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VCR & Off air signal differences

 
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VCR & Off air signal differences

Post by Spot-Wobble » Thu May 08, 2014 9:27 pm

Perhaps someone could explain the differences between the signal output by a VCR and an off air transmission.
I know there are differences but don't know what.
My 1960's Fergie 1400 suffers from line pulling at the top of the picture when fed from a VCR but is fine when fed from a set top box etc.
The same but much worse on my Bush CT197C (see other thread).
If I knew what the differences were then maybe I could come up with a fix for both sets.

Andy

 
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Re: VCR & Off air signal differences

Post by Red_to_Black » Thu May 08, 2014 9:51 pm

It is caused by mechanical 'jitter' or slight timing errors due to slight imperfections in the VCRs Drum/head bearings and head to tape interface as well as a slight imperfections with tape compatibility between different machines, this is normal and slightly affects the horizontal sync in the signal.

You often get less "hooking" or "flagwaving" when playing back a recording made on the machine that is playing back, and more on a tape recorded elsewhere as the timing jitters from both machines are slightly different from each other and tend to be cumulative and add up .

A shorter time constant in the flywheel sync circuit allows a TV to better track these 'jitters' and some late 70's - early 80's "VCR compatible" sets had a dedicated channel with a modified time constant for use with a VCR, sometimes marked as AV (different to later sets where AV was an external input/scart etc.).

Using a short time constant on all stations wasn't used because in poor signal areas this could cause corrugated or watery verticals on normal reception.

More modern TVs could of course automatically compensate for the various signals and compensate by adjusting the time constant of the line sync circuit to suit.
Last edited by Red_to_Black on Thu May 08, 2014 10:00 pm, edited 5 times in total.

 
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Re: VCR & Off air signal differences

Post by Spot-Wobble » Thu May 08, 2014 9:54 pm

Thanks R -> B

A good explanation.

Regards

Andy

 
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Re: VCR & Off air signal differences

Post by Red_to_Black » Thu May 08, 2014 10:02 pm

Hi Andy I was further editing the earlier post to try and clarify it a bit better and also add a bit more info. as I missed a few bits and corrected other bits :bba

 
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Re: VCR & Off air signal differences

Post by Cathovisor » Thu May 08, 2014 10:11 pm

One other little bit I'd like to add to R2B's explanation:

Red to black wrote:It is caused by mechanical 'jitter' or slight timing errors due to slight imperfections in the VCRs Drum/head bearings and head to tape interface as well as a slight imperfections with tape compatibility between different machines, this is normal and slightly affects the horizontal sync in the signal.

The other issue is that you are dealing with an elastic medium - tape. So it deforms when the heads hit it as they pass through the gap, and then there is the fact you are dragging this from one reel to another over a surface.

Red to black wrote:More modern TVs could of course automatically compensate for the various signals and compensate by adjusting the time constant of the line sync circuit to suit.


A lot of later TVs used counters in the sync processing chip to derive the field syncs...

Something people might like to consider if they have a large library of tapes they wish to use with their older tellies is that some semi-professional machines, and machines offering "World" playback had timebase correctors built into them. This stabilised the output signal by clocking the picture into a memory device and clocking it out using a stable internal reference. Panasonic made a few machines that did this, and Samsung certainly offered the "world" machines with digital standards conversion and off-tape timebase correction. Some S-VHS machines offer this too.

Given the rock-bottom prices VHS machines realise these days, it might be worth seeking them out.

Naturally, broadcast signals are timed to extremely precise amounts - the master oscillators used to derive the pulses that produced the signals were once derived from rubidium standards at BBC Television Centre - frequently pulse generators are now locked to GPS signals, which are extremely accurate indeed.
Last edited by Cathovisor on Thu May 08, 2014 10:24 pm, edited 1 time in total.

 
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Re: VCR & Off air signal differences

Post by Red_to_Black » Thu May 08, 2014 10:23 pm

Hi Cathy, I was having difficulty in trying to put into words what I meant and to try to keep it understandable as well as not too long and technical.

Of course you are correct about the tape being elastic and head bounce. :cca

Later sync processing/"jungle" ICs had dual or multi -loop timing which as you say was divided and counted down often from a master oscillator crystal at a frequency of some multiple of the colour sub-carrier, this worked on a line by line basis and was fully automatic.

ttt:

 
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Re: VCR & Off air signal differences

Post by Cathovisor » Thu May 08, 2014 10:41 pm

Although they won't be co-timed in any sense, it is very instructive to lock an oscilloscope to an off-air signal and then look at the signal coming off-tape. It's astonishing a TV set locks to a VCR at all!

 
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Re: VCR & Off air signal differences

Post by Michael Watterson » Thu May 08, 2014 10:53 pm

Other differences:
Exact relationship between colour subcarrier and line frequency is lost, partly because the colour signal is shifted to a lower frequency so it can be recorded below the FM carrier and sidebands the luminance is modulated on to (The luminance is FM carrier to make it immune to amplitude fluctuations).
The Luminance bandwidth is greatly reduced (even more on "long play"). Hence recorded teletext is garbled.
The colour bandwidth is reduced a lot and I I think an S-VHS only makes a big improvement to luminance bandwidth.

B&W always looks better than colour on a VHS, esp on an S-VHS.

The RF modulator is usually full double sideband, not vestigial sideband. Even the Chroma and Sound will appear twice!

When I first was taught about VCRs in 1970s I marvelled that they work as well as they do! The head not just going at correct speed but synchronised to frame such that the head follows the same skinny slanted / diagonal track on tape with alternate heads. No wonder tape playback recorded on a different machine is harder and why later home machines could make better recordings than pre-recorded tape.
Imagine high speed VHS duplication!

With time a tape stretches so a tape that played fine when originally recorded may "hook" a bit on top of picture on any machine on playback. Tracking is also problematic with age or different machine. If you have a undersized raster you can see the head switching. Even if tracking is over all acceptable, the head switching noise may become visible at bottom of screen.

My experience too is that the HiFi (not Nicam, that is only a tuner feature) FM helical recorded sound "breaks up" before obvious picture disturbance. VHS "normal" sound is linear on edge of tape and with the lack of space and low tape speed it's very poor frequency response and dynamic range (noisy). The "HiFi" system usually the Tuner also has Nicam decoder, is helical FM recording (is it always 2 extra heads at 90 degrees to video heads?) and can be near CD quality unlike the regular linear VHS audio which is often poorer than compact Cassette.

Of course no Domestic VHS has a NICAM coder, so Stereo playback is only on RCA audio jacks and/or SCART audio, the modulator is only ever mono.

 
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Re: VCR & Off air signal differences

Post by Red_to_Black » Thu May 08, 2014 10:55 pm

Cathovisor wrote:It's astonishing a TV set locks to a VCR at all!


True!
I suppose it was always a trade-off to try to keep the cost of the home machines reasonably affordable, especially in the early days of VCRs, whilst at the same time improving the TV technology so that it did not matter.

I suppose Andy could try and modify and shorten the time constant of his set just before the point of where off-air pictures are starting to be affected.

Mind not being familiar with the circuit details I am not sure how to attempt this at the moment. :aaq

Suggestions on a postcard ? :bba

 
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Re: VCR & Off air signal differences

Post by Cathovisor » Thu May 08, 2014 11:00 pm

Red to black wrote:
Cathovisor wrote:It's astonishing a TV set locks to a VCR at all!


True!
I suppose it was always a trade-off to try to keep the cost of the home machines reasonably affordable, especially in the early days of VCRs, whilst at the same time improving the TV technology so that it did not matter.

If I recall, the N1500 was substantially more expensive than the contemporary Philips television sets. I'm curious to know how well these old sets coped with the output from the old "battleship" machines like the Philips N1500/N1700 and the Sony open-reel machines for example, where the linear tape speed was much higher and the mechanics were more substantially engineered: I have a 1/4" Akai open-reel machine, which must have been "interesting" in its day!

 
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Re: VCR & Off air signal differences

Post by Red_to_Black » Thu May 08, 2014 11:02 pm

Hi Michael, I didn't think you could hi-speed duplicate VCR tapes, this is inherent to the helical scan process, as the moment you change the speed you thereby change the track angle.

Later more modern Hi-Fi machines did not always have the audio heads spaced at 90 degrees from the video heads, the head drum FG/PG and switching circuits had a much more complex pulse train waveform, this was achievable in part due to digital servo's relying more on counting techniques than the earlier analogue servos.

 
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Re: VCR & Off air signal differences

Post by Red_to_Black » Thu May 08, 2014 11:12 pm

Cathovisor wrote:If I recall, the N1500 was substantially more expensive than the contemporary Philips television sets. I'm curious to know how well these old sets coped with the output from the old "battleship" machines like the Philips N1500/N1700 and the Sony open-reel machines for example, where the linear tape speed was much higher and the mechanics were more substantially engineered: I have a 1/4" Akai open-reel machine, which must have been "interesting" in its day!


Is it possible these very early machines were used with professional grade monitors predominately ?
Not being familiar myself with these very early systems.

The N1500/1700 machines were just consumer grade machines that worked pretty much the same as early Beta and VHS with their complementary modified (AV channel) TVs of the day.

 
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Re: VCR & Off air signal differences

Post by TVJON74 » Thu May 08, 2014 11:30 pm

With regard to high speed recording and playback. I once owned a domestic JVC VCR where the entire head drum assembly was mounted on servo contoled motors and the head angle was adjustable (by the machine) so playback was possible IIRC @ 9x normal speed forward and 7x normal speed in reverse with a perfect picture. Some clever electonics also gave you normal sound in bursts, what ever direction you were playing the tape.

Jon

 
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Re: VCR & Off air signal differences

Post by Red_to_Black » Thu May 08, 2014 11:55 pm

TVJON74 wrote:With regard to high speed recording and playback. I once owned a domestic JVC VCR where the entire head drum assembly was mounted on servo contoled motors and the head angle was adjustable (by the machine) so playback was possible IIRC @ 9x normal speed forward and 7x normal speed in reverse with a perfect picture. Some clever electonics also gave you normal sound in bursts, what ever direction you were playing the tape.

Jon


I had the misfortune of trying to repair one of these beasts once :bba , it was JVC dynamic track following machines, the one I looked at was a hugely complex beast, it was a multi-standard type of machine three speed settings SP, LP and EP and also had MESECAM capabilities, the gentlemen it belonged to worked overseas.

Clever bit of technology though, must have been expensive.

Hi speed reproduction as far as I know was never practical because the helical scanning produces a spiral like track an exact length across the tape, it was just too awkward to attempt, hence the shift to digital Disc based recording techniques, much easier to mass produce.

The VHS-C format was a clever bit of Track geometry manipulation, some clever people at JVC :aad

Ps. I was Just looking through some of my technical documents for the geometry and theory of the depth multiplex Hi-Fi recording technique, I had an excellent article written by Panasonic that explained it all in relatively understandable language, sadly I cannot put my hand on it at the moment, but if I find it I will upload it to the library.

 
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Re: VCR & Off air signal differences

Post by Red_to_Black » Fri May 09, 2014 12:12 am

I was just thinking of the reply I made to Michael further back in this thread RE: Hi-Fi heads, I don't think these were ever spaced at 90 degrees to the video heads, they were off-set obviously, but until I find my data I cannot recall off hand by how much, they had to follow closely the video heads though.
The better machines had a seventh flying erase head, this allowed perfect insert edits.

Where the heads were off-set at 90 degrees was on the very early long play machines, the LP heads being off-set by 90 degrees from the SP ones, this was later changed to them being co-sited with their SP counterparts, and later being of the dual tip type.

 
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Re: VCR & Off air signal differences

Post by TVJON74 » Fri May 09, 2014 12:27 am

Just looking at some google images of head drums, it looks like the Hi-Fi heads are spaced from the video heads but only by about 30deg.

Jon

 
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Re: VCR & Off air signal differences

Post by Cathovisor » Fri May 09, 2014 1:13 am

TVJON74 wrote:Just looking at some google images of head drums, it looks like the Hi-Fi heads are spaced from the video heads but only by about 30deg.

Jon

That's how I remember them - and depth-multiplex recording on (IIRC) 1.4 and 1.8MHz carriers that led to an odd "burring" noise on one channel as the heads wore down...? As I understood it, the higher frequencies penetrated less into the magnetic layer so that as the heads wore and the head current rose, the video signal penetrated deeper and partially erased the hi-fi signals leading to the odd noise effect described.

Thing is, when it worked properly it was a superb analogue audio recording medium - incredible signal/noise and dynamic range, and eight hours on a tape!I recovered a large number of Alan Dell's Dance Band Days programmes from a tape I kept for that purpose quite recently.

Now, I may be imagining this, but wasn't there a contact-type method of reproducing video tapes at one time...?

Of course, the best dynamic track following system was the Philips one used in the V2000 system; which in turn was similar to the method used on Ampex's 1" machines - using a piezo bimorph to move the head itself whilst keeping the drum static.

 
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Re: VCR & Off air signal differences

Post by Cathovisor » Fri May 09, 2014 1:29 am

Red to black wrote:The N1500/1700 machines were just consumer grade machines that worked pretty much the same as early Beta and VHS with their complementary modified (AV channel) TVs of the day.

Don't forget the N1500 dates back to around 1972, so TV sets would not have had dedicated VCR channels by then: I don't really recall seeing those until about the time VHS and Betamax started creeping onto the market later in the 1970s... according to Wikipedia the first machines were about £600!

As a complete aside, Total Rewind makes a fascinating visit...

 
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Re: VCR & Off air signal differences

Post by Briancuff » Fri May 09, 2014 9:30 am

Cathovisor wrote:Now, I may be imagining this, but wasn't there a contact-type method of reproducing video tapes at one time...?

There was indeed. Ampex made the ADR150 high speed 2" quad tape duplicator in the early 1970s. This was a huge beast which could make up to, IIRC, six copies at 10 times real time. The master- mirror image - tape was recorded on an AVR-1 VTR with a modified video head on which the rotary heads ran in the opposite direction to the norm. It was also recorded on very high reluctance tape.
On the ADR-150, the master tape was routed round 1, 2 or 3 duplicator units. On each unit, two virgin tapes were loaded and the path of these tapes ensured that the master tape and the virgin tape were pressed in very close contact in a special block which was "excited" by an RF field. This is what transferred the magnetic track pattern onto the receptor tape.
These systems cost several hundred thousand dollars, even in those days!!
There were systems made for 1/2" tape systems too, but I have no recollection of those devices.

They were made for the US market with its many time zones and requirements to send programs to the main broadcasters' affiliates.

 
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Re: VCR & Off air signal differences

Post by Michael Watterson » Fri May 09, 2014 11:59 am

Yes, the HiFi machines came in after I was no longer involved with servicing. So I was in doubt as to exact offset.

The V2000 was so clever that the launch I think may have been delayed nearly 2 years as they couldn't mass produce the heads! If it had beaten VHS & Betamax to market it might have been a winner. But being a well behind third it was doomed.

I saw a colour 1/4" Akai portable Reel to Reel in the BBC. They got one out of curiosity to see if it could be used for news. They decided no. Today of course with low quality Skype and Mobile phone interviews dominating TV & Radio it would ironically be fine!

I left behind my EIAJ 1/2" B&W machine and camera in 1989 when I went abroad (from Ireland) to work. Sad. I took my Goldring / Lenco turntable system and all my LPs, but of course they got left abroad when the job fell through & I came back 13 months later (to UK).

I had an N1700 on loan for a while in 1979. It played fine on my late 1960s / early 1970s big B&W hybrid TV. I had fitted a video in and isolation transformer. It was 21" or larger and veneer cabinet (like 1970s sideboards / shelves, rectangular, not rounded, not dark or pale). Could it have been a Pye? I don't remember, but 625 UHF only. We had a colour set at my parents home, but my own 1st colour set was a 8" multistandard bought new in late 1982 with a portable VHS (lacking camera) and matching stacking tuner Unit. It all ran off batteries, mains or 12V (apart from the matching tuner for VHS) so we had it on holiday in our semi-converted Volkswagon Camper 1983 / 1984. I gave TV to someone that had knocked their Colour TV onto floor (it still worked, but disastrous colour purity!) and the VHS + tuner pair to a friend whose VHS had been stolen. It was stolen a week later. :)

 
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Re: VCR & Off air signal differences

Post by Doz » Fri May 09, 2014 12:25 pm

Cathovisor wrote:Now, I may be imagining this, but wasn't there a contact-type method of reproducing video tapes at one time...?



High speed duplication of commercial material was possible by making a mirror copy of the original. This was recorded on very high coercivity tape, and when passed through a scanning magnetic field left it's imprint , duly reversed, on the copy tape.

Saw this machinery once at Nimbus technologies in Swanage.

They guy said when it jammed, it produced "The autumn leaf effect" whereby the tape would shoot up into the air and come down in tatters....


... oh and the other difference in output of a domestic video machine, is the modulator is DSB and not VSB.

 
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Re: VCR & Off air signal differences

Post by Red_to_Black » Fri May 09, 2014 12:33 pm

The V2000 system was very clever and effectively a quarter inch format too, as you could turn the tape over.

It was unfortunately a bit of a step too far technology wise and being over complex for the time and as you say a bit late to the party.

Part of the V2000 system was re-incarnated in the later Sony Video 8 system, but without the bendy heads :qq1
The Dynamic Track Following (DTF) was incorporated into the Video 8 system as ATF or Automatic Track Following by recording four pilot tones in the with the video tracks and by using clever heterodyning could automatically track without the use of a seperate control track.
This system also used FM sound recording (similar to Hi-Fi recording discussed earlier) from the very beginning.

There were later developments to this system too such as PCM audio, and Hi Band which was the sort of S-VHS equivalent, finally culminating in a wholly digital system.

 
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Re: VCR & Off air signal differences

Post by Michael Watterson » Fri May 09, 2014 12:53 pm

I'm very very fond of my Sony DCR-TRV330E Digital-8
It plays my earlier Analogue 8mm tapes fine (I got the Analogue Camcorder free).

The Analogue camera was one of 2 on a palette of Samsung VHS "returns" (it wasn't Samsung though [edit WRONG, it was]) I was repairing for a friend back in 1997 approx. The dead camera the boards had come loose. The other wouldn't focus, zoom or iris. I was "given" the bad one. :)
I found the alloy block with motors at lens had cut most of the tracks at each corner of flexible PCB/Cable. So with 500W of light, jeweller's monocle and large magnifier on stand I repaired the cable by scratching and tinning the track traces on thin plastic either side of cracks, super glue a very fine piece of tinned wire across crack.
At 3rd attempt and nearly a week I got it all working. Still works today though about 2minutes battery life!

[Edit had to check loft. Samsung MyCam K70 PAL 8mm analogue Video, not a Hi-8]

I bought a big load of 8mm tapes in a Bargain Euro Store at €2 each years ago (2002?). They seemed fine on the Analogue and also seem to work OK for Digital 8mm recording.

Hard to beat a camcorder with good zoom lens for birds in flight vs still camera. I have some lovely Heron footage.

 
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Re: VCR & Off air signal differences

Post by ntscuser » Sat May 10, 2014 5:02 pm

Red to black wrote:The V2000 system was very clever and effectively a quarter inch format too, as you could turn the tape over.


The problem with that was, since the recording time was doubled, retailers charged twice as much for the blank tapes relative to VHS and Betamax!

That killed it in my opinion, along with the necessity of turning the tape over.

Philips might have done better with a quarter inch tape recording in one direction only.

 
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Re: VCR & Off air signal differences

Post by Michael Watterson » Sat May 10, 2014 6:13 pm

The deliberate aim was to have a flip over cassette:
1) Familiar like audio compact cassette.
2) potentially no rewinding.

1" or 3/4" tape was deemed too big. So the idea of dynamic track following to shrink it to 1/4" per side. It wasn't blank tape cost that killed it. It was simply too late, so rare that video libraries had the format. By then Video rental was becoming as important as home taping. It would be some years in the future before studios realised direct retail sales at low price was a good strategy.

The Compact Cassette wiped out the slightly older 8 Track cartridge (Lear Jet?), very fast in Europe though compared to USA. Of course an 8 Track cartridge is an endless loop and can't rewind at all, so not comparable to VHS or Betamax or the earlier Japanese single reel Video cartridge (Stiff self lacing leader and takeup spool inside the machine, not cartridge. Pressing Eject fully rewound it first obviously).

Otherwise better with a single sided 1/2" tape.
They already had done the N1500 and N1700 long before VHS and Betamax. It used the two spools stacked, for those too young to remember, to have fat stubby cartridge.

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