I am using the live555 library in a Linux application that receives
a low frame rate MPEG2-TS from a live source one frame at a time.
The source is not represented as a file by the OS. To make the
interface to the live555 library easier, I opened a pipe and write
each frame to it as I receive them. The read end of the pipe is
given as the source to a ByteStreamFileSource instance which then
feeds an instance of MPEG2TransportStreamFramer, which in turn feeds
a BasicUDPSink (unfortunately my requirements do not allow me to use
RTP/RTCP, which I would have preferred). This seems to work fairly
well with the following exceptions.
1. BasicUDPSink has a default packet size of 1450.
MPEG2TransportStreamFramer asks its source (ByteStreamFileSource in
this case) for 1450 bytes
You should instead be setting the "preferredFrameSize" parameter to
"ByteStreamFileSource::createNew()". See, for example, line 144 of
"testProgs/testMPEG2TransportStreamer.cpp", or line 199 of
"liveMedia/MPEG2TransportFileServerMediaSubsession.cpp". If you set
the "preferredFrameSize" parameter to 1316, then your
"ByteStreamFileSource" will deliver that many bytes.
2. Since data is given to me and thus written to the pipe
frame-by-frame, it is very common that a single write to the pipe
will not be a multiple of 7 188-byte TS packets (e.g. a frame might
be 25192 bytes = 134 188-byte TS packets = 19 UDP packets containing
7 TS packets each + 1 UDP packet containing 1 TS packet). While the
MPEG2-TS source continues to produce frames this does not cause any
problems. However, when the source is paused/stopped it can cause
the event loop to hang indefinitely. This is due to the fact that
ByteStreamFileSource::doReadFromFile is trying to read more data
than it can receive and thus blocks in the "fFrameSize = fread(fTo,
1, fMaxSize, fFid);" line.
Hmm, reads from the input file (in this case, a pipe) are supposed to
be non-blocking, returning only as many bytes as are currently
available (which will always be >0, because the read is happening
only in response to a return from "select()" on the input file's
socket). Perhaps there's some way you can configure your pipe to
ensure that reads from it don't block?
--
Ross Finlayson
Live Networks, Inc.
http://www.live555.com/
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