avdevice/decklink_dec: add option to align capture start time

This option is useful for maintaining input synchronization across N
different hardware devices deployed for 'N-way' redundancy.
The system time of different hardware devices should be synchronized
with protocols such as NTP or PTP, before using this option.

Signed-off-by: Marton Balint <cus@passwd.hu>

doc/indevs.texi

@@ -371,6 +371,20 @@ If set to @option{true}, timestamps are forwarded as they are without removing
 the initial offset.
 Defaults to @option{false}.
 
+@item timestamp_align
+Capture start time alignment in seconds. If set to nonzero, input frames are
+dropped till the system timestamp aligns with configured value.
+Alignment difference of upto one frame duration is tolerated.
+This is useful for maintaining input synchronization across N different
+hardware devices deployed for 'N-way' redundancy. The system time of different
+hardware devices should be synchronized with protocols such as NTP or PTP,
+before using this option.
+Note that this method is not foolproof. In some border cases input
+synchronization may not happen due to thread scheduling jitters in the OS.
+Either sync could go wrong by 1 frame or in a rarer case
+@option{timestamp_align} seconds.
+Defaults to @samp{0}.
+
 @end table
 
 @subsection Examples

libavdevice/decklink_common_c.h

@@ -56,6 +56,7 @@ struct decklink_cctx {
     int raw_format;
     int64_t queue_size;
     int copyts;
+    int64_t timestamp_align;
 };
 
 #endif /* AVDEVICE_DECKLINK_COMMON_C_H */

libavdevice/decklink_dec.cpp

@@ -703,6 +703,16 @@ HRESULT decklink_input_callback::VideoInputFrameArrived(
         return S_OK;
     }
 
+    // Drop the frames till system's timestamp aligns with the configured value.
+    if (0 == ctx->frameCount && cctx->timestamp_align) {
+        AVRational remainder = av_make_q(av_gettime() % cctx->timestamp_align, 1000000);
+        AVRational frame_duration = av_inv_q(ctx->video_st->r_frame_rate);
+        if (av_cmp_q(remainder, frame_duration) > 0) {
+            ++ctx->dropped;
+            return S_OK;
+        }
+    }
+
     ctx->frameCount++;
     if (ctx->audio_pts_source == PTS_SRC_WALLCLOCK || ctx->video_pts_source == PTS_SRC_WALLCLOCK)
         wallclock = av_gettime_relative();

libavdevice/decklink_dec_c.c

@@ -84,6 +84,7 @@ static const AVOption options[] = {
     { "queue_size",    "input queue buffer size",   OFFSET(queue_size),   AV_OPT_TYPE_INT64, { .i64 = (1024 * 1024 * 1024)}, 0, INT64_MAX, DEC },
     { "audio_depth",   "audio bitdepth (16 or 32)", OFFSET(audio_depth),  AV_OPT_TYPE_INT,   { .i64 = 16}, 16, 32, DEC },
     { "decklink_copyts", "copy timestamps, do not remove the initial offset", OFFSET(copyts), AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0, 1, DEC },
+    { "timestamp_align", "capture start time alignment (in seconds)", OFFSET(timestamp_align), AV_OPT_TYPE_DURATION, { .i64 = 0 }, 0, INT_MAX, DEC },
     { NULL },
 };
 

libavdevice/version.h

@@ -29,7 +29,7 @@
 
 #define LIBAVDEVICE_VERSION_MAJOR  58
 #define LIBAVDEVICE_VERSION_MINOR   4
-#define LIBAVDEVICE_VERSION_MICRO 104
+#define LIBAVDEVICE_VERSION_MICRO 105
 
 #define LIBAVDEVICE_VERSION_INT AV_VERSION_INT(LIBAVDEVICE_VERSION_MAJOR, \
                                                LIBAVDEVICE_VERSION_MINOR, \