Reduce FPS on OSMF stream - Issue with MPEG-2 header

Question

I've been searching all over and can't find a solution. I have a 25 FPS video that I'm playing on OSMF, but OSMF insists on playing with 29-31 FPS. This causes the video to play ~15% faster than real time. The result is extremely noticeable if you open the same video in VLC and play it side by side.

The problem comes in when I try to do a live stream. It will eat through the buffer and catch up to real time then the stream crashes because there's no new video waiting.

I've tried tracing the code to find out where the frames are actually output to the screen, but I hit a dead end at the SWC file. I also have tried searching online but I can't find anything about limiting the FPS - everyone is just interested in increasing it.

I'd rather play at 15 FPS and drop 10 frames per second than catch up to real time and crash tragically.

Edit - after an entire weekend spent staring at this issue I've made some incredible headway. First and foremost, the only way to limit the FPS in OSMF is by sending a custom FLV header with the timestamp set appropriately (1000 / FPS difference between each frame)

Realizing this I could solve this issue I'm having temporarily by manually setting the timestamps based on an internal counter. Each time a frame is processed set timestamp = last_timestamp + 40;. The problem is that I don't know if video will always be 25 FPS. Some day I may have 30 FPS or even 60 FPS video streams. To make this more robust I decided to decode the MPEG-2 header (read the PTS value) and convert it to an FLV header.

Now here's the issue… This video file (theoretically 25 FPS) plays perfectly in QuickTime. As a result I know the headers are fine because an expensive piece of software with billions of dollars behind it properly calculated the frame rate. But when I read the PTS from the header (as per this SO post) and divide by 90 (convert 90Khz clock to millisecond timestamp) each timestamp is 33 or 34 milliseconds apart - the 29~31 FPS I was getting.

So why is the PTS giving me timestamps that are 33-34 milliseconds apart when I know the video is 25 FPS (40 milliseconds apart)? More importantly, how is QuickTime reading the MPEG-2 header so that everything plays correctly?

Answer 1

First, to answer my original question:

Reducing FPS in OSMF

You must create your own implementation of the file handler which parses the video header and modifies the timestamps. There is no way to tell OSMF to play in "slow motion" or "fast forward". For an example of creating your own file handler, look at the FLVParser class. Notice that there are separate classes for parsing video and audio tags. The headers must be updated in EACH of these to ensure that video and audio play back in sync.

When a file is passed to the video parser, each timestamp is considered relative to the first. So if the first timestamp is 1234 then this will be set as "time zero" and all future timestamps will be relative to this. This is important. If you skip a video tag and the first timestamp you send is from later in the video, it will use the wrong value for "time zero" and things will not be sync'd properly.

This bring us to…

My issue

First and foremost, the durations in the M3U8 did not match up with the sum of the differences of timestamps. Starting at the first timestamp and labeling it "time zero", then looking at the last time stamp and subtracting time zero from it, the resulting time span was not equal to the expected duration of the TS file.

It turns out when VLC or QuickTime encounter this situation (the sum of PTS values does not equal the duration of the video) they generate new headers on the fly. Take the duration, divide by the number of frames, there's your new offset between PTS values.

That was my first problem. Once that was solved I was no longer gaining a second every 10 seconds, but instead gaining a second every 2 minutes. Turns out I was also loading the next TS file a bit too early (off-by-one error) which was causing me to drop a packet from each TS. This led to losing one frame every 10 seconds.

Additional Info

I ran into another issue with FPS shortly after this which I have found a solution for. Anyone experiencing OSMF playing videos too quickly, I urge you to grep your code for bufferTimeMax.

When bufferTimeMax > 0 and bufferLength >= bufferTimeMax, audio plays faster until bufferLength reaches bufferTime. If a live stream is video-only, video plays faster until bufferLength reaches bufferTime.

Depending on how much playback is lagging (the difference between bufferLength and bufferTime), Flash Player controls the rate of catch-up between 1.5% and 6.25%. If the stream contains audio, faster playback is achieved by frequency domain downsampling which minimizes audible distortion.

6.25% = 0.975 seconds per 15 (gaining a second every 10~20)

Answer 2

You would need to set the FPS manually.

In Flash Professional, you can change the framerate in Document Properties.

In Flash Builder, you can set the framerate using metadata, as described here and here.

Thus...

[SWF(frameRate='15')]