Re: [FFmpeg-devel] [PATCH v4] decklink: Add support for compressed AC-3 output over SDI

[Marton Balint <cus@passwd.hu>]

On Mon, 3 Apr 2023, Devin Heitmueller wrote:

> Extend the decklink output to include support for compressed AC-3,
> encapsulated using the SMPTE ST 377:2015 standard.
>
> This functionality can be exercised by using the "copy" codec when
> the input audio stream is AC-3.  For example:
>
> ./ffmpeg -i ~/foo.ts -codec:a copy -f decklink 'UltraStudio Mini Monitor'
>
> Note that the default behavior continues to be to do PCM output,
> which means without specifying the copy codec a stream containing
> AC-3 will be decoded and downmixed to stereo audio before output.
>
> Thanks to Marton Balint for providing feedback.
>
> Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
> ---
> libavdevice/decklink_enc.cpp | 97 ++++++++++++++++++++++++++++++------
> 1 file changed, 82 insertions(+), 15 deletions(-)
>
> diff --git a/libavdevice/decklink_enc.cpp b/libavdevice/decklink_enc.cpp
> index 8d423f6b6e..9ee1925fd0 100644

[...]

> --- a/libavdevice/decklink_enc.cpp
> +++ b/libavdevice/decklink_enc.cpp
> +/* Wrap the AC-3 packet into an S337 payload that is in S16LE format which can be easily
> +   injected into the PCM stream.  Note: despite the function name, only AC-3 is implemented */
> +static int create_s337_payload(AVPacket *pkt, enum AVCodecID codec_id, uint8_t **outbuf, int *outsize)

Actually you can remove the codec_id parameter as well...

> +{
> +    // Note: if the packet is an odd-number of bytes, we need to make
> +    // the actual payload one byte larger to ensure it ends on an S16LE boundary
> +    int payload_size = pkt->size + (pkt->size % 2) + 8;

FFALIGN(pkt->size, 2). But you'd want FFALIGN(pkt->size, 4) because you 
want the buffer size to be divisable by 4 because later decklink needs a 
sample count...

> +    uint16_t bitcount = pkt->size * 8;

Is this supposed to be aligned too? I see similar code in 
libavformat/spdifenc.c and FFALIGN(pkt->size, 2) << 3 is used there.

> +    uint8_t *s337_payload;
> +    PutByteContext pb;
> +
> +    /* Sanity check:  According to SMPTE ST 340:2015 Sec 4.1, the AC-3 sync frame will
> +       exactly match the 1536 samples of baseband (PCM) audio that it represents.  */
> +    if (pkt->size > 1536)
> +        return AVERROR(EINVAL);
> +
> +    /* Encapsulate AC3 syncframe into SMPTE 337 packet */
> +    s337_payload = (uint8_t *) av_malloc(payload_size);
> +    if (s337_payload == NULL)
> +        return AVERROR(ENOMEM);
> +    bytestream2_init_writer(&pb, s337_payload, payload_size);
> +    bytestream2_put_le16u(&pb, 0xf872); /* Sync word 1 */
> +    bytestream2_put_le16u(&pb, 0x4e1f); /* Sync word 1 */
> +    bytestream2_put_le16u(&pb, 0x0001); /* Burst Info, including data type (1=ac3) */
> +    bytestream2_put_le16u(&pb, bitcount); /* Length code */
> +    for (int i = 0; i < (pkt->size - 1); i += 2)
> +        bytestream2_put_le16u(&pb, (pkt->data[i] << 8) | pkt->data[i+1]);
> +    if (pkt->size % 2)

pkt->size & 1

> +        bytestream2_put_le16u(&pb, pkt->data[pkt->size - 1] << 8);
> +

And you likely want a bytestream2_put_le16(&pb, 0) in the end so even 
the end of the 4-byte aligned buffer is properly zeroed.

Thanks,
Marton
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