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Re: Information, Compression, and storage.



At 08:01 PM 7/6/99 -0400, Christopher Bacon wrote:
>There are a few types of video compression that are theoretically lossless
>(meaning that they can be completely reversed).  These include DCT, wavelet,
>and fractal.  ...

I have to disagree with Christopher's use of specific terms-- although I
agree with the concepts he expresses.

All current tape formats that use DCT based compression are lossy. They are
not constrained to prevent the selective elimination of information. At the
level of compression that I have heard attributed to Sony's Digital Betacam
(2.1:1), there are some video fields that do not require any bitrate
reduction to pass on to the tape. There are some video fields that
absolutely must have information removed to meet the recordable bitrate. As
others have pointed out, this decision must be made on every video field as
each field is presented to the recorder and so some of the information must
be sacrificed to meet real-time expediency. That said, the results are very
good and more than acceptable for a lot of the work we do. 

It is possible, in theory, to encode using DCT transforms and not loose
information. The information loss happens when the frequency components of
a block are quantized at less than the full scale value. There can be
compression using DCT without coefficient down-quantizing but there will
also be plenty of real world pictures that will not yield a bitrate
reduction without the use of entropy. The mere possibility of a
'pathological' picture failing to meet the recordable bitrate on tape
forces the system to allow losses. That pathological picture is often just
the one that the client insists is the only acceptable 'look' ;-)

If you want lossless recording, all the time, no matter what valid signal
you sent to the recorder, you must use D-1 (ignoring the 10 to 8 bit
truncation) or the D-5 format in standard definition. The only recorder I
know of for HD signals that is lossless is D-6. (Don't confuse D-5 format
with compressed high definition video encoded to 360Mb/sec at 10 bits which
is recordable on a D-5 tape. The Panasonic 2700 just packages the
compression with the D-5 record deck.)

Sorry about the rant but the use of lossless seems to me to be like the use
of 'pregnant' you can't be a little bit lossless...
----
David Tosh <dlt at earthlink.net>
Engineer, Complete Post Hollywood, CA USA

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