[FFmpeg-trac] #6974(undetermined:new): Improve handling of AAC PCE audio channels with segmented MPEG-TS

FFmpeg trac at avcodec.org
Mon Jan 22 06:49:54 EET 2018

#6974: Improve handling of AAC PCE audio channels with segmented MPEG-TS
             Reporter:  redmcg       |                     Type:
               Status:  new          |  enhancement
            Component:               |                 Priority:  normal
  undetermined                       |                  Version:  git-
             Keywords:  hls aac      |  master
  adts pce                           |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
 Summary of the bug:
 I had been happily transcoding from AC3 (5.1 side) to AAC with HLS muxing.
 But after a recompile from 'master' I discovered audio had stopped

 A 'git bisect' indicated that this stopped working with commit

 This commit marked '5.1 side' as not "normal" for AAC and thus a PCE entry
 was now required.

 The PCE entry seems to work fine for the first segment of a segmented
 MPEG-TS, but subsequent MPEG-TS files don't appear to have an ASC
 (AudioSpecificConfig data structure).

 Running ffprobe against the first segment (for example 'ffprobe hls0.ts')
 works fine and returns:
 Input #0, mpegts, from 'hls0.ts':
   Duration: 00:00:02.01, start: 1.400000, bitrate: 382 kb/s
   Program 1
       service_name    : Service01
       service_provider: FFmpeg
     Stream #0:0[0x100](eng): Audio: aac (LC) ([15][0][0][0] / 0x000F),
 48000 Hz, 6 channels, fltp, 378 kb/s

 But running it on the second segment (for example 'ffprobe hls1.ts')
 [mpegts @ 0x1476040] decoding for stream 0 failed
 [mpegts @ 0x1476040] Could not find codec parameters for stream 0 (Audio:
 aac (LC) ([15][0][0][0] / 0x000F), 0 channels, fltp, 426 kb/s):
 unspecified sample rate
 Consider increasing the value for the 'analyzeduration' and 'probesize'
 hls1.ts: End of file

 I discovered the failure occurs at libavcodec/aacdec_template.c:3126 -
 which has this check:
         if (!avctx->channels && elem_type != TYPE_PCE) {
             err = AVERROR_INVALIDDATA;
             goto fail;

 because avctx->channels == 0 and elem_type == TYPE_CPE.

 I'm not sure of the solution though. I figure either:
 a) Add the ASC to beginning of each TS segment (if possible);
 b) Encode each AAC frame with a TYPE_CPE (if possible);
 c) Have HLS default to fMP4 when an AAC CPE is in use (for which CPE seems
 to work); or
 d) Produce a warning

 Although I think option 'b)' breaks compatibility with older decoders (if
 it is even possible to do).

 How to reproduce:
 % ffmpeg -loglevel info -i in.ts -map 0:a -c:0 aac -f hls hls.m3u8
 % ffprobe hls0.ts # works
 % ffprobe hls1.ts # doesn't

 ffprobe version N-89870-gbda5ad3 Copyright (c) 2007-2018 the FFmpeg

Ticket URL: <https://trac.ffmpeg.org/ticket/6974>
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