From tristan Mon Oct 24 16:41:33 1994
To: mkato@av.crl.sony.co.jp, tahara@av.crl.sony.co.jp
Subject: Re: sony-ct3
Cc: confadhoc@la.tce.com

Dear Video Conformance Testing AHG members,

I have checked the bitstream sony-ct3, and I found it correct.  I get a
perfect match with the reconstructed frames.

However, I noticed that the encoder that was used to make this
bitstream does not follow the recommendation in Note-2, section 7.4.4.
My decoder now checks this point (this is not currently a conformance
point, but a recommendation).

In some cases (like when a fixed frame is encoded), a bitstream
containing some non-zero blocks of coefficients that reconstruct to
zero when a double-precision IDCT is used may cause very significant
drift when decoded by some decoders using an IDCT that reconstruct
non-zero blocks for those blocks.

An example of such bitstream is tcela-12 (see tcela-12.README).

This problem was discussed by John Morris some time ago.

Well-designed encoders should check the output of their IDCT for each
8x8 block in P-Pictures and if it is zero, should clear all DCT
coefficient of this block before generating the bits.  The check only
needs to be done for P-Pictures to avoid this drift problem.


Regards,

--Tristan (tristan@la.tce.com)