[Live-devel] Trick Mode Data Rate Limitation

[xcsmith at rockwellcollins.com]

Ross and Community,

I am hoping to implement a bandwidth limitation on trick mode streams 
only, such that the bitrate of the trick mode stream would not exceed the 
average bitrate of the original stream.  For the particular 5 Mbps streams 
that I  have, I cannot get above 4x speed-up without significantly 
exceeding the 5 Mbps data rate.  (I have stopped the filter from repeating 
I-Frames because my client's network buffers have no hope of handling 
these repetitions.  Also, I was told that the I-frames which were repeated 
had identical PCR if - I remember correctly - and so the decoder buffer of 
my client would become full before a new PCR was available in the buffer.)

I have figured out a very crude way to implement this for my specific 
system, but I am trying to figure out a better way to do the math to 
determine which I-frames to discard to implement a bitrate limitation. 
Initially, I think that if I add another count to 
MPEG2TransportStreamTrickModeFilter such that each GOP would be divided 
into 4 "time slots", and allow only 1 I-frame to be sent per "time-slot", 
that my trick-mode bitrate limitation would work most of the time.  I 
think this was the suggestion here: 
http://lists.live555.com/pipermail/live-devel/2007-March/006317.html

It would work something like this:
Each GOP I can think of as being divided into 15 time-slots, T0-T14.  I 
must send "scale" number of I-frames per GOP.  If "scale" > 15, then some 
I-frames will be discarded.
I can figure out which I-frame, IN, from the original stream goes in any 
given slot with: 

        floor function ( scale * slot-number / GOP-length) = N

To limit the bitrate on top of this, I would have to divide the GOP into 4 
and allow only one I-frame to be sent in each of T0-T3, T4-T7, T8-T11, 
T12-T14.

You can see that this implementation would not scale very well at all, and 
in fact might not even work for particular streams in my system.  I would 
most like to ensure that the trick mode bitrate would not exceed the 
average bitrate of the original stream.  But if that is not possible, then 
I would like to figure out a way to specify the maximum bitrate.  Can you 
provide any suggestions on how to implement either of these changes?  I 
see here, you have a different idea altogether: 
http://lists.live555.com/pipermail/live-devel/2007-April/006509.html

Thank you!
Xochitl Smith
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