[Libav-user] G.711 audio in mkv container

[Alex Grosu]

Dear all

I am having some issues in creating a G711 audio mkv container with 
libavcodec on windows.
I set the codec context (header) like this:
c->codec_id = CODEC_ID_PCM_ALAW;
c->codec_type = CODEC_TYPE_AUDIO;
c->bit_rate = 64000;
c->sample_rate = 8000;
c->channels = 1;
c->frame_size = 2650;
c->channel_layout = 1;
waveFormatEX = malloc(sizeof(WAVEFORMATEXTENSIBLE) + 
FF_INPUT_BUFFER_PADDING_SIZE);
waveFormatEX->Format.wFormatTag = WAVE_FORMAT_EXTENSIBLE;
waveFormatEX->Format.nChannels = 1;
waveFormatEX->Format.nSamplesPerSec = 8000;
waveFormatEX->Format.wBitsPerSample = 16;
waveFormatEX->Format.nBlockAlign = 2;
waveFormatEX->Format.nAvgBytesPerSec = 16000;
waveFormatEX->Format.cbSize = sizeof(WAVEFORMATEXTENSIBLE);
waveFormatEX->Samples.wSamplesPerBlock = 0;
waveFormatEX->dwChannelMask = KSAUDIO_SPEAKER_MONO;
waveFormatEX->SubFormat = KSDATAFORMAT_SUBTYPE_ALAW;
c->extradata = (uint8_t*)waveFormatEX;
c->extradata_size = waveFormatEX->Format.cbSize;

The mkv container is created, ffplay can play it without problems:

This is the what ffplay outputs when playing the file:

Input #0, matroska,webm, from 'c:\Users\alex\Bus 
standard-37-Enregistreur Xebra3
--2016-09-15(0h 1m 0s)\Cam 1 Flux audio.mkv':
   Metadata:
     encoder         : Lavf52.78.1
   Duration: 00:01:52.00, start: 0.000000, bitrate: 34 kb/s
     Stream #0:0: Audio: pcm_alaw ([6][0][0][0] / 0x0006), 8000 Hz, 1 
channels, s
16, 64 kb/s (default)
     Stream #0:1(fra): Subtitle: subrip (default)
    8.94 M-A: -0.000 fd=   0 aq=  421KB vq=    0KB sq=    0B f=0/0

The sound can be heard witthout problems. The file should have ~ 1 
minute, but ffplay say the duration is 1:52

The vlc player instead has issues in playing the file. The sound is 
heard but with "gaps", something related to pts?
This is the info from vlc about the file:
Type: Audio
Codec: PCM ALAW(alaw)
Channels: Mono
Sample rate: 8000 Hz
Bits per sample: 16
And here are some warnings while playing the file:
corewarning: playback way too early (-170000): playing silence

coredebug: inserting 1360 zeroes

corewarning: playback way too early (-150000): playing silence

coredebug: inserting 1200 zeroes

corewarning: playback way too early (-170000): playing silence

coredebug: inserting 1360 zeroes

corewarning: playback way too early (-150000): playing silence



Can somebody please guide how to properly set G.711 (CODEC_ID_PCM_ALAW) 
in libavcodec?
Thank you a lot for reading this long post
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