Correct!
That is until the engineers at Ampex figured out the rotating helical-scan head.

 

So I wonder , what the op wants to do , what is their knowledge level ,what research have they done ?
I do hope this is not another one of those posts that goes everywhere !

 

Correct!
That is until the engineers at Ampex figured out the rotating helical-scan head.

Actually, what AMPEX (an acronym, so all caps) figured out is transverse scan. It was wildly impractical to move the tape past the heads at the necessary speeds, so they decided to move the heads past the tape.

The headwheel has four heads 90 degrees apart. The plane of rotation is 90 degrees from the direction of tape travel. Rotation is 14,400 RPM, so while tape speed is 15 inches per second, the head-to-tape speed is around 100x that. The track angle is very close to 90 degrees. One scan of a head across 2" of tape records/plays a little over 16 lines of video. The tape is sucked up against a circular guide that matches the radius of the headwheel. A VTR requires a vacuum pump to operate.

AMPEX uses rotary transformers to couple the record and play signals, while RCA (the meatball) uses brushes. RCA and early AMPEX machines have high-impedance video heads that require frequency and Q compensation adjustments for each tape. 3rd and 4th gen AMPEX machines use low impedance heads that eliminate both adjustments.

Another thing AMPEX figured out was that even the best ball bearings had too much play, which showed up as an amplitude modulation of the FM carrier caused by radial movement, plus a phase modulation caused by axial movement. Both of these were almost completely eliminated by going to compressed air bearings, so besides the vacuum pump, a quadraplex video recorder also requires compressed air. We eliminated the vacuum pumps by picking off some of the compressed air for a venturi vacuum generator for the female guide.

https://en.wikipedia.org/wiki/Quadruplex_videotape

Various companies in Japan worked out the early helical scan system, turning it into the EIAJ standard. In the US, IVC developed the 9000 helical scan machine. It pulled 2" tape and made broadcast-legal video in both regular and still-frame playback. It did not do well in the US market, but better in Europe.

ak

 

Actually, what AMPEX (an acronym, so all caps) figured out is transverse scan. It was wildly impractical to move the tape past the heads at the necessary speeds, so they decided to move the heads past the tape.

The headwheel has four heads 90 degrees apart. The plane of rotation is 90 degrees from the direction of tape travel. Rotation is 14,400 RPM, so while tape speed is 15 inches per second, the head-to-tape speed is around 100x that. The track angle is very close to 90 degrees. One scan of a head across 2" of tape records/plays a little over 16 lines of video. The tape is sucked up against a circular guide that matches the radius of the headwheel. A VTR requires a vacuum pump to operate.

AMPEX uses rotary transformers to couple the record and play signals, while RCA (the meatball) uses brushes. RCA and early AMPEX machines have high-impedance video heads that require frequency and Q compensation adjustments for each tape. 3rd and 4th gen AMPEX machines use low impedance heads that eliminate both adjustments.

Another thing AMPEX figured out was that even the best ball bearings had too much play, which showed up as an amplitude modulation of the FM carrier caused by radial movement, plus a phase modulation caused by axial movement. Both of these were almost completely eliminated by going to compressed air bearings, so besides the vacuum pump, a quadraplex video recorder also requires compressed air. We eliminated the vacuum pumps by picking off some of the compressed air for a venturi vacuum generator for the female guide.

https://en.wikipedia.org/wiki/Quadruplex_videotape

Various companies in Japan worked out the early helical scan system, turning it into the EIAJ standard. In the US, IVC developed the 9000 helical scan machine. It pulled 2" tape and made broadcast-legal video in both regular and still-frame playback. It did not do well in the US market, but better in Europe.

ak

Actually, what AMPEX (an acronym, so all caps) figured out is transverse scan. It was wildly impractical to move the tape past the heads at the necessary speeds, so they decided to move the heads past the tape.

The headwheel has four heads 90 degrees apart. The plane of rotation is 90 degrees from the direction of tape travel. Rotation is 14,400 RPM, so while tape speed is 15 inches per second, the head-to-tape speed is around 100x that. The track angle is very close to 90 degrees. One scan of a head across 2" of tape records/plays a little over 16 lines of video. The tape is sucked up against a circular guide that matches the radius of the headwheel. A VTR requires a vacuum pump to operate.

AMPEX uses rotary transformers to couple the record and play signals, while RCA (the meatball) uses brushes. RCA and early AMPEX machines have high-impedance video heads that require frequency and Q compensation adjustments for each tape. 3rd and 4th gen AMPEX machines use low impedance heads that eliminate both adjustments.

Another thing AMPEX figured out was that even the best ball bearings had too much play, which showed up as an amplitude modulation of the FM carrier caused by radial movement, plus a phase modulation caused by axial movement. Both of these were almost completely eliminated by going to compressed air bearings, so besides the vacuum pump, a quadraplex video recorder also requires compressed air. We eliminated the vacuum pumps by picking off some of the compressed air for a venturi vacuum generator for the female guide.

https://en.wikipedia.org/wiki/Quadruplex_videotape

Various companies in Japan worked out the early helical scan system, turning it into the EIAJ standard. In the US, IVC developed the 9000 helical scan machine. It pulled 2" tape and made broadcast-legal video in both regular and still-frame playback. It did not do well in the US market, but better in Europe.

ak

You have great knowledge on the professional video recorderes , the size of large fridge freezers.

Think consumer machines had heads at more like 45 degrees and used the speed of the head to hold the tape tight .

I assume the op has a consumer machine . Let's hope they get back to the forum .

 

You have great knowledge on the professional video recorders , the size of large fridge freezers.

In round numbers, 3 feet by 4 feet by 6 feet, 1/2 ton. Everything from grunt linear power supplies to a 10 MHz FM signal system, plus voltage-variable, baseband video analog delay lines, plus multiple overlapping servo systems, and two motor drive amplifiers. Except for high voltage transmission lines, it is the world of EE in one big analog box. This is where I worked my way through school - EE classes during the day, and a room with seventeen (!!!!!) video machines at night. Absolute EE candy-land.

Think consumer machines had heads at more like 45 degrees

VHS decks started out with two heads on the drum, but added more as quality improved - flying erase heads for editing, separate record and play heads, etc.

and used the speed of the head to hold the tape tight .

The speed of the head was used to create an air cushion; the head did not touch the tape, or touched it very lightly. Consumer machines (3/4" U-Matic, 1/2" VHS and BETA, etc.) all have mechanical tape tension systems. Toward the end of the quad era, AMPEX produced the AVR-1. This monster had dual vacuum columns to maintain tape tension, and ***no pinch roller***.

I assume the op has a consumer machine .

If the OP has a quad deck, I'm on the next plane out of here.

ak

 

At this point we should ask just what format of video the TS is actually wanting to amplify/record/playback.
Please consider that HAM RADIO folks have been sending and receiving video over voice-frequency links prior to transistors being commonly available. SLOW-SCAN video, but actual video.
AND consider the BELL SYSTEM "PICTURE PHONE" that also delivered video images.
I SUGGEST that the TS do more research and try to find a way to watch a current standards VHS video player actually operating, with the covers removed so that the tape path is visible. Once you understand what is happening, it is very impressive indeed!! All of the folks who are at all interested, really should do that! To achieve an effective head-across- tape speed fast enough for OK video playback at a bulk tape speed of about 2 inches per second is totally brilliant! AND to do it cheap but reliable and long lasting is AWSOME!!
The actual magnetic track is diagonal across the tape, because the tape is wrapped diagonally around the spinning cylinder with the writing part centered between the two ends of the cylinder. So instead of straight down across the tape, it is a much longer path with a higher speed because the tape speed adds to the spin speed. I have no clues as to how they synchronise for playback. Probaby magic, is my guess.

One more detail: For most applications I have that the LM733 amplifier really should have a bipolar power supply, meaning both NEGATIVE and POSITIVE, relative to the system COMMON line. the LM733 is not like a common opamp at all. It IS a "Differential Video Amplifier", which is a quite different sort of animal.

 

JVC licensed the original patents that became VHS from ----- AMPEX.

ak

 

I have actually SEEN one of thos e 60 inches per second video playbacks at a TV station, back in about 1961, I think. My recollection was about how exciting a tape spill would be.

 

My recollection was about how exciting a tape spill would be.

Death trap video.

ak

 

It will be very interesting to learn what the TS is actually able to achieve!!

 

It has been a while since we heard from the TS!. We may never know what the original intent was. Certainly the evolution of video recording is quite impressive, and the evolution of the video recording scheme is totally awesome indeed. The advances in megnetic tape recording are especially impressive.

 

not sure what TS stands for, but that's pro'ly me. got caught in a whirlwind of other projects, but turning back to this one for the time being. basically the idea is to inject a composite signal into one side of the removed, non-rotating head...and pick up the signal on the other side and amplify it back to a "normal" usable composite video on the other half of the head...then, experiment with all sorts of devices to alter the stable transmission across the coils with such things as magnets, speakers, etal...and see if i can drum up some interesting distortions in a truly analog, hands-on way. something to pass the time until the next world cup game . 8o)

 

not sure what TS stands for, but that's pro'ly me. got caught in a whirlwind of other projects, but turning back to this one for the time being. basically the idea is to inject a composite signal into one side of the removed, non-rotating head...and pick up the signal on the other side and amplify it back to a "normal" usable composite video on the other half of the head...then, experiment with all sorts of devices to alter the stable transmission across the coils with such things as magnets, speakers, etal...and see if i can drum up some interesting distortions in a truly analog, hands-on way. something to pass the time until the next world cup game . 8o)

TS. Thread Starter
OP. Original Poster
just short hand , essecialy when on a small screen it can be hard to scroll back to top whilst replying

 

basically the idea is to inject a composite signal into one side of the removed, non-rotating head...and pick up the signal on the other side and amplify it back to a "normal" usable composite video on the other half of the head..

Video is not recorded the same way as audio. The video heads are designed to work with a modulated FM carrier, not baseband video.

Also, what do you mean by "one side" and "the other side" of a video head.

ak

 

Video is not recorded the same way as audio. The video heads are designed to work with a modulated FM carrier, not baseband video.

Also, what do you mean by "one side" and "the other side" of a video head.

ak

the two sides of the cylinder can be separated...top and bottom. the heads are spinning on the top side while the bottom side is stationary. the video signal seems to be transmitted by two similar antennae...quite a tech marvel if you ask me. i am guessing it is basically, the two sides of the head, without any other parts if the vcr attached could be used as a short distance video transmitter and reciever....hopefully that clears up the two-sides.

 

Early VHS decks might have used slip rings for the video signal, but later ones used rotary transformers (like quad machines). The tech was almost 20 years old when VHS was introduced.

Later decks still used slip rings for other functions. VHS-HiFi put the audio on an FM carrier, and that might have gone through rings, same for the flying erase head.

ak

 

And then came S-VHS.
Real improvement.

 

REcording COMPOSITE VIDEO is certainly amazing, especially at the slow tape speeds. THAT works because the speed of the video recording head past the tape is much faster.You will not be even close to recording composite NTSC video on a fixed head!! At least not at any speed close to what is used for any sort of magnetic tape recording.

The tape path scheme of a modern video recorder/player is actually a "Mechanical Marvel", considering how cheap those systems are made.
It would educate all of us if somebody who actually knew how the present VHS scheme works, IN DETAILS, would provide an explanation. How does that spinning head speed relate tothe frame rate?? And how is the audio track matched to the video track so perfectly?? Certainly it is not simple.

 

The TS doesn't want to record/play video in the usual way. I think he wants to use two video heads to project a video signal across a gap, kinda like an air-core transformer. But that is a guess.

VHS uses an "Omega-wrap" tape path. The tape makes drum contact for a little over 180 degrees of its circumference. Each head on the drum records one video field per pass, so one complete rotation of the drum covers one frame of video in two stripes. In original VHS, audio is recorded along the edge of the tape as in a standard audio tape deck. In SP mode the linear tape speed is slower than a standard audio cassette, which is one reason why the audio fidelity is so poor. Another is that the magnetic grain of the oxide coating is optimized for the video head tracking angle, not the 90 degree angle of an audio head. Quad tape systems have (had) an even worse track angle conflict, but it is not as noticeable because of the much higher tape speed.

https://en.wikipedia.org/wiki/VHS#Recording_process

ak

 

Projecting a VIDEO signal across a gap WILL NOT WORK.