[FFmpeg-devel] [PATCH v2] avformat/pcm: decrease delay when reading PCM streams.

[Tomas Härdin]

On 2018-02-14 15:11, Philipp M. Scholl wrote:
> Hi there,
>
>   thanks for the feedback. I made the following changes to the patch:
>
>   - hard-code to 40ms delay. Wasn't really sure what a good value is. But given
>     that the is mainly for encoding low-rate sensor data in audio streams,
>     together with a video recording, 40ms should be fine in most cases.
>
>   - fix issue when sample rate is smaller (<100, now 25).
>
>   - use ff_log2 for calculating a power of two read size. A power of two to make
>     efficient use of I/O buffers.
>
>   - remove the superfluous check for multiple input streams. PCM decoders are
>     always single-streams, right?!

There are certainly formats which support multiple audio streams. MOV 
and MXF comes to mind. Anywhere you need multiple languages.. But I see 
the patch doesn't change anything having to do with stream count anyway

> diff --git a/libavformat/pcm.c b/libavformat/pcm.c
> index 806f91b6b1..48c3c09bb4 100644
> --- a/libavformat/pcm.c
> +++ b/libavformat/pcm.c
> @@ -28,13 +28,21 @@
>   
>   int ff_pcm_read_packet(AVFormatContext *s, AVPacket *pkt)
>   {
> -    int ret, size;
> +    int ret, size = INT_MAX;
> +    AVCodecParameters *codec = s->streams[0]->codecpar;
> +
> +    /*
> +     * Compute read size to complete a read every 40ms.  Clamp to RAW_SAMPLES if
> +     * larger. Use power of two as readsize for I/O efficiency.
> +     */
> +    size = (codec->sample_rate/25 | 1) * codec->block_align;

FFMAX(codec->sample_rate/25, 1) would be nicer

> +    size = FFMIN(size, RAW_SAMPLES * codec->block_align);
> +    size = 1 << ff_log2(size);
>   
> -    size= RAW_SAMPLES*s->streams[0]->codecpar->block_align;
>       if (size <= 0)
>           return AVERROR(EINVAL);

This will never be true since ff_log() always returns >= 0 which makes 
size >= 1. I suggest putting it before the 1 << ff_log2.

>   
> -    ret= av_get_packet(s->pb, pkt, size);
> +    ret = av_get_packet(s->pb, pkt, size);

Doesn't looks like it belongs, but it makes all the assignments align so 
whatever

/Tomas