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Information, Compression, and storage.



Oh my god, Steven, what have you started?!

There will now be a torrent of information, packed with more or less
impenetrable math (or maths as we say over here), describing what each of
the different compression systems do and how they do it.

As a humble observer, can I just say one thing in (more or less) plain
English before the experts start throwing numbers at each other. All of
these compression systems - repeat, all of them - do horrid things. Some do
less horrid things than others, but they all do horrid things.

In simple terms, yes, you can have lossless compression in things like .zip
for PC or .sea for Mac, because you know you are asking the computer to do
something hard, and you can happily sit and wait for it to look at a file
and decide which bits of data it can compress. Unfortunately, television
does not work like that. Every 25th of a second (30th of a second for you)
another file - another frame - comes along.

The compression system, therefore, has to work in a different way. And the
way that most of the systems like MPEG work is to send the whole picture
every now and then, and use whatever remaining bandwidth to describe how
the picture has changed from frame to frame. And, yes, the result is jaggy
movement, loss of detail, terrible colour contouring and all the other
nasties that are becoming increasingly common in "broadcast quality"
television these days.

I won't get in the way of the experts and their compression ratios, but
just think about this one for the moment. You Steven, like I do, have the
opportunity to sit in a telecine suite and see the output straight out of
the back of a Spirit or an URSA or a C-Reality. That, as we all know, is
about as good as it gets.

In round terms, that is 720 pixels on 480 lines 30 times a second in three
colours (we'll live with 8 bits for the moment). Multiply that lot together
and you get around 30 megabytes per second. If you are going to get it down
to (for the purpose of this argument) 3 megabytes a second, you are
throwing away 90% of that beautiful picture. Anyone think that is a good
idea?

Dick

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