Function create_s337_payload

libavdevice/decklink_enc.cpp:323–358

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 */

/* 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, uint8_t **outbuf, int *outsize)
{
    /* Note: if the packet size is not divisible by four, we need to make the actual
       payload larger to ensure it ends on an two channel S16LE boundary */
    int payload_size = FFALIGN(pkt->size, 4) + 8;
    uint16_t bitcount = pkt->size * 8;
    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]);

    /* Ensure final payload is aligned on 4-byte boundary */
    if (pkt->size & 1)
        bytestream2_put_le16u(&pb, pkt->data[pkt->size - 1] << 8);
    if ((pkt->size & 3) == 1 || (pkt->size & 3) == 2)
        bytestream2_put_le16u(&pb, 0);

    *outsize = payload_size;
    *outbuf = s337_payload;
    return 0;
}

static int decklink_setup_subtitle(AVFormatContext *avctx, AVStream *st)
{