Dear MPEG colleagues,

tcela-11v2 is an MPEG-1 constrained bitstream that demonstrates that some
perfectly legal and compliant bitstreams cannot be decoded properly by
some perfectly compliant decoders.

IMPORTANT: This bitstream is NOT intended to used for Video Conformance
Testing.  However the information contained here may be relevant to the work
of this AHG.

The problem demonstrated here is that the domain for which an IEEE
compliant IDCT is required to give correct results is not sufficient to
decode all compliant MPEG (MPEG-1 or MPEG-2) bitstreams.

Several steps that can occur in an encoder can cause the DCT coefficients
reconstructed by the decoder (after inverse quantization) to be out of
this domain: Inaccuracy of the FDCT, quantization of coefficients,
coefficient clamping.

This problem is addressed by one of the Video Technical Comments of the USNB.
This problem exists both in MPEG-1 and MPEG-2.

This example is an extreme case, in which the first P-frame was coded
using a very inaccurate FDCT.  According to the MPEG-1 IS, this bitstream
is perfectly compliant.

You should be able to match the reconstructed frames perfectly if you use
a software decoder implementation of IDCT that never overflows (e.g.
floating point).

However, the images reconstructed by a hardware decoder may have a very
large mismatch (in the upper part of the frames).  Check with your
hardware decoder, if you have one.

Note: Not all software MPEG-1 decoder will decode this bitstream correctly (for
example, the "mpeg_play" MPEG-1 X viewer from Berkeley University does not).


Bit rate:               1.0 Mbit/sec 
Sampling dimensions:    352  x 240 x 29.97 Hz 



    Bitstream file             tcela-11v2.bits
    Reconstructed frames*      tcela-11v2.decoded
    Decoding trace **          tcela-11v2.trace
    picture structure          I P P P P...

     *frame format:

                4:2:0 frame ("display" order) 8 bit

                       width
                +---------------+
                |               |
                |      Y0       | height
                |width/2        |
                +-------+-------+
                |  Cb0  | height/2
                +-------+
                |  Cr0  | height/2
                +-------+-------+
                |               |
                |      Y1       |
                |               |
                +-------+-------+
                |       |
                  etc...

        ** Only the first row of MBs is traced.
           More complete traces are available on special order.


Best regards,

Tristan

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 Tristan Savatier                  E-mail: tristan@la.tce.com
 THOMSON CONSUMER ELECTRONICS            TEL: +1 310 568 9003
 Los Angeles Laboratory                  FAX: +1 310 568 9002
 5601 W. Slauson Ave., Suite 262, Culver City,  CA 90230, USA
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