Tvheadend

Further investigation of this -

Because of the wrong way round tagging (ADTS as LATM and vice versa), TVHeadend in-built transcoding for these audio tracks does not work for either type. No audio is delivered when trying to transcode.

Raw audio (without transcoding) does work in VLC, because the raw ADTS or LATM is just passed without having to be parsed by AVConv in TVH.

I don't understand the code very well, but I did find something that looks like a mix-up of the two!

In the esstream.h file we have -

SCT_AAC,     /* AAC-LATM in MPEG-TS, ADTS + AAC in packet form */

SCT_MP4A,    /* ADTS + AAC in MPEG-TS and packet form */

So ADTS as SCT_MP4A and LATM as SCT_AAC.

Whereas in streaming.c we have the opposite -

{ "AAC",        SCT_AAC },

{ "AAC-LATM",   SCT_MP4A },

I think one of these is the wrong way round which I guess could be causing the mix-up!

I also found in libav.c -

  case SCT_MP4A:
  case SCT_AAC:
    codec_id = AV_CODEC_ID_AAC;
    break;

I think whichever of them is the AAC LATM one should be mapped to 'AV_CODEC_ID_AAC_LATM'.

In the rest of the code, 'AV_CODEC_ID_AAC' is just being treated as ADTS, even though both types of AAC are mapped to it, with a function for transcoding to remove the ADTS header - this will never work for LATM!

src/muxer/muxer_libav.c -

} else if (codec_id == AV_CODEC_ID_AAC) {
      /* remove ADTS header */
      packet.data = pktbuf_ptr(pkt->pkt_payload) + 7;
      packet.size = pktbuf_len(pkt->pkt_payload) - 7;

Did some more investigation and finally got to the bottom/fixed these issues.

What I hadn't fully realised is that TVHeadend re-packages LATM audio with ADTS headers internally before outputting a raw stream (in parser_latm.c).

So the issue is actually - why does raw ADTS audio that TVHeadend creates itself from an AAC-LATM source work with my Sonos speaker/Internet radio, but raw ADTS audio that was already ADTS in the MPEG-TS not work?

The answer is in the content type/MIME headers! In src/muxer.c -

{ "audio/aac",                MC_AAC },
{ "audio/mp4",                MC_MP4A },

MC_AAC (which maps to AAC-LATM streams), is output with the MIME type of 'audio/aac', whereas MC_MP4A (which maps to AAC - ADTS in MPEG-TS) is output with the MIME type of 'audio/mp4'.

As both are being output as ADTS in the end, this doesn't make sense as they are the same stream structure!

So, I changed the MC_MP4A line to be 'audio/aac' (the same as MC_AAC) -

{ "audio/aac",                MC_MP4A },

And... bingo! MPEG-TS ADTS raw audio streams now work perfectly with my internet radios!

I also swapped the mapping in streaming.c which fixes the web interface flagging in the display of AAC and AAC-LATM -

{ "AAC-LATM",   SCT_AAC },
{ "AAC",        SCT_MP4A },

I've never contributed before, but I can commit these changes for review on GitHub if that would be helpful?

Changing the streaming.c labels also then requires this change in htsp_server.c so that AAC-LATM gets reported as 'AAC' in HTSP. (Otherwise there's no sound in Kodi on channels with AAC-LATM audio, such as UK DVB-T2 HD channels) -

-if (ssc->es_type == SCT_MP4A)
+if (ssc->es_type == SCT_AAC)
      type = "AAC"; /* override */