[mythtv] AGC for audio

[John Fulton]

Hi devs,

With a recent upgrade to 0.21 I found that audio channels from my DVB 
tuners and the DVD were low - in my setup this was obvious with 
mythmusic starting mp3 playback after exiting a video recording and the 
speaker cones nearly flying across the room.  I've also noticed that 
other users complain about varying levels between channels or tuners.

I've coded up a simple Auto Gain Control (AGC) based on this post 
http://www.dsprelated.com/showmessage/21943/1.php .  It's modified to 
take into account that the gain can go too high during periods of silence.

I placed the call to ManageAGC() in AddAudioData() before any calls to 
AddSamples() in NuppleVideoPlayer.cpp . It seems to be working well.  In 
normal operation it has two floating point multiplications and 2 
add/subtracts per an audio sample.

I've coded it against the release-0-21-fixes branch and placed a svn 
diff patch here if anyone would like to try it out: 
http://people.connect.net.au/bf/john/agc_svn_diff

Regards,
John