SSBdemod: updated documentation

debian/changelog

@@ -1,8 +1,9 @@
 sdrangel (3.5.3-1) unstable; urgency=medium
 
   * SSB demod: anded optional AGC
+  * ChannelAnalyzerNG: better handling of LSB in the UI
 
- -- Edouard Griffiths, F4EXB <f4exb06@gmail.com>  Sat, 28 Jul 2017 09:14:18 +0200
+ -- Edouard Griffiths, F4EXB <f4exb06@gmail.com>  Mon, 31 Jul 2017 09:14:18 +0200
 
 sdrangel (3.5.2-1) unstable; urgency=medium
 

plugins/channelrx/demodssb/readme.md

@@ -96,10 +96,46 @@ Values are expressed in kHz and step is 100 Hz.
   - In SSB mode this is the lower cutoff (USB: positive frequencies) or higher cutoff (LSB: negative frequencies) of the in channel signe side band bandpass filter.
   - In DSB mode it is inactive and set to zero (double side band filter).
 
-<h3>11: Volume</h3>
+<h3>11: Volume and AGC</h3>
+
+![SSB volume and AGC controls](../../../doc/img/SSBDemod_plugin_vol.png)
+
+<h4>11.1: Volume</h4>
 
 This is the volume of the audio signal from 0.0 (mute) to 10.0 (maximum). It can be varied continuously in 0.1 steps using the dial button.
 
+<h4>11.2: AGC toggle</h4>
+
+Use this checkbox to toggle AGC on and off.
+
+If you are into digging weak signals out of the noise you probably will not turn the AGC on. AGC is intended for medium and large signals and help accomodate the signal power variations from a station to another or due to QSB.
+
+This AGC is based on the calculated magnitude (square root of power of the filtered signal as I² + Q²) and will try to adjust audio volume as if a -20dB power signal was received.
+
+<h4>11.3: AGC time constant</h4>
+
+This is the time window in milliseconds of the moving average used to calculate the signal power average. It can be varied in powers of two from 16 to 2048 ms that is: 16, 32, 64, 128, 256, 512, 1024 and 2048 ms. The most practical values are between 128 and 512 ms.
+
+<h4>11.4: Signal power threshold (squelch)</h4>
+
+Active only in AGC mode.
+
+This threshold acts as a squelch and will mute the sound below this average signal power. To prevent short transient drop outs the squelch gets active only if the power has been below the threshold for a period equal to the AGC time constant (11.3)
+
+This feature is mostly useful when more than one SSB channel is active. When there is no transmission the level of noise rises at the level of a normal signal due to the AGC and adds to the noise of other channels. Therefore it is desirable to shut down the audio when there is no signal in the channel.
+
+To turn off the squelch completely move the knob all the way down (left). Then "---" will display as the value and the squelch will be disabled.
+
+The signal power is calculated as the moving average over the AGC time constant (11.3) of the power  of the filtered signal as I² + Q².
+
+<h4>11.5: Signal power threshold (squelch) gate</h4>
+
+Active only in AGC mode with squelch enabled.
+
+To avoid unwanted squelch opening on short transient bursts only signals wilth power above threshold during this period in milliseconds will open the squelch.It can be varied from 0 to 20 ms in 1 ms steps.
+
+When the power threshold is close to the noise floor a few milliseconds help in preventing noise power wiggle to open the squelch.
+
 <h3>12: Audio mute</h3>
 
 Use this button to toggle audio mute for this channel.