ABOUT CLOCK PULSE
The clock pulse is keyed either to specified frames in ordinary (continuous) modes
6H) or to the beginning of the recording of
video fields in the time-lapse recording modes
48H 960H).
Output signals can be used to synchronize the VCR with other equipment, such as sequential switchers. These signals
are generated
just after the VCR begins to record either fields or frames of video.
In
ordinary recording
6H) is done in the frame
mode, while in time-lapse operation
48H
only one-field is recorded at each time-lapse interval. in real-time
playback, a frame consists of two interlaced fields of video, and in VHS-compatible recorders, each field of a single frame of video
is processed as part of a frame. During playback in one of the time-lapse modes
960H modes), a single recorded field is read
twice from the tape to produce a complete video image. This has the effect of reducing the vertical resolution to one-half its normal
value.
The clock pulse output has great versatility. If the clock output mode is set to 1 and the VCR is set to the 2H or 6H mode, the clock
pulse will be timed for a frequency of 30 Hz. If the VCR is operating in a time-lapse mode, the clock pulse will occur slightly after the
recording of each video field. The time-lapse modes are named relative to the 6-hour real-time duration of a standard VHS T-l 20
cassette on EP mode. For instance, if the
time-lapse mode is selected, it means that a
cassette will last 18 hours. The
ratio 18 hours/6 hours 3 shows that one field of video will be recorded in the
mode for every 3 fields in the 6H ordinary mode.
Since the normal field rate is 60 fields/second, the time elapsed between recorded fields in the
mode is:
18 hours
1
6 hours
X
60 fields
0.05
In the
the clock pulse may be selected to be output every time a field is recorded, or once every two fields, or every three
fields, etc. This number is called the frequency division ratio (FDR), and the ratios available are
10,
and 60, along with a “field” rate, to be explained below. If the frequency division ratio of 1 is selected, then the clock pulse will be
generated shortly after each recorded frame (in one of the continuous modes) or each recorded field, if the VCR is set to a
lapse
mode.
If
an
FDR of 2 is selected, then a clock pulse will be generated for every second frame or field. An FDR of 3 will
produce a pulse for every third frame or field, etc. In our example of operation in the
mode, we saw that a field of video is
recorded every 0.05 second. If we select an FDR of 3, then the clock pulse will be generated shortly after every third recorded field,
or
0.05
1 field
X
0.15
In the
recording mode, then, if an FDR of 3 is selected, the clock pulse will be generated every 0.15 second.
The clock pulse frequency can be selected as follows:
Desired Frequency
of Pulse
Everv frame*
second frame
Every third frame
Every fourth frame
Every fifth frame
Every tenth frame
Every 15th frame
Every 20th frame
Every 25th frame
Every 30th frame
Every 50th frame
Every 60th frame
Number to Select
from Screen Display
1
2
3
4
5
1 0
1 5
2 0
2 5
3 0
5 0
6 0
l
When the clock pulse is keyed to one of the ordinary recording modes
it is generated just after the recording of a frame;
when it is keyed to a time-lapse mode
48H
it is generated just after the field that is about to be recorded.
One additional option is available in the frequency division ratio selection. If the word
(field) is selected in the screen display
instead of one of the numbers from 1 to 60, then the clock pulse will be generated each time a field is about to be recorded in one
of the continuous modes, or 60 times per second.
1 5
