[mythtv] Corrupted ac3 audio handling in 0.22

Tom Dexter digitalaudiorock at gmail.com
Thu Feb 25 20:58:55 UTC 2010

Back in 0.21 I submitted an ffmpeg patch that prevented loud audio
chirps in corrupted ac3 audio:


That essentially replaced the loud chirps with silence (complete
dropout of the audio).

It appears that since then, ffmpeg now has the some sort of error
concealment (as was noted in the TODO comment on the original fix).  I
believe this is controlled via this bitmask in

enc->error_concealment = FF_EC_GUESS_MVS | FF_EC_DEBLOCK;

...as per this (in libs/libavcodec/avcodec.h):

     * error concealment flags
     * - encoding: unused
     * - decoding: Set by user.
    int error_concealment;
#define FF_EC_GUESS_MVS   1
#define FF_EC_DEBLOCK     2

...though I don't know anything about either one.

I've noticed that since upgrading, when I have any bad ac3 (from bad
weather reception for example), rather than getting silence I get a
real annoying mid-range tone.  While it's much better than the full
scale chirps that would about knock you off your seat, it's _way_ more
annoying than dropouts.

Has anyone else noticed this, or is anyone clear on those flags?  Thanks.


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