static int spdif_write_packet(struct AVFormatContext *s, AVPacket *pkt)
{
    IEC61937Context *ctx = s->priv_data;
    int ret, padding;

    ctx->out_buf = pkt->data;
    ctx->out_bytes = pkt->size;
    ctx->length_code = FFALIGN(pkt->size, 2) << 3;
    ctx->use_preamble = 1;
    ctx->extra_bswap = 0;

    ret = ctx->header_info(s, pkt);
    if (ret < 0)
        return ret;
    if (!ctx->pkt_offset)
        return 0;

    padding = (ctx->pkt_offset - ctx->use_preamble * BURST_HEADER_SIZE - ctx->out_bytes) & ~1;
    if (padding < 0) {
        av_log(s, AV_LOG_ERROR, "bitrate is too high\n");
        return AVERROR(EINVAL);
    }

    if (ctx->use_preamble) {
        spdif_put_16(ctx, s->pb, SYNCWORD1);       //Pa
        spdif_put_16(ctx, s->pb, SYNCWORD2);       //Pb
        spdif_put_16(ctx, s->pb, ctx->data_type);  //Pc
        spdif_put_16(ctx, s->pb, ctx->length_code);//Pd
    }

    if (ctx->extra_bswap ^ (ctx->spdif_flags & SPDIF_FLAG_BIGENDIAN)) {
    put_buffer(s->pb, ctx->out_buf, ctx->out_bytes & ~1);
    } else {
    av_fast_malloc(&ctx->buffer, &ctx->buffer_size, ctx->out_bytes + FF_INPUT_BUFFER_PADDING_SIZE);
    if (!ctx->buffer)
        return AVERROR(ENOMEM);
    ff_spdif_bswap_buf16((uint16_t *)ctx->buffer, (uint16_t *)ctx->out_buf, ctx->out_bytes >> 1);
    put_buffer(s->pb, ctx->buffer, ctx->out_bytes & ~1);
    }

    /* a final lone byte has to be MSB aligned */
    if (ctx->out_bytes & 1)
        spdif_put_16(ctx, s->pb, ctx->out_buf[ctx->out_bytes - 1] << 8);

    put_nbyte(s->pb, 0, padding);

    av_log(s, AV_LOG_DEBUG, "type=%x len=%i pkt_offset=%i\n",
           ctx->data_type, ctx->out_bytes, ctx->pkt_offset);

    put_flush_packet(s->pb);
    return 0;
}