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Re: Digi Beta versus D1



At 04:38 PM 9/17/98 EDT, KA2IQB at aol.com wrote:
>Lossless video compression schemes ..., which are capable of restoring
compressed
>data files back to their exact, original state.  In video, DCT and Digital
>Betacam fall into this category, ...

I don't intend to destroy the context of your remarks. My understanding of
Digibeta compression was that it is exclusively applied within each field,
and is a form of discrete cosine transform compression that limits the data
rate to no more than about 2.1 to 1. Any time the frequency components of a
field cause the quantized data to exceed the maximum, information is lost.
It is less a matter of time than a matter of how much energy exists in
detail and in how much of the whole field. Losses happen, usually, though
not necessarily. Worse, losses happen differently in the same element from
field to field depending on what may be happening in another part of the
field. This makes fine, spatially detail vary over time depending on the
picture content. As I understand the process.

The compression on Digibeta holds up quite well to real world pictures and
the compression ratio is a very moderate compromise that seldom results in
objectionable artifacts. I think that is a suitable choice for most
situations that don't require absolute, no compromise, lossless storage.

>...you might want to suggest D-5 next time.  While the
>format uses mild audio compression, standard 601 digital video goes through
>untouched by it.

I haven't looked at the documentation for the D-5 format but am concerned
if the AES audio source fed to the D-5 is modified in _any_ way by the
machine. We are having troubles recording Dolby AC-3 streams on D-2
format-- ANY concealment by the transport cause the AC-3 coding to suffer
grievously. It would be a disaster for some up-coming projects to have
deliberate compression applied to the audio elements. Could you elaborate
on the details of the compression?


David Tosh <dlt at earthlink.net>
Engineer, Complete Post Hollywood, CA USA

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