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Technical Services Unit, Carlton
12-07-1994, 01:25 AM
I was interested to read Gideon Ariel's posting re APAS. The concept of
digitising the video signal direct to Hard-disc and then analysing it will
surely be the way of the future, as the technology to record a reasonable
video rate at acceptable quality matures. Video recorders, for their
designed purpose, are useful and reliable machines, with known limitations.
Unfortunately, as Dr. Ariel pointed out, those limitations become quite a
problem when using standard, broadcast video units in precision
measurement.

However, the comments re frame rates are quite extraordinary.

Speaking as a video engineer of 20 years experience, I assure readers of
this list that video CAMERAS do not vary in speed. They use a quartz
crystal oscillator to set the timing of the horizontal and vertical sync
and, after a warmup pariod of a few minutes, will run at a stable speed for
a very long time. Such is the stability of even basic domestic camcorders
that I have had two camcorders produce tapes that ran in sync. for nearly
30 minutes.
By using one camera or a master oscillator and "gen-locking" the remaining
cameras or video processors, a whole system runs "in-step". The stability
of the master oscillators used in broadcasting is at least 1 in 10^^8.

What does vary is the speed of the video tape.

There are several control systems in a typical recorder. During recording,
the speed of the tape and the speed and position of the video head drum are
servo locked to the incoming video signal. The video signal track is laid
down on tape as a series of evenly spaced stripes, with a sync signal
(control track) derived from input vertical syncs.

When the recorder starts up, the tape goes from rest to normal speed within
one second, and the speed and position servos set the head drum correctly
within 2 seconds, or less. Depending on the type of recorder, and on
whether the tape is blank or not, this period of settling will occur during
blank or pre-recorded tape and recording doesn't start until settling
occurs. After this, the recorder runs at constant speed. It is quite easy
to record 10 seconds of anything before beginning your trial and in fact
this is a standard procedure in video production, to give a stable section
of tape for the "Pre-roll" of the editing machines.

During playback, the tape speed is set by comparing the frequency of
control track pulses with an accurate crystal oscillator (or external
video). Any variation of recording speed is then tracked by this servo
system, giving a constant frame rate output. The control track pulse also
provides a position signal for the head drum ( the "Tracking" control
adjusts this) to put the video heads at the correct place on the tape to
read the video signal.

The effect of mechanical tape speed variation (wow and flutter) is to
slightly vary the replay frame rate (by less than 0.2% in a new machine)
and to introduce time errors in the horizontal TV lines. These "time base
errors" are less than about 1% of the length of a line and are only
important when trying to mix two video tape signals together.

To see the effect of speed errors on your VCR, use an underscan monitor and
look at the bottom of the picture. There is a point about 6 lines from the
bottom where head switching takes place. The image below this point, that
is the signal from the start of the sweep of the head onto the tape, may
have a horizontal phase shift, ie a time error, compared with the signal
above the point which is from the end of the sweep of the previous field.
This time shift is due to errors of tape movement and is usually a small
fraction of a line (eg a few nS).

If the VCR is correctly adjusted, there will be a small "Y" shaped blurring
on vertical images and there may be some small to and fro movement. If the
image appears to be greatly displaced horizontally, the tape tension of
either the recorder or the player is incorrect. Some VCR's eg Umatic have
a "SKEW" control which adjusts tape tension to correct this shift. The "Y"
shaped blurring is also caused by the minute error in positioning the heads
during manufacture. ie they are not precisely 180 degrees apart on the
drum.


John Yelland