Origins of the 405 line system.

[Murphyv310]

I've an old Sony 2mp camera with sports mode Michael that might work, I'll try tomorrow.

[Geordie McBoyne]

The original 405 line system had 385 active lines. Interestingly, the 441 line system in use in France until 1956 had the same number of active lines, the frame blanking period occupied many more lines than our 405 line system.
Unlike the 405 line system where the frame blanking period started at the commencement of the eight broad frame pulse chain, in the 441 line system six lines were blanked out before the start of the frame broad pulses. Both systems had eight broad frame pulses. With 405 lines max video bandwidth need only be 2.7Mhz, max 3Mhz, whereas in the 441 line system max video bandwidth has to be almost 3.5Mhz.
From the "Black Book" The 1939 frame divider chain was designed to replace earlier apparatus. The new circuit employed three blocking oscillators which functioned as two divide by nine and one divide by five counters. Valves used for each counter were a D63 double diode and a KTZ63 pentode. The 20,250Hz pulse chain from the master oscillator was supplied to the first blocking oscillator, nine pulses were counted and the output pulse supplied to next divide by nine blocking oscillator. The divide by 81 signal is finally supplied to the divide by five oscillator. The output from the divider chain is amplified by a great big N34 pentode. This is the trigger pulse to initiate the start of the eight broad pulse chain.

The American 343 system did indeed have a line frequency which was close to 405 lines. 10290 for 343/60. 10125 for 405/50.
343 = 7 X 7 X 7.

Geordie McBoyne.

[Mike Watterson]

Presumably the French thought the larger blanking interval was "needed". I remember visible blanking interval lines on some old 1950s 405 sets I "restored" (well got working!) in 1970s. I don't remember seeing it on any dual standard sets or 625 sets. A smaller flyback time on line or frame means higher voltages and more losses?

[Geordie McBoyne]

Actually it wasn't a French television system but in fact it was the apparatus was installed by the Germans during the occupation of Paris. Original pre-war French transmitter, 30KWatts, the most powerful television transmitter in Europe. AP was 17Kwatts. 405 lines frame blanking period is 1.4milliseconds and my calcs come out with 2.26milliseconds for 441 lines.

Geordie McBoyne.

[ppppenguin]

Flyback time with a CRT will always be a compromise. The pioneers had to find out what worked by a mixture of theory and experiment. Electrostatic CRTs can do flyback very swiftly without expending much power. Magnetic deflection can't. There's that old V = 0.5 * L^2 at work to stop you. As for why magnetic deflection became universal that's easy to answer. If you want wider deflection angles and hence shorter CRTs then the deflection plate geometry becomes impossible very quickly. The beam just hits the plates. Perhaps even more important is when you increase the EHT to get higher brightness the ES deflection voltage increases linearly with EHT while the current for EM scanning only goes up as the square root of EHT. For small 'scope tubes they developed PDA (Post Deflection Acceleration) but I doubt that was feasible for big CRTs.

Coming right up to date with the assortment of HD standards the 1.5GHz serial digital interface can carry about 15 different formats. From 1080/60i and 1080/30p right down to to 720/24p. The last of these has H blanking several times longer than active line.

[beamcurrent]

All this talk of division ratios, reminded me of RCA solution for a portable SPG in 1939.

http://www.tvcameramuseum.org/rca/1939fcam/1939fp4.html

BTW I have a valve 405L SPG by EMI c. 1960 with 5x3x3x9 = 405

[Synchrodyne]

and USA changed to 525 to get closer to 16mm film quality I think, they decided 343 wasn't good enough.

The reasons behind the NTSC’s choice of 525 lines in 1941 is well documented in the book “Television Standards and Practice”, edited by Donald G. Fink, and published by McGraw-Hill in 1943.  This is available on-line (also in .pdf format) at:  https://archive.org/stream/televisionsta...h_djvu.txt.  See page 218ff.

 
NTSC had initially chosen 441 lines, but took a second look.  The justification for 525 lines – accepted by the committee – was written by Fink himself.  (In)visibility of the line structure and flatness of field were key factors considered in the choice.  One may infer that within reason, that adequately addressing these items took precedence over horizontal definition, given that NTSC was confined to working with a 6 MHz channel with 4 MHz vision bandwidth.
 
This book is I think essential reading for anyone interested the history of analogue TV transmission standards.  Also in that category, also edited by Fink, is “Color Television Standards”, McGraw-Hill 1955, with LCC #55-5684.  (I haven’t found it on-line, but then I haven’t searched too hard for it.)
 
Much, much later, Fink was on record as saying that the 6 MHz was too narrow, and that 8 MHz would have been ideal.
 
 
Cheers,
 
Steve

[ppppenguin]

All this talk of division ratios, reminded me of RCA solution for a portable SPG in 1939.

http://www.tvcameramuseum.org/rca/1939fcam/1939fp4.html

The Hammond Organ method

ISTR reading that a mechanical SPG was used at Ally Pally at one point. Is this true?

[colly0410]

Than you Synchrodyne for that link, I'll have a read through it..

[Gryphon]

ISTR reading that a mechanical SPG was used at Ally Pally at one point. Is this true?

Depends on what you mean by "mechanical SPG".

AAUI, there was no equivalent of a central SPG in the Baird system, as the sources made their own syncs, so in this sense each source had its own mechanical SPG.  As an aside, this has interesting implications for the idea of having co-timed sources at a mixing point, and therefore here is another reason why the Baird system was a dead-end.

The M-EMI SPG is in Birkenshaw and is electronic.  However, I believe that, at one time, mains lock was achieved by comparing a pulse at frame rate with local mains and using the integrated error to drive a d.c. servomotor whose shaft was ganged to a component controlling the frequency of the master oscillator - so in this sense, there can be said to have been a mechanical element to the SPG.