120 line conversion

[Steve Anderson]

Yep, if it has a TBC (Time-Base Corrector) then any other standard aside from what it was designed for will fail. Though the vast majority of domestic machines didn't have a TBC. On your machine when recording onto DVD it is essential, TV's/monitors are quite tolerant of input line-time variations, the DVD recording process isn't.

It may be possible if not doing a VHS to DVD transfer, just playing a tape, to bypass the TBC, if possible it's probably buried somewhere deep in some obscure menu.

Steve A.

[Harry Dalek]

As to using VHS for recording formats other than the 625-line (or whatever) for which it is designed, my understanding is that it only works successfully on earlier machines that don't attempt clever post-processing of the signal. This is, however, only what I have heard from others. I have never tried recording 405-line output (such as from my standards converter) - maybe I should give it a go some time.

I had some multi system machines infact towards the end of VHS it was very common for them to do either NTSC or Pal well here in Australia and some with SECAM still have one with that ..may be they were just making the same machine for different countries but came in handy swapping tapes with some buddies in the usa at the time, for recording i did have systems converter ,i know i tried 525 625 visa visa apart from no colour there was really just an aspect ratio difference as i recall as you were recording at either correct tape speed...i would bet the 405 would work i am sure others to would be interested in the result .
Back on to 120 line
Looking at Garys list of what makes up hes video signal it say no line or frame sync its a wonder i managed to display any thing stable enough i was wondering why the line locking was dreadful at 120 shows you how good those sawtooth oscillators in Steve's circuit are .

[Klaas Robers]

Be aware that a VCR records and plays back fields, not frames. As you know the standard resolution TV systems all use interlace, with 50 (60) fields per second. VCR records this as fields, so it expects 50 (60) frame syncs per second. So in fact it records 312.5 lines at 50 Hz or 262.5 lines at 60 Hz.

If you use 120 lines at 25 Hz, which is called "progressive scan", that is "not interlaced" the frame sync comes at 25 Hz, not 50 Hz and a VCR will have problems with that in synchronising the head wheel.

[Harry Dalek]

Be aware that a VCR records and plays back fields, not frames. As you know the standard resolution TV systems all use interlace, with 50 (60) fields per second. VCR records this as fields, so it expects 50 (60) frame syncs per second. So in fact it records 312.5 lines at 50 Hz or 262.5 lines at 60 Hz.

If you use 120 lines at 25 Hz, which is called "progressive scan", that is "not interlaced" the frame sync comes at 25 Hz, not 50 Hz and a VCR will have problems with that in synchronising the head wheel.

Steve mentioned has to be 50 or 60 hZ what ever system i will keep that in mind when i get around to the flying spot scanner Klass.

Heres more on the B&K TV Analysts i find them very interesting nice set of videos on it .

https://www.youtube.com/playlist?list=P ... 0amX0KX869


youtu.be/D7LwSWCLUEM

[Steve Anderson]

Yep, for an EU VCR the frame or field frequency must be 50Hz, 60Hz for the US and some other countries. A 25Hz (30Hz) field or frame (without interlace) rate will not work. Basically vertical sync must be at 50Hz (60Hz) whether interlaced or not. What happens between the vertical pulses the VCR doesn't really care about - unless it has a TBC that you cannot bypass.

The other thing to consider, and this may vary from machine to machine, is the low-frequency response. 625 requires a response down to at least 25Hz, most machines go somewhat lower, but it's worth thinking about. Active clamping on the back-porch part of the signal is usually the way to preserve this. The luminance signal on VHS/Betamax is recorded via FM which could have a response down to DC, not that dissimilar to SSTV.

The other little 'gotcha' can be the head switching glitch about six lines before the end of the frame/field. depending on your monitor or TV you may see a small white dot wobbling around at the bottom of the screen. This is where the heads are switched 180 degrees apart on the head-drum. If the switch causes a spike into the sync region this can cause problems with simpler downstream electronics.

Steve A.

[Harry Dalek]

Yep, for an EU VCR the frame or field frequency must be 50Hz, 60Hz for the US and some other countries. A 25Hz (30Hz) field or frame (without interlace) rate will not work. Basically vertical sync must be at 50Hz (60Hz) whether interlaced or not. What happens between the vertical pulses the VCR doesn't really care about - unless it has a TBC that you cannot bypass.

The other thing to consider, and this may vary from machine to machine, is the low-frequency response. 625 requires a response down to at least 25Hz, most machines go somewhat lower, but it's worth thinking about. Active clamping on the back-porch part of the signal is usually the way to preserve this. The luminance signal on VHS/Betamax is recorded via FM which could have a response down to DC, not that dissimilar to SSTV.

The other little 'gotcha' can be the head switching glitch about six lines before the end of the frame/field. depending on your monitor or TV you may see a small white dot wobbling around at the bottom of the screen. This is where the heads are switched 180 degrees apart on the head-drum. If the switch causes a spike into the sync region this can cause problems with simpler downstream electronics.

Steve A.

A question i have to ask does the 50 or 60hz vertical have to there ? i am thinking yes as i would think the old vcr circuits would have a sync detection circuit for both looking for it .
Another Question i have not really read any where or asked is why does a LD video signal have no framing pulse i know you and other have done some clever circuits to work out this problem missing pulse and all but why don't we just have one now and please your self which you use ?...make life easier on the electronic monitor side of it ....i am not sure when a both vertical and horizontal syncing pulses were first used but it must of been around the time crt's took over from the mechanical systems ?
Another Question on the last youtube video how the hell is hes PMT working with the case open i found with a pin prick light hole it causes static washed out picture on my PMT camera ...i know they say theses are UV sensitive but aren't they all ?

[Klaas Robers]

If you have no more than 30 lines, it is seen as waste if you spoil one line for a frame pulse. The more lines you have the less you spoil. Indeed, it is not simple to detect a missing sync pulse, but for the mechanical scanning this is not needed. And for CRT, 405 lines was the first practical system, and a frame pulse was inserted. Of course with magnetic deflection you need some fly back time, so the missing lines are not a problem.

I also wondered about the brightly lit open case with the PMT. It must have been that for the photos the high voltage of the PMT was switched off.

[Harry Dalek]

If you have no more than 30 lines, it is seen as waste if you spoil one line for a frame pulse. The more lines you have the less you spoil. Indeed, it is not simple to detect a missing sync pulse, but for the mechanical scanning this is not needed. And for CRT, 405 lines was the first practical system, and a frame pulse was inserted. Of course with magnetic deflection you need some fly back time, so the missing lines are not a problem.

OK i can see why Klass yes i forgot 405 Line did start out when it won out over the Baird so that was pretty early on it started .. funny when when trying Garys sync less 120 line on the devil i find at around 1800hz 2 frames are seen but the locking is very good don't know why it does lock well but at 3600hz one image its very hard .

I also wondered about the brightly lit open case with the PMT. It must have been that for the photos the high voltage of the PMT was switched off.

Looking at the other videos i am sure he has it working as the testcard is on the monitor ,i looked into this and am trying to find schematics for the BK TV ANALYST which made these i came across one or 2 models ahead and the power supply to the PMT is very low -270 v that makes sense as the flying spot tv screen is pretty bright for a PMT

[Steve Anderson]

For a VCR a 50/60Hz field or frame pulse is essential as this divided by two (25/30Hz) is recorded onto a longitudinal control track on the tape. This regulates the playback capstan speed to maintain a modicum of mechanical synchronisation. Finer synchronisation is also done via another PLL for the head-drum. As much as I know there are those here who shun the PLL, the VCR, DVD or even a simple audio CD player all use them, they do work! You just need to design them well, that's not such an easy task when it involves mechanical components within the loop. The maths involved can become complex but not impossible.

I've said before that a PLL properly designed is as reliable as any flip-flop. op-amp or any other electronic component, when you involve mechanical moving parts - that's when the headaches start.

You can understand why people like me design out as much electro-mechanical parts as possible. They're a pain. Switches, pots, connectors, motors, all sources of unreliability.

Steve A.

[Klaas Robers]

Looking at the other videos i am sure he has it working as the testcard is on the monitor ,i looked into this and am trying to find schematics for the BK TV ANALYST which made these i came across one or 2 models ahead and the power supply to the PMT is very low -270 v that makes sense as the flying spot tv screen is pretty bright for a PMT

No Harry, it is a flying spot scanner. What you see is a (large) testcard slide closely in front of the CRT, which displays a white field. The PMT then picks up the flying brightness of the slide. That is how a flying spot scanner works. The trick is that the line rate and the frame rate can be adjusted from the front. In this way it can generate a video signal of whatever standard. In real use the cabinet is closed of course. But then you cannot make a photo.

[Harry Dalek]

No Harry, it is a flying spot scanner. What you see is a (large) testcard slide closely in front of the CRT, which displays a white field. The PMT then picks up the flying brightness of the slide. That is how a flying spot scanner works. The trick is that the line rate and the frame rate can be adjusted from the front. In this way it can generate a video signal of whatever standard. In real use the cabinet is closed of course. But then you cannot make a photo.

Yes its a Flying Spot Klass the people that made it call it later Television Analyst i was told by some else to look for that and as there was little on the BK1000 thats in the youtube videos i linked to a few posts back ...... did track down the schematic for that one the other models are in the link i just posted up in Resources .
The one here in the pdf the 1000 model the first model i have been trying to work out the voltage to the PMT i see that its similar to the later model so must use a lower voltage ,i was thinking this PMT would not work at 270 volts .

i want to use my spare scope and feed the little monitors sawtooth to it have a hand drawn slide or such flat on the crt display have a pmt or light sensor pick up the video and feed it back to the little monitor and display the image they would both be locked in sync and if i adjust the monitor line rate frame the scope flying spot with follow very much like The 73 magazine early 70s SSTV flying spot idea