittleEndian<quint16>(header.wave.bitsPerSample);
+            m_fileFormat.setChannels(qFromLittleEndian<quint16>(header.wave.numChannels));
+            m_fileFormat.setCodec("audio/pcm");
+            m_fileFormat.setFrequency(qFromLittleEndian<quint32>(header.wave.sampleRate));
+            m_fileFormat.setSampleSize(qFromLittleEndian<quint16>(header.wave.bitsPerSample));
+            m_fileFormat.setSampleType(bps == 8 ? QAudioFormat::UnSignedInt : QAudioFormat::SignedInt);
         } else {
-            break;
+            result = false;
         }
     }
-
+    m_headerLength = pos();
     return result;
 }
-
diff --git a/demos/spectrum/app/wavfile.h b/demos/spectrum/app/wavfile.h
index fc14b08..935e935 100644
--- a/demos/spectrum/app/wavfile.h
+++ b/demos/spectrum/app/wavfile.h
@@ -46,38 +46,22 @@
 #include <QtCore/qfile.h>
 #include <QtMultimedia/qaudioformat.h>
 
-/**
- * Helper class for reading WAV files
- *
- * See https://ccrma.stanford.edu/courses/422/projects/WaveFormat/
- */
-class WavFile
+class WavFile : public QFile
 {
 public:
-    WavFile(const QAudioFormat &format = QAudioFormat(),
-              qint64 dataLength = 0);
+    WavFile(QObject *parent = 0);
 
-    // Reads WAV header and seeks to start of data
-    bool readHeader(QIODevice &device);
+    bool open(const QString &fileName);
+    const QAudioFormat &fileFormat() const;
+    qint64 headerLength() const;
 
-    // Writes WAV header
-    bool writeHeader(QIODevice &device);
-
-    // Read PCM data
-    qint64 readData(QIODevice &device, QByteArray &buffer,
-                    QAudioFormat outputFormat = QAudioFormat());
-
-    const QAudioFormat& format() const;
-    qint64 dataLength() const;
-
-    static qint64 headerLength();
-
-    static bool writeDataLength(QIODevice &device, qint64 dataLength);
+private:
+    bool readHeader();
 
 private:
-    QAudioFormat m_format;
-    qint64 m_dataLength;
-    qint64 m_dataPosition;
+    QAudioFormat m_fileFormat;
+    qint64 m_headerLength;
+
 };
 
 #endif