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From: ugfailau@sunybcs.UUCP (Fai Lau)
Newsgroups: net.video,sci.electronics
Subject: Re: Number of VCR heads
Message-ID: <1287@sunybcs.UUCP>
Date: Tue, 28-Oct-86 11:05:09 EST
Article-I.D.: sunybcs.1287
Posted: Tue Oct 28 11:05:09 1986
Date-Received: Wed, 29-Oct-86 04:07:16 EST
References: <3855@nsc.UUCP>
Organization: SUNY/Buffalo Computer Science
Lines: 44
Xref: watmath net.video:3562 sci.electronics:15
Summary: Head gaps

In article <3855@nsc.UUCP>, misha@nsc.UUCP (Michael Umansky) writes:
> I am trying to get TRUE and COMPLETE information on function of
> number of video heads in a VCR.  The best theory I heard so far
> is that one set of heads (2) has optimized head gap for Standard
> Play (SP) mode and the second set of heads (2) has optimized head gap
> for Extended/Long Play (EP/LP) and Super Extended/Long Play (SEP/SLP).
> I would like to know if that is TRUE?  If NOT, then how are the four (4)
> or three (3) or five (5) heads used?  If YES, then how do the three (3)
> and five (5) head machines work? Please post to the net for general
> audience information.  Thanks in advance.
> --
	That is TRUE. The only reason VCRs have more than one set of heads
is because the additional set has different size head gaps. In EP/LP
mode, the video tape is travelling at a slower speed, and the tracks written
by a standard set of video heads tend to overlap. (I assume you know the
mechanics of video recording) So a set of heads with a smaller head gaps
work better. Also, in slow mode with SP, the EP/LP heads are often used
instead to read the tracks created by the SP heads. It is because when the
tape speed slows down, the offset angle of the tracks originally written
in regular speed can no longer agree with the angle of the video drum.
And a head with smaller gap (OOPS! not just different gap size, but different
THICKNESS too) and smaller thickness could minimize the distortions
created by a SP head crossing over two adjacent tracks during scanning.
That is, the smaller heads can fit into the path of a wider track
even when it does not travel at exactly the same angle.
	I can not speak for ALL machines with odd number video heads,
however, I can speak for some of them.
	In a three head machine, the additional head works WITH the
standard pair of heads. It is only used during slow mode (or fast mode,
as long as the speed is different). This head is usually mounted
close to either one of the standard heads and is identical (maybe thinner)
to it EXCEPT that its gap angle (video heads have tilted gaps, unlike audio 
eads) is aimed at the opposite direction, just like the other video head
in the opposite side (should I go into details explaining why,
I guess not, it would make the article too long). In these machines,
every other track is skipped during fast mode. Since all tracks scanned
this way were written with only one video head and are now scanned
by two, the two heads used for scanning should have same gap angles.
	The mechinics of the five head machine is roughly the same.

Fai  Lau
ECE / CS  SUNY at Buffalo (The Arctic Wonderland)
BI: ugfailau@sunybcs
that the additional