[FFmpeg-devel] [PATCH v4 3/3] aadec: improve seeking in mp3 content

[Karsten Otto]

Ping - What about this one? I tested it with about 20 files and it works perfectly
for all of them - except one which has tag/padding. In its case, playback quality
is virtually the same as without the patch, i.e. no harm done.

Cheers, Karsten

> Am 07.07.2018 um 19:41 schrieb Karsten Otto <ottoka at posteo.de>:
> 
> MP3 frames may not be aligned to aa chunk boundaries. When seeking,
> calculate the expected frame offset in the target chunk. Adjust the
> timestamp and truncate the next packet accordingly.
> 
> This solution works for the majority of tested audio material. For
> some rare encodings with mp3 padding or embedded id3 tags, it will
> mispredict the correct offset, and at worst skip an extra frame.
> ---
> libavformat/aadec.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/libavformat/aadec.c b/libavformat/aadec.c
> index e3c03bc522..2b9e4e526c 100644
> --- a/libavformat/aadec.c
> +++ b/libavformat/aadec.c
> @@ -37,6 +37,7 @@
> #define TEA_BLOCK_SIZE 8
> #define CHAPTER_HEADER_SIZE 8
> #define TIMEPREC 1000
> +#define MP3_FRAME_SIZE 104
> 
> typedef struct AADemuxContext {
>     AVClass *class;
> @@ -50,6 +51,7 @@ typedef struct AADemuxContext {
>     int64_t current_chapter_size;
>     int64_t content_start;
>     int64_t content_end;
> +    int seek_offset;
> } AADemuxContext;
> 
> static int get_second_size(char *codec_name)
> @@ -230,6 +232,7 @@ static int aa_read_header(AVFormatContext *s)
>     ff_update_cur_dts(s, st, 0);
>     avio_seek(pb, start, SEEK_SET);
>     c->current_chapter_size = 0;
> +    c->seek_offset = 0;
> 
>     return 0;
> }
> @@ -295,12 +298,13 @@ static int aa_read_packet(AVFormatContext *s, AVPacket *pkt)
>     if (c->current_chapter_size <= 0)
>         c->current_chapter_size = 0;
> 
> -    ret = av_new_packet(pkt, written);
> +    ret = av_new_packet(pkt, written - c->seek_offset);
>     if (ret < 0)
>         return ret;
> -    memcpy(pkt->data, buf, written);
> +    memcpy(pkt->data, buf + c->seek_offset, written - c->seek_offset);
>     pkt->pos = pos;
> 
> +    c->seek_offset = 0;
>     return 0;
> }
> 
> @@ -344,7 +348,12 @@ static int aa_read_seek(AVFormatContext *s,
>     c->current_chapter_size = chapter_size - chapter_pos;
>     c->chapter_idx = 1 + chapter_idx;
> 
> -    ff_update_cur_dts(s, s->streams[0], ch->start + chapter_pos * TIMEPREC);
> +    // handle extra offset for unaligned frames
> +    if (s->streams[0]->codecpar->codec_id == AV_CODEC_ID_MP3) {
> +        c->seek_offset = (MP3_FRAME_SIZE - chapter_pos % MP3_FRAME_SIZE) % MP3_FRAME_SIZE;
> +    }
> +
> +    ff_update_cur_dts(s, s->streams[0], ch->start + (chapter_pos + c->seek_offset) * TIMEPREC);
> 
>     return 1;
> }
> -- 
> 2.14.3 (Apple Git-98)
> 
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel