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From: feb@cblpe.UUCP (Franco Barber)
Newsgroups: net.video,sci.electronics
Subject: Re: Number of VCR heads
Message-ID: <272@cblpe.UUCP>
Date: Thu, 30-Oct-86 09:11:46 EST
Article-I.D.: cblpe.272
Posted: Thu Oct 30 09:11:46 1986
Date-Received: Fri, 31-Oct-86 20:00:51 EST
References: <819@bnrmtv.UUCP>
Reply-To: feb@cblpe.UUCP (55216-Franco Barber)
Organization: AT&T Bell Laboratories - Columbus
Lines: 53
Xref: mnetor net.video:2484 sci.electronics:34

In article <819@bnrmtv.UUCP> perkins@bnrmtv.UUCP (Henry Perkins) writes:
>[... explanation of function of heads in VHS machines]
>Frequently a single extra head is added to better support special
>effects like freeze frame and slow motion.  This gives 3 heads on the
>cheap machines, and 5 on the better ones.
>[... a little more stuff]
>Except for the very-top-of-the-line-with-all-the-editing-effects
>models, SuperBeta HiFi machines still only have two heads.  The
>fanciest machines (like my Sony SL-HF900 SuperBeta) have 4 heads.
>
>
>A two-head SuperBeta VCR (costing about $350) has 20% more resolution
>than a 5+2 head $1400 VHS HQ machine.  It's a shame Sony can't market
>their products as well as JVC.
>-- 
>{hplabs,amdahl,3comvax}!bnrmtv!perkins        --Henry Perkins
>
>It is better never to have been born.  But who among us has such luck?
>One in a million, perhaps.

I thought I would take a minute to point out how the 900 uses
its 4 video heads (I have one, too.)

Two heads are used for BI, two heads are used for BII/BIII,
optimized for BII (I've noticed picture search at BII is perfectly
clear, but at BIII there are small snow bars in the picture.)
(I haven't been able to test BI: I don't have any BI tapes to play. Maybe
next month when I get my SL-HF1000 which records at BI I'll find
out what BI is like on the 900.)

Apparently, special effects and freeze frame on the 900 is done with
only one head. There is a control on the top labeled "still adjust."
With the machine in freeze frame, turning this control makes the two
fields in the picture on the screen move in relation to each other;
the scan lines in one field move up and down while the other field
stays still. The only explanation I have for this is that perhaps
one head (whichever head happens to line up with the current field)
is used to read one field, which is displayed and stored in a CCD
array (the kind of analog storage device also used in some video
cameras) to be redisplayed on the next field after a time delay
which is controlled (partially) by the "still adjust" control.

This is of course, only an educated guess, based on the effects of
the "still adjust" control and an old "VIDEO" magazine review which
said two heads were used for BI and two for BII/III.

This same 'storage' effect could be used on VHS, I suppose (and
indeed, the just introduced 'digital' (:-) VHS vcr's do just that,
except that they digitize the field into a frame buffer instead of
using an analog delay line. )
-- 
Franco Barber
..!cbosgd!cbuxc!cblpe!feb