[FFmpeg-devel] [PATCH v2] avdevice/decklink: Add option to align Capture start time

[Karthick J]

From: Karthick Jeyapal <kjeyapal at akamai.com>

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.
---
 doc/indevs.texi                 | 13 +++++++++++++
 libavdevice/decklink_common_c.h |  1 +
 libavdevice/decklink_dec.cpp    | 11 +++++++++++
 libavdevice/decklink_dec_c.c    |  1 +
 4 files changed, 26 insertions(+)

diff --git a/doc/indevs.texi b/doc/indevs.texi
index ed2784b..694bac9 100644
--- a/doc/indevs.texi
+++ b/doc/indevs.texi
@@ -371,6 +371,19 @@ If set to @option{true}, timestamps are forwarded as they are without removing
 the initial offset.
 Defaults to @option{false}.
 
+ at 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 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 by even @option{timestamp_align} seconds.
+Defaults to @samp{0}.
+
 @end table
 
 @subsection Examples
diff --git a/libavdevice/decklink_common_c.h b/libavdevice/decklink_common_c.h
index 32a5d70..8e3bbeb 100644
--- a/libavdevice/decklink_common_c.h
+++ b/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 */
diff --git a/libavdevice/decklink_dec.cpp b/libavdevice/decklink_dec.cpp
index 7fabef2..1f4d68b 100644
--- a/libavdevice/decklink_dec.cpp
+++ b/libavdevice/decklink_dec.cpp
@@ -703,6 +703,17 @@ 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_make_q(ctx->video_st->r_frame_rate.den,
+                                              ctx->video_st->r_frame_rate.num);
+        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();
diff --git a/libavdevice/decklink_dec_c.c b/libavdevice/decklink_dec_c.c
index 6ab3819..2e6fcb6 100644
--- a/libavdevice/decklink_dec_c.c
+++ b/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 },
 };
 
-- 
2.7.4