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We are trying to get super accurate timings in an flv video in relation to javascript. For example, we are using javascript to display an element in a canvas on the screen, and we need to pinpoint the exact frame that the image occurs in on the netstream/flv.

We are publishing the netstream to a wowza media server, and doing no transcoding. Theoretically (at least in my limited knowledge), using ns.send to send a message down the line to Wowza is appending into the same flv file and I can see it there. However, we are seeing that the inserted message is ~100ms off of when we actually sent it.

Does anyone have any experience in doing something like this? A potential solution would be to append an all black frame into the actual video frame, but I can't seem to find any information on how to manipulate a netstream during publish.

Roger Hsiao
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  • The delay is gonna be inevitable since there is travelling & waiting to receive/append involved. A suggestion to try, if you haven't, is maybe using a (javascript?) timer to play video (after ~100 ms delay) or send message (~100 ms before video play) to attempt some basic syncing.. But can message be sent before video playing? – VC.One Jul 22 '14 at 23:36
  • Thanks for the response! I unfortunately can't do this because the delay isn't always constant and varies from 50ms - 200ms. I was actually under the impression that there shouldn't be any delay because its all being put into the netstream buffer. Thus, there will be a delay in the travel latency to the Wowza server, but in terms of relative time to the start of the video/flv, it should remain constant. ie.....If I send a message 5000 ms after calling ns.publish, it should occur 5000ms +/- 30ms after the start of the recorded flv. – Roger Hsiao Jul 22 '14 at 23:56
  • I was pulling an all-nighter when 1st response and should have been quicker to say.. When you send messages they are a separate thing to the video stream itself (you could pause your video and still have code that sends an NS message every 2000ms forever until close or crash..). As for appending black frames you would need to do that before publishing. Using NS.appendBytes. A hint is here: [Simple FLV Writer](http://www.zeropointnine.com/blog/simpleflvwriteras-as3-class-to-create-flvs/). The bitmapData would be your black frame(s).. A bit complicated cos the metadata bytes need an update also – VC.One Jul 23 '14 at 14:49
  • The easier (and better) option is to not append frames per assumed delay (too much math+bytes but not reliable on all devices) but concentrate on getting a sync between your Flash app & Javascript, The receiving side is lagging so let it control the playback. **1)** Publish and immediately pause the NS video --> **2)** Send message that video ready--> **3)** Receiver (JS) acknowledges message and sends a `ns.resume` command to begin video.. This should have tighter sync and its up to your testing to find potential improvements (JS is not my strength, just suggesting what I would try) – VC.One Jul 23 '14 at 15:07

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