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Dear Charlie,
Could you explain the differences between a 24 hour time lapse VCR and a 24
hour real time VCR. I know true 24 hour real time is impossible, so is there any
real difference? Thanks for your help.
Sign me,
Not quite real
Dear Not Quite,
In reality, you are right on the mark. 24 hour real time is impossible, using
the current technology that is available to us today. The differences between a
time-lapse machine in the 24 hour mode and a 24 hour machine in the same mode
are real. So are the advantages.
First, let's talk about time-lapse and what it is all about. When we go into
the time-lapse mode (anything over 12 hour speed), we enter into the realm of
incrementing the video tape. What this means is that we literally start to move
the video tape forward in specific increments of time. Consequently, we are able
to record long periods of time on a small amount of video tape. For instance, if
I put the time-lapse video recorder into the two hundred and forty mode, I will
be able to record ten days of continuous video on a two hour cassette.
Obviously, the math says that I can't record two hundred and forty hours of
continuous information on a two hour cassette. So what happens. We increment the
video tape across the video heads as such a speed as to allow the two hour tape
to go from Recording time divided by tape length in minutes (120) equals time lapse
between pictures.
Let's try it with the two hundred and forty hour mode on our time lapse
recorder.
240 / 120 = 2 seconds.
This means that we will record a single video image each two seconds. So what
about a time-lapse video recorder in the twenty four hour mode. 24 / 120 = .2
seconds or five pictures per second. Since real time recording runs at about 30
pictures per second, you can guess the result. Play back of a twenty four hour
recording in real time would look like an old Charlie Chaplin movie, but even
faster.
When recording video information onto a video tape, you must picture the
video tape moving past a set of rotating video heads. The resulting sweep of
magnetically recorded information on the tape would not be straight up and down
on the tape, but rather on a slant like a slash ( \ ). Additionally we require a
space between each slash of video information to prevent picture overlap. All in
all, a very complicated procedure of precise timing and advanced electronics.
The biggest problem comes into view when you consider all of the twenty four
recording applications in Security. Unfortunately, five pictures per second is
just not enough information in most of these applications. Consequently, the
new, high density, 24 hour recorders were developed. With these new machines, we
changed the pitch or slant of the magnetically recorded information, cut down on
the spacing between images and increased the length of the tape. The end result
is more recorded images per inch of video tape. How much more? The average 24
hour video recorder runs at 17 to 21 images per second. This is not real time,
but it is a vast improvement over the time-lapse video recorder.
The same technology and reasoning applies to the new 72 hour recorders. A
time-lapse recorder in the seventy two hour mode records a video image each
point six seconds. A 72 hour, high density recorder records an average of 5 to 7
images per second. Big difference.
In the end, your application will dictate which recorder you use in the
field.
At any rate, good luck. If I or my staff can be of any additional service to your CCTV needs, please feel free to holler. We are in your service.