Hedghog 625-405 standards converter

[Murphyv310]

Hi.
I had a look at the waveforms last night and I have a question??
Before I had a couple of problems with the aurora giving a little bit of foldover on the left hand side of the picture. Now this was on two converters and on both the stored image and with a video input. It wasn't seen on some sets and I thought it was most likely that the timing was on the edge on two converters.
The effect is not seen on the RT Russell nor on the Imogen or on the hedgehog.
I've asked about this before both on the UKVRRR & VRAT, if I get time I'll get a couple of photos off the scope from the Hedgehog and the Aurora that is the worst and see if anyone can come up with a definitive answer. I'm aware that my TV22 which has been modded hasn't got perfect flyback time and it did have foldover issues, adding flywheel sync and altering the phasing has made a cure but this is a different issue.

[`FRANK.C]

Hi Trevor
It would be good to see the scope photos of the line blanking periods of the affected and unaffected ones, although if the problem was to do with too short of line blanking I would expect Hedghog to show the problem too as its line blanking is particularly short in 5:4 mode.

Frank

[Murphyv310]

Hi Frank.
Yes there is considerable foldover on 5:4. Hopefully get some pictures this afternoon.
I'm trying to talk Anne into going to Galway this year, obviously to see you as well, will keep you posted.

[ppppenguin]

A few years ago I remember looking at the H parameters* of the 405 signal and how they varied. I can't find my notes at present. The official spec has varied over time. ISTR there was some variation between different versions of the Aurora. All the variations were fairly minor as I recall. If the flyback time of a receiver is close to the H blanking interval then H phasing will be very critical if foldover is to be avoided.

As a designer, like Frank or Darryl (or myself), you are free to vary the H (and other) parameters if you wish. On the whole, as designers we like to stick to published specs as far as possible, unless they are known to give trouble. We are also human and sometimes get things wrong.

*Front porch, sync width, back porch. Together these add up to the H blanking interval.

[`FRANK.C]

Hi Trevor
It would be great to see ye this year.

As far as I understand the 405 timings are given as a range as opposed to exact figures, the range is quite wide and has changed over time. So there is quite a large variation in acceptable figures.

Frank

[Murphyv310]

Absolutely agree.
Snag is that some sets are on the "edge" and this problem can show up.
The worst set was the Murphy V659 and eventually after two line output transformers the 6/30L2 line multivibrator cathode resistor was found to be 10% high, just at the limit of spec, replacing this tidied up the drive waveform and reduced the foldover, now I only see half the castleations on left being distorted on the worst aurora. Its not a big problem by any means, took long enough though to find the issue on this particular set and overlooked as the component was just in spec.

When I looked before at this there was only marginal differences between individual waveforms, no doubt enough though to be seen on some sets.

[Terry]

Alan Pemberton gives the following figures:

Line blanking period 18.5µs
Front porch 1.7µs
Line sync pulse width 9 ±1µs
Time reference point to line blanking end 16.8 ±0.5µs

All measured between the 50% points. I think we can take these as being the parameters in use up until the end of 405-line transmissions but he does add the following footnote:

The c.1950 diagrams give the line blanking period of the 405-line standard as 16.5 ±0.5µs, rather than 18.5µs. Blumlein defines it as 1/200 + 1/10 + 1/20 of a line period = 0.494 + 9.88 + 4.94 = 15.314µs.

It might be interesting to compare the actual timings of the various generators and converters to see how they compare with these figures.

[`FRANK.C]

Hi Trevor
Well done on finding that resistor not an easy one when just in spec.

The H blanking intervals in the Hedghog are 18.52 uS on 4:3 and 15.87uS on 5:4
I used This BBC document as a reference for 4:3.
The 5:4 timings are based loosely on the Marconi 405 line system drawings on Peter Scott's web page. I thought this might more fitting for the older 5:4 standard. I expected though that the shorter blanking interval may cause problems on some sets.
Photos below of of both 4:3 and 5:4 H blanking intervals at 2uS/div. also a photo of 5:4 on my Ekco TSC91.

Frank

[Murphyv310]

Hi
I'll power up the Ekco TSC30A tomorrow and do some screen shots.

[Synchrodyne]

The following attachments document some of the changes in the 405-line waveform standard.

First, Wireless World (WW) 1961 April showed the original 1935 specification:

   

Second, WW 1948 December showed the waveform as it then was:

   

Third, WW 1950 March recorded the forthcoming aspect ratio change:

   

Fourth, WW 1952 June showed the waveform as promulgated by the CCIR:

   

Fifth, WW 1956 January recorded changes since 1952:

   

Sixth, the waveform as recorded by Hutson in “Television Receiver Theory, Part 1” of 1966:

           

Additionally, from WW 1948 April, a commentary about the then-new Test Card C as it applied to the 5:4 aspect ratio:

           



Cheers,

Steve