Re: [FFmpeg-devel] [PATCH] ffprobe/eac3/mlp/dca: add detection of spatial audio extensions

[Hendrik Leppkes <h.leppkes@gmail.com>]

On Sun, Feb 12, 2023 at 1:53 AM Marth64 <marth64@proxyid.net> wrote:
> diff --git a/libavcodec/dca_syncwords.h b/libavcodec/dca_syncwords.h
> index 4d2cd5f56d..200702f89e 100644
> --- a/libavcodec/dca_syncwords.h
> +++ b/libavcodec/dca_syncwords.h
> @@ -33,4 +33,7 @@
>  #define    DCA_SYNCWORD_SUBSTREAM_CORE       0x02B09261U
>  #define    DCA_SYNCWORD_REV1AUX              0x9A1105A0U
>
> +#define    DCA_SYNCWORD_XLL_X                0x00020008U
> +#define    DCA_SYNCWORD_XLL_X_IMAX           0x00F14000U
> +
>  #endif /* AVCODEC_DCA_SYNCWORDS_H */
> diff --git a/libavcodec/dca_xll.c b/libavcodec/dca_xll.c
> index fe2c766d98..efbbae67f8 100644
> --- a/libavcodec/dca_xll.c
> +++ b/libavcodec/dca_xll.c
> @@ -19,6 +19,7 @@
>   */
>
>  #include "libavutil/channel_layout.h"
> +#include "avcodec.h"
>  #include "dcadec.h"
>  #include "dcadata.h"
>  #include "dcamath.h"
> @@ -1043,6 +1044,7 @@ static int parse_band_data(DCAXllDecoder *s)
>  static int parse_frame(DCAXllDecoder *s, const uint8_t *data, int size, DCAExssAsset *asset)
>  {
>      int ret;
> +    int extradata_peek_pos;
>
>      if ((ret = init_get_bits8(&s->gb, data, size)) < 0)
>          return ret;
> @@ -1054,10 +1056,23 @@ static int parse_frame(DCAXllDecoder *s, const uint8_t *data, int size, DCAExssA
>          return ret;
>      if ((ret = parse_band_data(s)) < 0)
>          return ret;
> +
> +    extradata_peek_pos = (get_bits_count(&s->gb) + 31) & ~31;
> +    if (s->frame_size * 8 > extradata_peek_pos) {
> +        unsigned int extradata_syncword = show_bits_long(&s->gb, 32);
> +
> +        if (extradata_syncword == DCA_SYNCWORD_XLL_X) {
> +            s->x_syncword_present = 1;
> +        } else if (extradata_syncword == DCA_SYNCWORD_XLL_X_IMAX) {
> +            s->x_imax_syncword_present = 1;
> +        }
> +    }
> +


I was testing this, and the DTS detections were not very reliable for
me. This is what I came up with instead:

#define    DCA_SYNCWORD_XLL_X                0x02000850U
#define    DCA_SYNCWORD_XLL_X_IMAX           0xF14000D0U


    if (s->frame_size * 8 > FFALIGN(get_bits_count(&s->gb), 32)) {
        unsigned int extradata_syncword;

        // align to dword
        skip_bits_long(&s->gb, -get_bits_count(&s->gb) & 31);

        // get sync code
        extradata_syncword = show_bits_long(&s->gb, 32);

        if (extradata_syncword == DCA_SYNCWORD_XLL_X) {
            s->x_syncword_present = 1;
        } else if ((extradata_syncword >> 1) ==
(DCA_SYNCWORD_XLL_X_IMAX >> 1)) {
            s->x_imax_syncword_present = 1;
        }
    }

I don't have many DTS:X discs, but this worked on them, the old code did not.
Aligning to DWORD for a new section is a typical DTS thing to do,
which then also resulted in the syncwords to shift a bit, and actually
include more digits.

The IMAX case is a bit weird, there seems to be an extra bit in there
thats not stable, so shifting it out improves the detection (or it
could be masked out, but same difference).

- Hendrik
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