[mythtv] "Extra Audio Buffering" setting

[Peter Bennett]

Hi Devs

There is a Frontend General Playback setting called "Extra audio 
buffering" which mentions certain types of tuners that cause crackly 
audio. Actually it enables setting DecodeExtraAudio which then maybe 
sets a flag called lowbuffers in the demuxer (class AvFormatDecoder). 
There is lots of complicated logic around whether or not that flag is 
set and in some cases its value is saved and reset.

The lowbuffers flag is only tested once in the system and if set the 
program buffers 100ms of video packets in AvFormatDecoder::GetFrame, so 
that audio is not left behind.

Bug #13172 describes how playback becomes extremely jerky if you adjust 
audio sync negatively more than a small amount. The solution is very 
simple, just buffer more than 100ms in AvFormatDecoder::GetFrame. I 
tested with 1000ms and then you can successfully set audio sync to 
somewhere around -1200 before it becomes jerky. In the patch for the fix 
I settled on a figure of 250 as a good default and added for a setting 
called AudioReadAhead for the value.

I would also like to get rid of the complex logic and flag surrounding 
the DecodeExtraAudio flag and replace that setting in frontend setup 
with AudioReadAhead with a default value of 250. With a value of 250 or 
1000 it works on everything I have tried. There is no adverse impact 
that I can see, although it will take some extra memory for the 
additional buffering. I do not know why there is all the extra logic 
around whether or not to set the flag, it is safe and easy to set it all 
the time.

I propose a setting that specifies number of milliseconds of audio 
read-ahead with an explanation that says to increase this value if the 
audio is jerky especially when adjusting audio sync negative.

Any comments?

Peter