Re: Capture quality modes

"Simon Slavin" wrote...
Matti Lamprhey wrote:
"Agamemnon" <agamemnon@xxxxxxxxxxxxxxxx> wrote...
"Matti Lamprhey" <matti@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote...
"JohnT" <johnhillriseDONOTSPAM@xxxxxxxxx> wrote...

The Bit Rate.

Er.. yes, it can quickly calculate the bit-rate limit based on
4.77GB and the number of hours required, but my question is,
how does it achieve that bit rate? I'm assuming there's some
standard algorithm which all such devices adopt...

Yes. Its called MPEG-2.

Is that a compression algorithm, or just a container format?

MPEG-2 is a data format. It can contain video data in one specific
compression format, and sound in another format. The compression
format used for video data is flexible in that you can choose compress
at different qualities and the decompression algorithm will work no
matter what quality you've used.

I would have thought that the methods used by these DVD recorders are
really quite simplistic and primitive. For example, my Humax 9200T
stores a 1-hour programme from ITV3 in a 1GB (approx) MPEG-2 file at
a quality which matches my DVD recorder's HQ mode. The HQ mode
takes about 4.77GB to store that file, so the algorithms must be very
different.

Your Humax, the equivalent Topfield model, and all digital-only PVRs
sold in the UK do no compression of their own. The digital TV signal
is transmitted in MPEG-2 format, already compressed. The PVR just
records this already-compressed data onto its hard disk. That's why
you don't notice any loss in quality when playing recorded programmes.

A DVD recorder can't do that, since it receives its signal in an
uncompressed form, usually over a SCART cable carrying analogue
signals. So DVD recorders do need to do their own compression and they
can be told to do so at different qualities.

After doing a bit more investigation, I believe that "compression" is a
misleading term for what they do. They simply digitze the captured
video at a resolution which guarantees the required bit-rate, using none
of the intelligent features supported by MPEG-2 such as difference
frames.

I captured an hour of BBC THREE during the day, when it produces a
static image, at the DVD recorder's HQ (1 hour per DVD) mode. Sure
enough, the file was large enough to exactly fill the DVD!

Matti


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