Added documentation for Spectrum Analyzer demo

Task-number: QTBUG-12720
Reviewed-by: David Boddie
(cherry picked from commit 8adeb2742a701080c7e6568a6aca1f27079e1909)

1 files changed, 30 insertions, 4 deletions

diff --git a/doc/src/demos/spectrum.qdoc b/doc/src/demos/spectrum.qdoc
index b720ce1..d5a3f85 100644
--- a/doc/src/demos/spectrum.qdoc
+++ b/doc/src/demos/spectrum.qdoc
@@ -28,8 +28,34 @@
 /*!
     \example demos/spectrum
     \title Spectrum Analyzer
-This application is a demo which uses the QtMultimedia APIs to capture and
-play back PCM audio. While either recording or playback is ongoing, the
-application performs real-time level and frequency spectrum analysis,
-displaying the results in its main window.
+
+    The Spectrum Analyzer demo shows how the \l{QtMultimedia Module} can be
+    used in Qt applications to capture and then play back an audio stream.
+
+    \image spectrum-demo.png
+
+    Because QtMultimedia allows the application to access the raw audio
+    stream, the data can either be inspected or modified by the application.
+    The Spectrum Analyzer demo displays three pieces of information while
+    audio is being either captured or played back:
+
+    \list
+    \o Information about the raw audio stream, shown in the uppermost widget:
+    \list
+        \o The amount of data currently in the buffer, shown in blue
+        \o The segment of data which was most recently analysed to compute
+        the frequency spectrum, shown in green
+        \o The raw audio waveform, shown in white and scrolling from right to
+        left
+    \endlist
+    \o A representation of the frequency spectrum, shown at the lower left
+    \o The current RMS level of the audio stream, and the recent 'high
+    watermark' level, shown at the lower right
+    \endlist
+
+    Spectrum analysis is performed by calculating the Fast Fourier Transform
+    (FFT) of a segment of audio data.  An open-source library,
+    \l{http://ldesoras.free.fr/prod.html}{FFTReal}, against which the
+    application is dynamically linked, is used to compute the transform.
 */
+