[FFmpeg-devel] [PATCH] Properly store sampling rate for FLAC in mp4

[Jean-Yves Avenard]

Note that that it's also not correct for other codecs when the
sampling rate is greater than 65536. Right now it stores 0.
If in quicktime mode, it could use a SoundDescription v2 box, but in
mp4 that doesn't exist.

Per ISO 14496-12 , it should be using a AudioSampleEntryV1 along a
SamplingRateBox ('srat') which uses a 32 bits unsigned integer.
There's no handling of that in the current ffmpeg.

If time permit I will submit something for this.

Rgds
JY


On 25 August 2017 at 13:25, Jean-Yves Avenard <jyavenard at gmail.com> wrote:
> From 9baa7166fa96ed6beac9146c7e3b4dcf425a67d0 Mon Sep 17 00:00:00 2001
> From: Jean-Yves Avenard <jyavenard at mozilla.com>
> Date: Fri, 25 Aug 2017 13:11:28 +0200
> Subject: [PATCH] Properly store sampling rate for FLAC in mp4
>
> Fixes ticket #6609
>
> Signed-off-by: Jean-Yves Avenard <jyavenard at mozilla.com>
> ---
>  libavformat/movenc.c | 28 +++++++++++++++++++++++++---
>  1 file changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> index 10b959ad02..aa4a9c962a 100644
> --- a/libavformat/movenc.c
> +++ b/libavformat/movenc.c
> @@ -1028,9 +1028,31 @@ static int mov_write_audio_tag(AVFormatContext
> *s, AVIOContext *pb, MOVMuxContex
>          avio_wb16(pb, 0); /* packet size (= 0) */
>          if (track->par->codec_id == AV_CODEC_ID_OPUS)
>              avio_wb16(pb, 48000);
> -        else
> -            avio_wb16(pb, track->par->sample_rate <= UINT16_MAX ?
> -                          track->par->sample_rate : 0);
> +        else {
> +            uint32_t rate;
> +            if (track->par->codec_id == AV_CODEC_ID_FLAC) {
> +                /* When the bitstream's native sample rate is greater
> +                 than the maximum expressible value of 65535 Hz,
> +                 the samplerate field shall hold the greatest
> +                 expressible regular division of that rate. I.e.
> +                 the samplerate field shall hold 48000.0 for
> +                 native sample rates of 96 and 192 kHz. In the
> +                 case of unusual sample rates which do not have
> +                 an expressible regular division, the maximum value
> +                 of 65535.0 Hz should be used. */
> +                rate = track->par->sample_rate;
> +                while (rate > UINT16_MAX && (rate & 1) == 0) {
> +                    rate = rate >> 1;
> +                }
> +                if (rate > UINT16_MAX) {
> +                    rate = UINT16_MAX;
> +                }
> +            } else {
> +                rate = track->par->sample_rate <= UINT16_MAX ?
> +                       track->par->sample_rate : 0;
> +            }
> +            avio_wb16(pb, rate);
> +        }
>          avio_wb16(pb, 0); /* Reserved */
>      }
>
> --
> 2.11.0 (Apple Git-81)