🚀 go-pugleaf

4958snz@longley.demon.co.uk>
Message-ID: <54269189wnr@ttelmah.demon.co.uk>
Date: Saturday, Apr 27, 1996 13.02.34
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Reply-To: roger@ttelmah.demon.co.uk
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In article: <830544958snz@longley.demon.co.uk>  David Longley <David@longley.demon.co.uk> writes:
> In article <830540997snz@myth.demon.co.uk> zeus@myth.demon.co.uk "Mike" writes:
> 
> > In article <830283694snz@longley.demon.co.uk>
> >            David@longley.demon.co.uk "David Longley" writes:
> > 
> > > I'd be grateful if someone who has the figures could explain in detail how
> > > the image loss comes about from say VHS --> half PAL capture --> MPEG
> > 
> > Depending on the specific system, ISTR that VHS has a luminance bandwidth of 
> > less than 3.5MHz (chrominance is much less than this), hence, approximately 
> > 140,000 pixels per full frame, or 224 pixels per line. For a moving image, 
> > it appears much greater than this as the eye and brain interpolate the images.
Actually, when digitising, you have to consider that you are converting from
an analogue 'bandwidth', to a digital sample. Hence the reason, why audio
CD's, use 44kHz sample rate, for 'only' a 20kHz used bandwidth. The comments
about moving images, are _very_ right. However, I would have to say, that a
sample at 220 pixels/line, definately 'throws away' some of the useable
information from VHS. Sampleing at @320pixels/line, seems to give a real
resolution that is as good as you can expect from VHS.

> >                                                        
> > Further loss is imposed when the signal is digitised to a finite number of 
> > levels and then again when the MPEG compression removes parts that it 
> > considers unimportant.
> > 
> > Mike.
> > 
> This is helpful. 
> 
> What would also be useful is some help to compute what sort of bandwidth
> is required to capture full PAL. I reckon it's over 5MB sec to disk which
> means a going for something like a 4GB barracuda which does not thermally
> recalibrate, hog CPU resources, and can sustain sequential throughput at
> over 7MB/sec.
A lot depends here, on how you are sampleing/storeing. If you are compressing
on the fly, you might manage with this little bandwidth. However, I capture
'full' S-VHS directly to disk, and the disk array that I use, uses triple F/W SCSI-II
busses, driving five 7200rpm drives. I have to 'pre-allocate' the space for video,
before capture, and storage is used up at @20Mb/sec. 

Best Wishes

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| EMail roger@ttelmah.demon.co.uk                                   |
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