[Mp4-tech] [Video][H.263] Non byte-aligned start codes

[Shailendra Singh]

Hi All,
The H.263 Recommendation states : "All picture, slice, and EOSBS (End Of
Sub-Bitstream code) start codes *shall* be byte aligned, and GOB (Group Of
Blocks) and EOS (End Of Sequence) start codes *may* be byte aligned."
My question is:
What is the possible reason that the GOB and EOS start codes are allowed to
be non-byte aligned, given the fact that it is so much easier to search for
a byte aligned start code in a bitstream?
With a non-byte aligned start code, use of stuffing bits is unnecessary,
which would result in some savings in the overall bitcount, but to me the
loss of ease of search for a byte aligned start code seems too big a price
to pay for this saving.
I am particularly perplexed about the GOB start code, given the fact that a
Slice Start Code (SSC) *has* to be byte aligned, and the GOB start code and
the SSC serve a similar purpose (and both are words of 17 bits, with the
same value - 0000 0000 0000 0000 1).
Thanks,
Shailendra
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