Re: Capture quality modes
- From: Simon Slavin <slavins.delete.these.four.words@xxxxxxxxxxxxxxxxxxx>
- Date: Sun, 27 Aug 2006 21:41:58 +0100
On 25/08/2006, Matti Lamprhey wrote in message
<4l9885Fsl3jU1@xxxxxxxxxxxxxx>:
"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.
Simon.
--
http://www.hearsay.demon.co.uk
.
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