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

Jeyapal, Karthick kjeyapal at akamai.com
Thu Sep 27 14:14:24 EEST 2018

On 9/27/18 2:44 AM, Marton Balint wrote:
> On Tue, 25 Sep 2018, Jeyapal, Karthick wrote:
>> On 9/24/18 7:42 PM, Devin Heitmueller wrote:
>>> Hello Karthick,
>>>> On Sep 24, 2018, at 7:49 AM, Karthick J <kjeyapal at akamai.com> wrote:
>>>> 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. 
>>> I can certainly see the usefulness of such a feature, but is the decklink module really the right place for this?  This feels like something that should be done through a filter (either as a multimedia filter or a BSF). 
>> Hi Devin,
>> Thank you very much for the feedback. I agree with you that if it can be done through a filter, then it is certainly a better place to do it. But I as far I understand it can't be implemented reliably in a filter, without imposing additional restrictions and/or added complexities. This is primarily due to the fact the frames might take different times to pass through the pipeline thread in each hardware and reach the filter function at different times and hence losing some synchronization w.r.t system time. In other words, some modules in the pipeline contains CPU intensive code(such as video decode), before it reaches the filter function. The thread that needs to do this should be very lightweight without any CPU intensive operations. And for better reliability it needs to be done as soon as the frame is received from the driver. For example, the video frame(captured by decklink device) could take different times to pass through V210 decoder due to HW differences and/or CPU load due to other encoder threads. This unpredictable decoder delay kind of rules out multimedia filters for this kind of operation. Now a bitstream filter(BSF) can mitigate this issue to some extent as it sits before a decoder. We will still need to insert a thread(and associated buffering) in the BSF, so that the decoder is decoupled from this time-sensitive thread. But still it doesn't provide any guarantee against CPU intensive operations performed in the capture plugin. For example, the Decklink plugin performs some VANC processing which could be CPU intensive in a low-end 2-core Intel processor. Or even if we assume Decklink plugin doesn't perform any CPU intensive operations, we cannot guarantee the same for other capture device plugins. Another option to implement this in filters would be to use "copyts" and drop the frames based of PTS/DTS value instead of system time. But then this imposes a restriction that "copyts" needs to be used mandatorily. If somebody needs to use it without "copyts", then it won't work. My understanding on ffmpeg is limited, and hence the above explanation may not be entirely correct. So, please feel free to correct me. 
> How about adding such an option to ffmpeg.c? You still can use wallclock timestamps in decklink, and then drop the frames (packets) in ffmpeg.c before the timestamps are touched.
Yes, that's true. But, I will have to set the decklink options abs_wallclock and decklink_copyts for it work correctly. This will be the restriction that we will impose on the user of this option.
If I have to set those two decklink options, then an additional copyts option along with a f_select expression that you had suggested below will do the job without any changes to the code. For example, I used the select expr "lte(mod(pts*1000000*tb, 6*1000000), 41666)+selected_n" where 6 is the segment size and 41666 is the frame duration of 24fps video. This seems like an easiest workaround for this issue. Thank you very much for suggesting the usage of f_select. 
> Another approch might be to store the wallclock frame time as some kind of metadata (as it is done for "timecode") and then add the possiblity to f_select to drop based on this. However the evaluation engine has no concept of complex objects (like frame, or frame metadata) so this probably needs additional work.
This involves a lot of extra work for a feature that can be implemented very easily on the capture plugin. And still other capture plugins will have to add the relevant metadata/sidedata for this feature to work for them. If you still think that decklink plugin is not the right place to add this feature, then I respect that decision. I will live with the f_select solution with extra restrictions on timestamping options (
Thanks again for your valuable suggestions.

> Regards,
> Marton
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