Why did 405 look so good?

[Mark Hennessy]

Quite simply, I'd like to think that it debunks the write-speed idea...

But if anyone wants to hang on to the idea, then I suppose the phosphors themselves might be an unknown quantity here

Of course, adjusting the timebase while remaining triggered to a TV line will alter the brightness because the duty cycle changes. It won't be all that noticeable. But in the interests of eliminating variables, I was assuming the timebase was free-running in BBL mode, where the hold-off time is the only thing that affects duty cycle.

Set a CRT VGA monitor to 640x480 and marvel at how sharp it looks. It's what was said in post #3. Simples...

[Murphyv310]

Yes it is a different conundrum Jeffrey. As you have a mains derived prewar set it will easy to see what I'm inferring when adjusting the line speed, disabling the frame timebase will make it more noticeable, you don't need a lot of brightness as it's not advisable with a frame collapse anyway.
I may just have a go with the 1946 Ekco and do a YouTube clip.

[Murphyv310]

The reference in post three was that all dual standard sets were substandard. That is so far from the truth. What about the Philips 210, the Thorn 1400 and others, once the "Broadcasters" sorted out the issues they quite clearly had the 625 line pictures were excellent. Even the pictures on colour dual standard sets like the Gec 2028 BRC 3000 massively improved after November 1969.

[Murphyv310]

I've tried to edit my last post with no luck.
I should have said Many dual standard sets were excellent, not all had valve tuners and indifferent IF alignment.

[Mark Hennessy]

But before drawing any firm conclusions from this, double-check that the hold-off period is the same at both settings. On the Telequipment DM63 - using that as an example because I know it well - there are 3 discrete values of hold-off timing capacitance (and no change in resistance) as you go around the timebase switch. As a result, there are noticeable steps changes in brightness between certain adjacent settings because a different capacitor has been switched in - nothing to do with write speed, just duty cycle. Easy to check with a second 'scope connected to the "Gate" output on the front panel.

Just checked mine - the second change in hold-off time happens between 10 and 20us. The two values of hold-off are 37us and 2.8us, which leads to a duty cycle change of 85% to 97% as you turn from 10 to 20us/DIV. The change in brightness is actually more subtle than I'd remembered it. As you speed the timebase up, the duty cycle falls to 44% at the fastest setting (200ns/DIV - it's only a 20MHz 'scope), and that obviously results in a dimmer trace.

As mentioned, the 'scope is free-running in BBL mode. When triggered to a fixed-frequency signal, then the duty cycle of the trace will be a product of the timebase speed and the trigger event frequency. As a TV shows "full" lines rather than just the first part of them, what I've done makes most sense.

So, a bit tangential, but very easy to measure with the Rigol, so why not? Either way, having looked into this, I'd like to think that the write speed angle is not a big issue in the difference between 405 and 625. Not unless there's something special about the phosphors used (but a dual-standard set would be the way to test for that).

[Murphyv310]

Don't forget that many 110 degree CRTs started life in 405 line only sets and progressed right through the dual standard era and other than getting square corners and rimband essentially they were identical. The A50-120WR of the very late 60's being the last incarnation of large screen mono tubes along with its 24 inch sibling

[Murphyv310]

TBH, i don't think scan speed has any real meaning for the milky pictures on 625. The French 819 system was constrained by its wideband IFs and it did look flat especially in cut down later sets. It was positive modulation like 405 was.
Incidentally why was there never a problem with the French dual standard sets, allegedly their 625 line pictures were far better than ours.

[Terry]

Don't forget that many 110 degree CRTs started life in 405 line only sets ...

Surely more 110° tubes were used in 625 line only sets abroad than were used in UK 405 line sets ?

[colly0410]

.
Incidentally why was there never a problem with the French dual standard sets, allegedly their 625 line pictures were far better than ours.

I'm wondering if it's because in French dual standard sets both their 819 & 625 systems used positive modulation, where as British dual standard sets had to cope with both positive & negative modulation. Just a thought..

[Synchrodyne]

Another thought – if one could provide 405-line and 625-line video feeds (to a dual-standard monochrome monitor with say a normal (23 inch) screen size) in which just one active line was at or near peak white (but anyway the same for each case) and all others were blanked, would the single 405-line appear to be brighter than the single 625-line?

If so, and let’s say for argument’s sake that the single 625 line was roundly two thirds as bright as the single 405 line, then repeat the experiment with two consecutive 405 lines at peak white, the rest blanked, as compared with three consecutive 625 lines (as near as possible on the same place on the screen) at peak white, the rest blanked.  From a reasonable viewing distance, would the 405 and 625 line sets look to be of similar brightness?  And so on in a few coarse steps up to a full raster.

Again if the individual 405 lines are brighter than the individual 625 lines, does this exacerbate the lininess issue?  One might expect that a brighter line will make an adjacent “black” space more apparent than a less bright line.  If that holds, then the perceived lininess would appear to increase in a non-linear manner (maybe approximately second order) as the number of lines is progressively decreased below the threshold minimum number required to produce a reasonably flat field.  Fewer lines means brighter lines and wider black spaces, both of which would contribute to increased perceived lininess.

Lininess, with bright individual lines abutting visible black spaces, might also contribute to an  impression of greater contrast than for a picture with the same overall contrast but with minimal lininess.

And yet another thought:  to that same monitor, let’s apply switchable spot wobble to the 405 scan, such that with spot wobble switched in, the 405 field has about the same flatness as the 625 field.  Then repeat the above experiment.  What would be the expected outcomes?

I have also pondered the situation where the transmitted signal had “wrong” black and peak white levels.  Then let’s assume a good receiving system with high-gain black-level agc, a contrast control as a gain control in the video amplifier, and black level clamping.  So the black level at the tube should be correct, and could be adjusted in the normal way with the brightness control.  Incorrect white level would show up as deviation of the peak white video signal amplitude from its nominal value.  “Low” white level could be offset by turning up the contrast, assuming that there was enough spare video gain.  So the transmission errors would not appear to be fundamentally inimical to obtaining a “full contrast” picture, at least with a properly specified receiving system.  But with domestic receivers built to a lesser standard, perhaps there would be problems.  And if, because of transmission errors, 625 typically required a higher video gain setting than 405, how many users adjusted the contrast control accordingly when changing from one to the other?  (But then, back in the monochrome days, how often did one see any domestic receiver with properly adjusted brightness and contrast controls?  (Even though in those days there were plenty of test card transmission hours when it could be done quite easily.)

The French case is interesting.  Certainly the choice of positive/AM for its 625-line system L was made to align with system E, with a view to simplifying the design of dual-standard receivers.  I don’t think that made any difference to the performance achievable with full specification receivers, but it might have made a given performance level easier and less costly to reach.  Bear in mind that the French setmakers had several years experience with “Strasbourg” and other border-area multi-standard receivers that had to deal with both positive/AM and negative/FM systems.  (And there were one or two models that also covered System A.)  And of course four-standard receivers (B, C, E and F) had been the norm in Belgium since 1953.  Then with the arrival of French UHF transmissions, these moved to five-standard (B/G/H, C, E, F and L).  It is reasonable to assume that the vast majority of the basic problems associated with valve-based multi-standard domestic receivers had been well-addressed by the European setmakers by the time that the UK requirement for dual-standard receivers arrived.  To what extent those lessons were known or applied by the UK setmakers is another question, though.

Perhaps one advantage in the French case is that their wideband IF strips designed for System E would have provided more than enough gain for System L.  I understand that in the 819-line only days, video bandwidth was a parameter that was often quoted for domestic receivers, with 9 MHz and 10 MHz being common numbers.  In the dual-standard era, there is some evidence that there were two basic approaches, one which used the same bandwidth (6 MHz or near to it) for both systems, and the other with 6 MHz for 625 and 9 or 10 MHz for 819.

My impression is that the UK setmakers generally settled for 625 video bandwidths that were on the low side.  Pye’s prototype dual-standard receiver (with 38.9 MHz IF) had a 4.25 MHz bandwidth on 625.  I suspect that other makers also chose a basic dual-Nyquist IF curve (with additional traps for 405) which would have allowed only a 4.85 MHz bandwidth with the 39.5 MHz IF.  On the other hand, full (3 MHz) bandwidth for 405 would have been readily obtainable.  So such receivers did a much better job on 405 than they did on 625.

That brings in the question as to what, if any effect did horizontal definition have on the perception of picture contrast.  Lower-than-desirable video bandwidth would result in somewhat smeared horizontal transitions, without say visibly sharp black to white changes.  Would the existence of sharp changes create the impression of greater contrast?  If so, then 625 was at a perceptual disadvantage as compared with 405 on some/many UK receivers.


Cheers,

Steve