From: mre@ukc.ac.uk (M.R.Ellis)
Newsgroups: rec.video,rec.video.production
Subject: Re: Video out = S-video out?
Date: 11 Dec 92 20:23:47 GMT
Reply-To: mre@ukc.ac.uk (M.R.Ellis)
Followup-To: rec.video
Organization: Computing Lab, University of Kent at Canterbury, UK.

In article <Bz1y9F.LoD@icon.rose.hp.com> victorm@hprnd.rose.hp.com (Victor Morris) writes:
>Kurt Haverstock (kh@sei.cmu.edu) wrote:
>: In article Video out = S-video out? of 8 Dec 1992 12:28:11 -0800 kimd@gizmo.usc.edu (Kim Daugherty) writes:

>: >A sales reptile at Good Guys told me that the video out is equivalent to the
>: >S-video out on a Hi8 camcorder going into a high resolution monitor?    

>: Equivalent? Perhaps...if equivalent means that the two produce a picture.

>: Video outputs are encoded NTSC with all of its convolutions and
>: shortcomings.  S-video is video in its pre-NTSC, component form.
>: Component video viewed on a component-capable monitor is quite a bit
>: better than that same signal viewed in NTSC.

>Interesting. I have never heard of pre-NTSC, or pre-PAL, or even pre-SECAM. 
>Even in component form, the signal is still NTSC (30 frames per second, 526 
>lines, 115v, etc). Its the signal that has been split from its composite
>form (as when trasmitted) to a component form (of which S-VHS is only one).

ARGGGGHHHH!

In component form the signal is most definitely *not* NTSC.  NTSC,
PAL and SECAM are methods which define the way the YUV (YIQ)
components are combined to form composite video.  It has nothing
what-so-ever in any way shape or form to do with the scan rates
(which is either 30 fields/sec or 60 frames/sec) and the number of
lines (which should be 525).  

NTSC has even less to do with the voltage - all standard video
signals, whether NTSC, YUV or RGB are 1V sync-to-white (or 0.7V
black-to-white) when matched into a 75R load.  Where on earth 115V
came from I have no idea - it's not even the mains voltage in
America!

I admit that the phraseology of pre-NTSC etc is a bit non-standard,
but it does describe what the original poster was trying to convey -
the correct term is in fact either decoded or unencoded, depending
on whether the signal has ever been coded.

>The benefits of reverting the signal to a component form is less
>signal noise which translates into higher resolution. In the case of
>S-VHS, the video and chroma info (Y-R if believe) are taken off the
>same sub-carrier (or more accuratley never put on the same
>subcarrier).

There is absolutely no advantage to decoding a signal - in fact many
broadcast engineers adhere to the rule that the worst thing you can
do with composite video is decode it.

You lose information in the coding process (from component to PAL,
NTSC and SECAM) no matter how much you pay for your coder.  You lose
even more information when decoding the composite video.  Subsequent
coding/decoding cycles only degrade the signal (and the signal/noise
ratio) even further.  In most professional environments the ideal
is to code the signal once and once only - the TV receiver decodes
it and the highest possible quality is obtained.  

It is for this reason a lot of effort has been put into the design
of D2 and D3 digital format VCRs - these allow the composite video
to be recorded and editted without significant loss of detail
(because of the digital nature of the format).  However, even at
only the fourth generation a D3 tape is noticeably worse if the
intervening dubs have taken place in the analogue domain not the
digital domain (ie using the CVBS not the digital inputs and outputs
on the VCRs).  D3 dubs made in the digital domain are almost
indistinguishable from the original master tape after over 500
generations!

D1 is a digital component format, and all three digital formats
exist as both video formats and as recording formats.  Each has it's
own strengths and weaknesses.

S-VHS is purely a RECORDING format.  However, since it is an analogue
format it's performance in multi-generation S-video dubbing is
significantly worse than either D2 or D3 composite dubs.  S-VHS
gives a much worse picture than the original signal,  it's only better
than standard VHS video recorder - whether it beats the original
BetaMax is open to debate.

Of course, YUV (YIQ) is only one possible component format.  Some of
the others include RGB (produced by all cameras - though often only
internally at the bottom end of the market) and YC (virtually the
same as S-video, but not defining the connector).  In the domestic
marketplace, S-video is rapidly becoming the most common video
format for camcorders, but CVBS is still favourite elsewhere.  

All of these formats can exist in NTSC, PAL and SECAM variants since
they convey no information about the format (excepting and possible
weighting value variations).  They can all exist at all possible
scan rate combinations. Not all the possible combinations actually
do exist in "the real world", but there is no technical reason why
any of them could not exist.

Cheers,

-- 
Mike Ellis, Darwin College, The University, Canterbury, Kent, England, CT2 7NY.

    ********* Views expressed are my own and in no way represent *********
    *********     the views of any past or present employers.    *********
Newsgroups: rec.video