[FFmpeg-devel] [PATCH 1/2] avdevice/decklink_dec: mark the field flag if framerate > 30FPS

[Marton Balint]

On Sat, 18 Jul 2020, lance.lmwang at gmail.com wrote:

> On Fri, Jul 17, 2020 at 07:13:28PM +0200, Marton Balint wrote:
>> 
>> 
>> On Fri, 17 Jul 2020, lance.lmwang at gmail.com wrote:
>> 
>> > From: Limin Wang <lance.lmwang at gmail.com>
>> > 
>> > In SMPTE ST 12-1: 2014 Sec 12.2, we need to mark the frame flag if the frame rate > 30FPS to
>> > avoid interoperability issues. It will be used by the encoder to identify even or odd frames
>> > and correctly calculate the frame number of the SEI TC.
>> 
>> This feature looks like it belongs to av_timecode_get_smpte_from_framenum
>> and not into decklink. Also checking previous timecode and guessing
>> something is a hack, especially since you have the exact source timecode.
> If I understand correctly, it's not hacky, for the framerate > 30, the source timecode will be
> frame pair(it's limited by 2bits limit of frame number ten digital. Below is one exmaple for 50fps
> if you tested with SDI with TC.
> 00:00:00:24
> 00:00:00:24
> 00:00:00:25
> 00:00:00:25
> 00:00:00:00
> 00:00:00:00

Have you actually tested this with a real SDI signal containing timecode?

Thanks,
Marton

>
> That's why I check the last TC to get the frame is even or odd.
>
> Why not use av_timecode_get_smpte_from_framenum(), for it's calculated by the string TC by one time,
> so it lacks the information whether it's odd or even frame.
>
>> 
>> Regards,
>> Marton
>> 
>> > 
>> > ---
>> > libavdevice/decklink_common.h |  1 +
>> > libavdevice/decklink_dec.cpp  | 14 ++++++++++++++
>> > 2 files changed, 15 insertions(+)
>> > 
>> > diff --git a/libavdevice/decklink_common.h b/libavdevice/decklink_common.h
>> > index bd68c7b..8ddc411 100644
>> > --- a/libavdevice/decklink_common.h
>> > +++ b/libavdevice/decklink_common.h
>> > @@ -151,6 +151,7 @@ struct decklink_ctx {
>> >     int channels;
>> >     int audio_depth;
>> >     unsigned long tc_seen;    // used with option wait_for_tc
>> > +    uint32_t last_tc;
>> > };
>> > 
>> > typedef enum { DIRECTION_IN, DIRECTION_OUT} decklink_direction_t;
>> > diff --git a/libavdevice/decklink_dec.cpp b/libavdevice/decklink_dec.cpp
>> > index dde68ff..a60c01b 100644
>> > --- a/libavdevice/decklink_dec.cpp
>> > +++ b/libavdevice/decklink_dec.cpp
>> > @@ -884,12 +884,26 @@ HRESULT decklink_input_callback::VideoInputFrameArrived(
>> >                         int metadata_len;
>> >                         uint8_t* packed_metadata;
>> >                         AVTimecode tcr;
>> > +                        AVRational rate = ctx->video_st->r_frame_rate;
>> > 
>> >                         if (av_timecode_init_from_string(&tcr, ctx->video_st->r_frame_rate, tc, ctx) >= 0) {
>> >                             uint32_t tc_data = av_timecode_get_smpte_from_framenum(&tcr, 0);
>> >                             int size = sizeof(uint32_t) * 4;
>> >                             uint32_t *sd = (uint32_t *)av_packet_new_side_data(&pkt, AV_PKT_DATA_S12M_TIMECODE, size);
>> > 
>> > +                            /* set the field flag if frame rate > 30FPS */
>> > +                            /* Refer to SMPTE ST 12-1:2014 Sec 12.2 */
>> > +                            if (av_cmp_q(rate, (AVRational) {30, 1}) == 1) {
>> > +                                /* Odd frame */
>> > +                                if (ctx->last_tc == tc_data) {
>> > +                                    if (av_cmp_q(rate, (AVRational) {50, 1}) == 0)
>> > +                                        tc_data |= (1 << 7);
>> > +                                    else
>> > +                                        tc_data |= (1 << 23);
>> > +                                }
>> > +                            }
>> > +                            ctx->last_tc = tc_data;
>> > +
>> >                             if (sd) {
>> >                                 *sd       = 1;       // one TC
>> >                                 *(sd + 1) = tc_data; // TC
>> > -- 
>> > 2.9.4
>> > 
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>
> -- 
> Thanks,
> Limin Wang
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