[Live-devel] Live H264 Streaming Timing Problem

[Ross Finlayson]

> I’m having difficulty streaming live H264 video captured from a PCI Express board. It appears that the data acquisition is too slow for the Live555 streaming application and hence the displayed video (in VLC) is sticky. For the time being I am assuming that the hardware is okay and would like to investigate the possibility that the Live555 application is attempting to grab the data too quickly for the hardware.
>  
> Could you tell me how Live555 calculates the time period between calls to ByteStreamFileSource::doReadFromFile() and also where this is implemented in the code.

You haven't said exactly how you're reading H.264 data from your video capture device, but because you mentioned "ByteStreamFileSource", I'm assuming that you're treating the device as a file (a byte stream), and feeding the output from a "ByteStreamFileSource" (i.e., your video capture device) into a "H264VideoStreamFramer" (which parses the byte stream into a sequence of H.264 NAL units).

In this case, the code (in "H264VideoStreamFramer") figures out the frame rate (and thus, how long to delay between frames) from the "time_scale" parameter in "Sequence Parameter Set" (i.e., SPS) NAL units.  If this parameter is not present, then the code can't figure out the frame rate, so it uses a default frame rate of 25 fps.

If, however, your video capture device delivers 'discrete' H.264 NAL units - i.e., one at a time - rather than an unstructured byte stream, then you should *not* attempt to read it as a file.  Instead, you should write your own subclass of "FramedSource" (perhaps, based on the "DeviceSource" model) that delivers the NAL units, and feed these into a "H264VideoStreamDiscreteFramer" - *not* a "H264VideoStreamFramer".  If you do this, then your 'input device' class - i.e., the subclass of "FramedSource" that you'll write - *must* set "fDurationInMicroseconds" for each NAL unit, before it delivers it to its downstream object (a "H264VideoStreamDiscreteFramer").


Ross Finlayson
Live Networks, Inc.
http://www.live555.com/

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