                                  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?

Zephyr Magazine is © Gene Williams. All rights reserved.