Zephyr Magazine - Issue 37

                              T H E
  
                           Z E P H Y R
  
                  __     M A G A Z I N E
                 {__]++++++++++++++++++++++++++[] 
                 Issue #37                2-12-87
 
            A weekly electronic magazine for users of 
                        THE ZEPHYR II BBS 
                    (Mesa, AZ - 602-894-6526)
                owned and operated by T. H. Smith
 
                    Editor - Gene B. Williams 
 
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                            (c) 1987
  
THIS ISSUE:

     Ahhhh, an appropriate issue number (I just turned 37) and an 
appropriate date (my son, Danny, turned 1-year-old today).

     But none of you care about that stuff. So let's get right 
into this issue. It's a continuation from the last issue on home 
video. As I said then, there is enough information on this topic 
to fill all the issues of 1987 - enough that I'm doing a book for 
Chilton on the subject (#9 in the series). Just how many such 
articles I actually DO for Zephyr Magazine depends primarily on 
the interest and curiosity shown by the readers. 
     In any case, here we go again.
     Last time we covered the basics of video cameras and 
formats. This time around let's start with some of the basics 
about how all this works.


                       Home Video - Part 2

     Light comes, from whatever source, strikes the subject being 
photographed (or, in this case, videographed), bounces off, hits 
the lens of the camera, gets refracted and focused, strikes the 
pickup, stimulates electronic signals which in turn stimulate the 
recording heads which in turn magnetize a specific pattern within 
the coating on the tape. Later, those magnetic patterns stimulate 
the playback heads, which generate the pulses which turn back 
into the video and audio.
     That's it in brief. Obviously it's not quite that simple. 
     Magnetism and electricity are closely allied. Make a coil 
from a piece of wire and pass a magnetic field along that coil. 
The result is electricity. (The reverse also works - move the 
coil of wire through a magnetic field and you still get 
electricity. All that really matters is the relative motion of 
the coil and the magnet.) 
     This is done all the time in the generation of electrical 
power. Build a casing that is lined with magnets and spin inside 
a core made up of coils of wire. Those coils will kick out 
electricity. Make the generator large enough and it will power a 
city. Refine it and you have video and audio playback.
     The recorded images and sounds are held on the tape in a 
pattern of magnetic fields by tiny particles. (The same is also 
true of any audio tape player or disk drive.) All those patterns 
pass by the playback heads, which are actually nothing but coils 
of wire. This causes electricity to be generated, with the 
pattern of that electricity being governed by the stored magnetic 
fields. As the pattern of the stored fields changes, so does the 
electrical output of the heads.
     Those electrical pulses are amplified and otherwise handled 
by the circuits. Eventually they come out of the television set 
as sound and pictures.
     Recording them in the first place is just the opposite.
     Pass electricity through a coil of wire and you'll create a 
magnetic field. This is the principle behind electro-magnets, and 
that's just what a recording head is. 
     The incoming signals vary in strength, and again have a 
pattern specified by the light and sound that are coming in. As 
those incoming signals vary, so does the magnet field. Pass a 
tape across the head and those varying magnetic patterns will 
become impressed on the tape.
     That's all there is to it.

     Last time we talked about how the video pickup device works. 
The device is photo-sensitive. This means that it builds an 
electronic charge as light strikes the surface. 
     Think of a solar battery. Now think of a solar battery that 
is a tiny, tiny fraction of a square inch. Now think of thousands 
of those all glued to a single surface. Once you've done that, 
the amount of electricity generated at any given spot will be 
determined by the amount of light that is hitting that spot. Once 
again you have a pattern - one that can be processed and handled 
by the circuits.
     And then we're back at the recording heads again.

     An audio signal is relatively simple. It's a fairly 
straight-forward analog signal with a relatively narrow 
bandwidth. Accurate recording and playback of the audio is also 
fairly simple. You can get good audio even if the tape is moving 
as slowly as 1 7/8ths ips (inches per second) - the standard 
speed of audio cassette decks. Studio audio decks use a much 
higher speed (15, 30, 60, etc. ips) since a higher speed results 
in better recording and playback.
     The video signal is different. It contains more information 
and also has a much broader bandwidth. Try to record the complex 
video signal at a slow tape speed and all you're going to get is 
a jumbled mess on the screen. While audio tape speeds can be 
measured in inches per second, the tape speed needed to record a 
good video signal is measured in feet per second.
     The first video recorders had powerful motors that yanked 
the tape across the heads at incredible speed. A large reel of 
tape could hold only a few minutes of recorded image. But the 
high speed also caused problems. Imagine taking something 
flexible and somewhat stretchy like plastic (Mylar - see the last 
issue) and zooming it along. Every start or stop is going to put 
a tremendous strain on that plastic. Within a short time it is 
going to stretch, which will in turn distort anything recorded on 
the tape.
     Besides all that, all that tape and those powerful motors 
get expensive in a hurry. They are also heavy and bulky.
     I wish I knew his name - or their names. Someone had the 
smarts to realize that what really mattered was relative speed. 
Perhaps it came from thinking in reverse. You can move the tape 
over the heads - or you can move the heads over the tape. Whoever 
it was figured out that you can also do both and come up with a 
suitable and realistic compromise.
     Tape speed was dropped into a more rational range. To make 
up for the difference in relative tape speed the heads had to be 
moved across the tape in the opposite direction at the same time. 
Since it was obvious that nobody would want a machine that was a 
mile in length, spinning the video heads was the obvious 
solution. The video heads in a VTR (video tape recorder) spin 
along at 3600 rpm.
     Think about it for a moment and you'll see a problem. Move 
the tape in a straight path across the heads and what you'll get 
is a compressed, and smeared, recording. It just won't work. To 
make it work only a minor modification is needed. Tilt the angle 
of the tape slightly and the recorded image will go on in 
diagonal stripes.
     As the tape moves forward, its angle in effect causes the 
head to move downward. The video image is thus recorded in the 
center part of the video tape.
     The audio signals are separate, as they must be. Being more 
simple, they can be recorded in a linear fashion, just as they 
are on standard audio tape.
     On the far side of the video head assembly (the big shiny 
thing) is an audio head. This places the sound on a linear track 
at the top of the tape.
     If you haven't already noticed, all this can get to be 
complicated. The video and audio heads are in two physically 
different spots. Playback there is simple since those heads also 
handle the playback and are in the same relative places. However, 
how does the VCR know where to begin scanning all those diagonal 
lines of recorded information? You have countless stripes of 
data, a moving tape and a spinning head.
     The control for all this exists as electronic pulses 
embedded on the tape on another linear track - the control track. 
This is located at the bottom of the tape. If you ever have or 
rent a tape where the picture jumps, open the protective lid. 
Chances are 99.9% that the bottom of the tape is wrinkled and 
damaged. (NOTE: To open the tape, push in the tiny square on the 
right hand side by the lid. This is the catch release.)
     This control track synchronizes all the activity going on 
inside the VCR. The playback machine is going to look for this 
track immediately. If it's not there, or disappears for a moment 
(such as when switching between two different sources) the synch 
will cause the playback machine to flutter until the control is 
found and put back into use again.
     Get more involved in video and you'll hear the terms 
"genlock" and "time-base coordinator." They mean essentially the 
same thing. This provides a single synch signal so that the 
recording deck gets only one, regardless of the number of inputs.


                        Signal Enhancers

     Read the ads and you might come to believe that a lousy 
recording can be improved if only you buy that $496 FX-1 with a 
built-in signal amplifier.
     Bull! It doesn't work that way.
     Right off, for any enhancer to do anything, it has to have 
the signal to work with. Feed the enhancer a totally blank tape 
and it won't come out with a fine quality movie, no matter which 
buttons you push. It has no signal to work enhance.
     Give it a recording where the original signal is washed out, 
with no detail, and it can't put those details in. They're not 
there to begin with, and the best enhancer in the world won't be 
able to create those details.
     The trick is to get the very best possible recording as a 
starting place. The worse that starting place, the worse the 
outcome. The enhancer won't replace or rebuild what wasn't there.
     Last time I cautioned you to not expect that home video 
camera to provide the quality you'd find in a studio. This time 
the caution is to not put your trust in an enhancer. They serve a 
purpose, but a minor one. They exist to transform an existing 
image.
     Many enhancers also claim to have the capability to reduce 
noise. Noise is any unwanted signal. Distortion. In video it 
exists primarily in the high frequencies. To reduce noise, those 
high frequencies are reduced. This also results in loss of image 
purity and quality. (Think about the results of taking out the 
highs in an audio recording to remove the cracks and pops of an 
old record. As the pops disappear, so does the high end audio.)
     If the enhancer is used to improve image resolution, it does 
it by increasing those high ends. That in turn also increases the 
noise.
     You can't have both. If you have a poor image that needs to 
be improved, you'll be introducing noise on the high end which in 
turn degrades the image. If you have a tape that has a lot of 
video noise in it, you'll be reducing clarity to get rid of that 
noise.

                              Audio

     The microphone that comes with the camera is usually of fair 
quality only. Worse, it's mounted right on the camera. If the 
scene you're shooting is 50 feet away, the audio is going to be 
very poor.
     An easy solution is to buy a remote microphone. Here you 
have two choices. One is to get a microphone and a long cable. 
The other is to get a wireless microphone. The first is a little 
less expensive - although not much once you buy that long cable. 
The second brings with it the chance of interference getting into 
(and onto) the audio track.
     A wireless microphone has a small transmitter built into the 
microphone housing and a receiver that connects to the camera. 
Between the two are radio waves, and that means that other radio 
waves can cause interference. Conditions can also degrade the 
audio signals.
     Unfortunately, the range of most home wireless remote mikes 
is rather short. Beyond 50 feet there will almost always be noise 
creeping in. Get to 100 feet and the noise is going to start to 
get irritating. Beyond that you might as well forget it. 
     Whichever microphone you get, there are two factors to 
consider. One is that the microphone has the kind of plug you 
need. With home video cameras this is usually an RCA-type pin 
jack. All you have to do is to look at the camera and see where 
the remote plugs in.
     Second is the impedance.
     Impedance is AC resistance. Due to the way amplifiers work, 
the input impedance must match the impedance of the amplifier 
section. If it doesn't, the amplifier won't be able to 
effectively handle the signal.
     If the amplifier is of high impedance and is fed a low 
impedance signal, the amplifier will become overloaded and the 
signal will be badly distorted. If you feed a low impedance 
amplifier with a high impedance source, the level of sound will 
be so low that you won't be able to hear it over the noise.
     Somewhere in the owner's manual for the camera - usually on 
the last page - will be a list of specifications. This will tell 
you the impedance of the audio input(s). The microphone will also 
have a listing as to its impedance - in the manual, sometimes on 
the box and sometimes right on the microphone. A few of the 
better microphones even have an impedance switch (generally just 
a "hi-lo" switch, but that's usually enough).


Until Next Time

     Should this be the last article on this topic? Or shall I go 
into other related topics? Some simple special effects? Maybe an 
article on lighting techniques and tricks? Or maybe even one on 
repairing and maintaining the equipment?
     Yeah! That would be a good one, wouldn't it? Quite a few of 
you have - or will have - VCRs at home. For those of you keeping 
track, "Chilton's Guide to VCR Repair and Maintenance" was #3 in 
that series.)
     A while back I flew to Mississippi to give a seminar on the 
subject, and a couple of weeks ago I did yet another radio 
interview (over Mutual Broadcasting out of Chicago). There were, 
as expected, some common questions, and some common problems. 
     You might be surprised at how many of those problems you can 
cure yourself, and without any technical background.
     So, whaddya think? Want to learn all about the care and 
feeding of your VCR? And how to triple its life? And prevent it 
from eating your tapes?