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Old 22nd Sep 2016, 10:12 am   #21
Nicklyons2
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Default Re: When did synchronization to the mains end?

The 405 transmissions were only mains locked up-until the 405 line output was derived from the 625 feed. AFAIK the 625UHF service was always asynchronous (in the UK) in preparation for colour; because the subcarrier needed to be accurate to around 1 ppm (in practice often an order of magnitude better than that), there wasn't much prospect of syncing thousands of tons of spinning machinery with it.

Remember when the TV system was standardised, c.1935, crystal drives were complex and expensive - maybe OK at Alexandra Palace but difficult at outside broadcast locations. Further, 'mains lock' meant the equipment required less smoothing, because any hum was static and could be largely adjusted out; in an era when a 32uF electrolytic was big this was a big help.

This wasn't just a British trait; when the USA adopted colour they moved the frame rate to accomodate the subcarrier to 29.97 (59.94Hz).
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Old 22nd Sep 2016, 12:25 pm   #22
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Default Re: When did synchronization to the mains end?

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Originally Posted by Vintage Engr View Post
The more unscrupulous guys would often hard-wire the frame sync to the mains, taking a feed at the low end of the heater chain! It did work, after a fashion.
What I do remember, is a whole group of customer calling to say that they had frame-roll, - this was when the transmissions ceased to be locked to the mains. There were a few red faces, as these customers were mainly on the area covered by said 'engineer'.
I'm sure there was an article in a 1950s Practical Television magazine about the subject of locking the frame timebase to the mains.

DFWB.
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Old 22nd Sep 2016, 12:34 pm   #23
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Default Re: When did synchronization to the mains end?

In 'Lo-Def' days, the mirror-drum scanner at Portland Place was said to be crystal-controlled at 750 rpm, rather than by the London AC power supply. Most experimenters seemed to think otherwise, and Major Radiovision even offered an 8-pole sync. wheel 'for the London area', to keep receivers in step with the mains.
I suppose at most times, the difference would be within the range of adjustment at the 'viewing-end'.
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Old 22nd Sep 2016, 3:36 pm   #24
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Default Re: When did synchronization to the mains end?

So how was 405-line colour transmitted then?
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Old 22nd Sep 2016, 3:52 pm   #25
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Default Re: When did synchronization to the mains end?

There is lots of info on British NTSC on this web site:

http://www.earlytelevision.org/british_ntsc_color.html

It reproduces (only) the first page of an article from the December 1955 "Wireless World", indicating at the final paragraph that asynchronous transmission was indeed used, but details of the effects of asynchronous transmission are evidently to be found on a subsequent page.

The Marconi booklet describing their 1954 (closed circuit) experiments does, on page 33, envisage that synchronous operation might have been possible.

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Old 22nd Sep 2016, 4:04 pm   #26
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Default Re: When did synchronization to the mains end?

Another oddball chassis I was involved with at Baird TV was a version of the 660 for testing CRTs. It only had a line timebase, the frame was 50Hz from the mains. No idea who they were supplied to.
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Old 22nd Sep 2016, 4:07 pm   #27
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Default Re: When did synchronization to the mains end?

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There is lots of info on British NTSC on this web site:
http://www.earlytelevision.org/british_ntsc_color.html
Okay thanks!

It must have created problems for any B&W sets locked to the mains?

PAL was also transmitted on 405-lines as late as 1965.
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Old 22nd Sep 2016, 8:34 pm   #28
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Default Re: When did synchronization to the mains end?

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It must have created problems for any B&W sets locked to the mains?
I thought the ctv tests were done after normal programs shutdown, no 24 hour tv then. Also b/w sets would only give trouble with poor smoothing. I presume not many would be watching and those that did in b/w would need a faulty set to show the problem.
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Old 23rd Sep 2016, 1:27 am   #29
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Default Re: When did synchronization to the mains end?

The link "Autumn 1955 Colour Tests (405 Alive article)" on the early TV web site leads to a PDF of a questionnaire that was sent to the trade, asking for specific feedback about the experimental broadcasts, and specifying their transmission times.
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Old 23rd Sep 2016, 10:54 am   #30
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Default Re: When did synchronization to the mains end?

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So how was 405-line colour transmitted then?
Most likely not locked to the 50Hz mains supply. The master generator would have operated at 4X fsc = 10.631250Mhz. Division by 525 produces the twice line frequency of 20,250Hz to generate the frame sync broad pulses. Further division of 20250 by 405 (9 X 9 X 5) will produce the trigger for the frame sync, 50Hz. The 20250 is divided by 2 for the line sync.
The master oscillator output is divided by 4 to produce the correct colour sub-carrier frequency of 2,657,812Hz = 262.5 X 405.
So it follows the sub-carrier, line and frame TB frequencies are generated from a single stable frequency source. Whether or not it was referenced to the mains frequency I do not know.

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Old 23rd Sep 2016, 11:38 am   #31
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Default Re: When did synchronization to the mains end?

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Originally Posted by FERNSEH View Post
The master oscillator output is divided by 4 to produce the correct colour sub-carrier frequency of 2,657,812Hz = 262.5 X 405.
Correction: fsc = 262.5 X 10,125 = 2,657,812.5Hz.

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Old 23rd Sep 2016, 11:56 am   #32
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Default Re: When did synchronization to the mains end?

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Originally Posted by Nicklyons2 View Post
Remember when the TV system was standardised, c.1935, crystal drives were complex and expensive - maybe OK at Alexandra Palace but difficult at outside broadcast locations.
True, but for OB applications, there would also be practical difficulties of syncing to a mains supply frequency where there was no mains supply! Bearing in mind that the mains frequency can very slightly on a short-term basis, how was mains locking contrived in practice? From an off-air standard running on a private frequency? It wouldn't be possible to do a mains-synced live OB from a generator-equipped OB van when the only broadcast signal available is derived from that van!
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Old 23rd Sep 2016, 1:23 pm   #33
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Default Re: When did synchronization to the mains end?

In practice, even as late as the 1970s some SPGs had a mains lock position (as well as Xtal & 'genlock'). I was thinking mainly of the early years of TV OBs and I think you'll find most, though I can't guarantee all, were from locations in and around London where it wouldn't have been too difficult to persuade the LEB to provide a supply from the nearest feeder pillar. Another ploy used later was to phase lock an audio tone to the mains and divide it back down at the other end; in this way the reference could be sent by either radio link or phone line. Viewing of archive telerecordings will reveal there was much that wasn't locked; vision cuts accompanied by a rolling shutter bar as the telerecording camera re-synced and, occasionally rolling hum bars.
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Old 23rd Sep 2016, 3:03 pm   #34
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Default Re: When did synchronization to the mains end?

From the "Black Book": The Master Oscillator- Type 3a and 3b.

A valve V1 constitutes a sinusoidal oscillator generating a sine wave of a frequency of 20,250c/s. The oscillator is of the Hartley type.
The tuning capacitor shaft is coupled to a synchronous motor and therefore automatically controlled by the difference frequency generated by the Master Oscillator Mains hold unit.
The Master Oscillator Mains Hold Type 4a.
The function of the master Oscillator Mains hold apparatus is to generate in 3-phase form a frequency whose value in cycles per second is equal to the difference, if any between the master frame frequency and the frequency of the mains. This 3-phase frequency will then be supplied to the synchronous motor in the Master Oscillator, and will rotate it, thereby equalising the mains and frame frequencies
.

So, from this notes gleaned from the Black Book, in 1936 it was not an easy matter to control the frequency of the master oscillator.

DFWB.

Last edited by FERNSEH; 23rd Sep 2016 at 3:23 pm.
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Old 23rd Sep 2016, 3:47 pm   #35
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Default Re: When did synchronization to the mains end?

Sounds very much like a frequency-locked loop, it's not even a phase-locked loop (such as in TV flywheel sync).

It means that while the TV frame frequency will on average be equal to the mains frequency, they're not actually synchronised, cycle-by-cycle. The mains frequency could change, in which case the motor starts turning and alters the frequency of the master oscillator. The two frequencies become equal again only as time goes to infinity.
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Old 23rd Sep 2016, 9:11 pm   #36
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Default Re: When did synchronization to the mains end?

Wasn't the need to remove mains lock from 405 when 625 started (1964) due to the lack of field-store standards converters? Most programs were originated on 625 and down-converted (on 3rd floor TVC line store converters) for BBC1 transmission. The field-store converters were somewhat later, 1968, just in time for the Mexican Olympic Games.
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Old 23rd Sep 2016, 9:12 pm   #37
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Default Re: When did synchronization to the mains end?

Because the master oscillator was locked to the mains frequency which can vary slightly within defined limits the precise 2H figure of 20250c/s will seldom be achieved. But that shouldn't matter as long as the correct divider chain figures of 9 X 9 X 5 can be maintained the correct number of lines and interlace will be displayed on the CRT. For example, if the mains frequency falls to 49.5c/s the line timebase frequency will be 10,023.75c/s 405 interlaced lines will still appear on the screen.

DFWB.
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Old 23rd Sep 2016, 9:26 pm   #38
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Default Re: When did synchronization to the mains end?

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The field-store converters were somewhat later, 1968, just in time for the Mexican Olympic Games.
From what I remember those pictures from Mexico were excellent.
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Old 24th Sep 2016, 1:33 am   #39
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Default Re: When did synchronization to the mains end?

Indeed the pictures were. The original 1963 design 625 to 405 converter was full of germanium transistors, very clever piece of equipment, much better than any optical converter. The 1968 colour 525/60 to 625/50 field store converter was for the time it was designed a really marvellous engineering achievement, nothing like it anywhere else. Unlikely there was any integrated circuits in it, most certainly silicon transistors, maybe glass delay lines but no programmable devices.

DFWB.

Last edited by FERNSEH; 24th Sep 2016 at 1:42 am.
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Old 24th Sep 2016, 10:21 am   #40
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Default Re: When did synchronization to the mains end?

Picture of the Mexico Olympics 525 to 625 converter, said to contain more tan 5000 transistors.

DFWB.
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