[Mp4-tech] Clock rate used for RTP timestamps

[Shailendra Singh]

Hi All,
    For H.264 and H.263, the corresponding RFCs mandate that a clock rate
(for RTP timestamping) of 90 kHz MUST be used.
    For MPEG-4, RFC 3016 (RTP Payload Format for MPEG-4 Audio/Visual
Streams) says that the resolution of the RTP timestamp "is set to its
default value of 90kHz, unless specified by an out-of-band means", i.e. a
clock rate which is not 90 kHz is allowed if explicitly specified, for
example, in a Session Description sent out with the "rate" parameter in the
"a=rtpmap" line appropriately set.
My questions are:
(1) What is the need to allow a non 90 kHz clock rate for MPEG-4 when the
other coding systems restrict it to 90 kHz?
(2) RFC 3016 does not say anything explicitly, but is there an implied
restriction on the range in which the clock rate should lie for MPEG-4?
Any pointers will be appreciated.
Thanks,
Shailendra
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