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Name:  Devin
Status: student
Grade: 6-8
Location: CA
Country: N/A
Date: 2/20/2005


Question:
How many gigabytes of information is stored on the average 6-hour VHS videotape? I realize that it is analog, not digital, but how many gigs?


Replies:
Here is a rough estimate:

I figure VHS is roughly equivalent to 30 340X400 pixel frames per second, each pixel representing 24 bits of information (one byte each of red, green, and blue). According to this estimation, six hours of VHS represents 30 X 340 X 400 X 3600 X 6 X 3 = 2.6e11 bytes = 260 GB.

Tim Mooney


Devin-

Since it is not digital, you have to make a fuzzy decision on the kind of "exchange rate" you want to believe in.

A) If your interest is TV, you want to decide how many digital bytes per second it would take to replicate the quality of the TV picture as played back from the tape.

B) If your interest is computer storage, you want to decide how many bytes could be encoded in the average TV frame on VHS tape, and still be readable without errors by the computer. The exchange is "lossy" going both ways: the number of bytes needed to replicate a TV picture is more than the number of stored digital bytes you normally could get back out of the picture.

The typical difference is at least a factor of two. Another thought is that the eye expects to see at least 256 different colors (8 bits, 1 byte) in a poor TV picture, up to about 64,000 colors (16 bits) in a relatively good TV picture. Many simple encodings of digital data into a TV picture will only put 1 bit into each TV pixel, using in effect only 2 colors. This kind of encoding will have less impressive density, but better encodings are pretty difficult to implement well. And computer data needs a very low error rate, even with a layer of error-correction-coding.

The quality of 6-hour "EP" recording on VHS tape is less than the quality of 2-hour "SP", so the implied data is less than 3 times that in the 2-hour recording.

counting TV frames:

6 hr/tape x 60 min/hr x 60 sec/min = 21600 sec/tape
21,600 sec/tape x 30 frames/sec = 648,000 frames/tape

picture resolution:
512 lines/frame x 512 pixels/line = 262144 pixels/frame
(many people will say the right number is about 4x smaller)
256 lines/frame x 256 pixels/line = 64,000 pixels/frame

color quality: (assume low quality)
poorest: 64 intensities/pixel x 8 "hues" = ~512 colors/pixel = ~9

bits/pixel
fair: 256 intensities x 32 "hues" = ~4096 colors/pixel = ~13

bits/pixel
good: 16 bits/pixel

A) (poorest TV resulting)

6.5e5 frames/tape x 6.4e4 pixels/frame x 9 bits/pixel =
= 3.8e11 bits/tape = 4.8e10 bytes/tape = 50 GB.

Most parts of the picture don't change very fast, and the picture has many low-detail places, so you could expect to be able to invent a 10:1 compression method. Then the 6 hours of poor TV could be replicated from 5 GB of digital data. You can see it might fit on a 2-layer digital DVD. On the other hand, if you need pristine viewing of 6hr of complex pictures, you could imagine needing 500 GB. Since a 6-hr tape is not that great, perhaps 200 GB is a more reasonable estimate.

B) simple method of storing digital data as light/dark spots in a TV picture:

648,000 frames/tape x 100x100 spots/frame x 1 bit/spot =
= 6.5 Giga-bits = 6.5 GBytes

It is possible that successive frames on a 6-hour EP tape are in effect blurred together, so the real number of changed pictures available might be more like that in a 2-hour tape, 3 times less, about 2 GB. I think I have heard of storing as little as 2GB on VHS cassettes. I have heard of more, too, but I am not up-to-date on how much more has been done.

Jim Swenson

PS- I admit, my arithmetic is not accurate. But I do not think it needs to be, for these estimations.



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