Number of VCR heads
Michael Umansky
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.
--
NAME: Michael Umansky (misha)
E-MAIL: ucbvax!hplabs!nsc!misha
WORK:
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Henry Perkins
> number of video heads in a VCR.
Video magazine has an article about video heads roughly once a year.
There are several different head designs, but here's the basic story
for VHS.
The minimum number of heads necessary for a VCR is two. Each head
does one half frame, and the heads are 180 degrees apart on the head
cylinder. However, a given head width can only be optimized for one
speed of play. Cheap machines will perform well at only one speed
(often EP), with adjacent tracks bleeding at a slower speed, and
inter-track noise showing up at a higher speed. That's why all the
machines with decent performance at more than one speed have a second
pair of heads. Then you have one pair for SP and one pair for EP.
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.
Things get more complicated from there. In some machines LP is done
by combining one SP head with one EP head. Many otherwise good
machines don't even allow recording at LP speed. As far as I know no
manufacturer has three separate pairs of video heads to support the
three recording speeds.
For HiFi, where the two audio tracks are laid down "under" the video
tracks, add two more heads. That's how you can get 6 or 7 heads.
More properly, these are 4+2 or 5+2 heads, since you don't get any
better picture by adding the HiFi heads. (Normal linear audio is
recorded on the edge of the tape by a single non-moving head.)
Beta is much simpler. You still have two opposed heads to record
the picture. However, the tolerances are not as restrictive, so
you can handle both Beta II and III speeds with the same pair of
heads.
Beta HiFi records the two audio tracks with the video signal, not
"under" it. You need more circuitry, but you use the same two heads.
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.
Fai Lau
> 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.
> --
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
Franco Barber
>[... 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.
>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