Re: PXL tapes

[Ryan Schweitzer <ryan@altie.org>]

On Wed, 24 Jul 1996, Alexander the Great jr. wrote:



> I do know for sure. The recording format is linear, NOT HELICAL scan. 

> A stationary head is being used. So unlike your home VCR which lays 

> its tracks diagonally on the tape and can therefore benefit by a wider

> tape because it can record longer tracks, your PXL track length is

> limited by the length of the tape not the width. If they wanted to 

> increase the bandwidth they would have to run the tape faster than

> they do. This would further reduce the recording time. 



Yep, some of the first prototype video tape recorders made by RCA and Bing

Crosby Enterprises (BCE) in the early 1950's used a linear transport,

before Ampex wowed them all with the helical scan transport in their

recorder they introduced in 1956, and because of this landmark innovation,

helical is what's used in video today.



The RCA prototype which came out around '52 or so used half-inch tape

(Ampex used 2"), had humongous reels that had to be stopped by technicians 

using gloves (no joke!), and had a tape speed of around 300 ips.  It

actually produced good video, though, although the max recording time was

about 15 minutes.



> The bandwidth of a cassette tape at 1+7/8 inches per second is about

> 16KHz. If the C-60 tape lasts 5 minutes per side, it means they run

> the tape six times faster than normal. That is 1+7/8 * 6 = 11.25 ips.

> The bandwidth can be expected to be approximately six times higher

> as well, so you get 96 KHz on your PXL per track. (I don't know

> if they use more than one track) 

>

> If the C-90 tape lasts 5 minutes per side, it means they run the 

> tape nine times faster than normal. That is 1+7/8 * 9 = 16.875 ips.

> The bandwidth can be expected to be approximately nine times higher

> as well, so you get 144 KHz per track. 



Well, the PXL doesn't automatically adjust the tape speed according to the

length of the tape to get 5 mins. on a side no matter what the length of

the tape, you just get less recording time :). The figures you

mentioned above for C-90 tapes are the correct specs, since PXLs are 

said to record 5 minutes of video/audio on a 90 minute cassette in the

PXL2000 manual, although I timed it out once using 60 & 90 min. tapes, and

I found that PXLs record 6 minutes 02 seconds of video per side on a 90

min. tape, and about 3 minutes 55 seconds of video per side on a 60 min.

tape, according to my findings.



> I don't have a PXL, so I cannot verify the frequency response estimates.

> If someone in the Los Angeles area is willing to lend me his PXL

> for 5-10 minutes, I could verify all that. I mean I can verify

> while you wait, you don't have to give me your precious camera.

> If you can tell me for sure how many tracks they use, (you can find it

> by carefully looking at the recording head) it would be helpful as well.



Well, after some experimenting, I found out that the PXL uses the right

stereo channel of the tape for the video information, and the left channel

of the tape for the audio information.  I found this out by actually

playing a PXL-recorded tape in a regular cassette player running at 1 7/8 

ips, and sure enough, the audio (which was slooooow, obviously) was on the

left channel, and the video information (which had a quiet segmented

buzzing sound at regular 1 7/8 ips speed) was in the right channel.



> Alex Kanaris



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Ryan Schweitzer--N0SXY (Ham radio)

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