Why did 405 look so good?

[Terry]

That improved when aluminised screens were introduced. No more ion burn and 50% more light output. But that happened before 625 line transmissions started in the UK, so your memory has jumped forward in time a few years.

Some tubes also had tinted screens - that would make a difference, too.

[ppppenguin]

Nowadays even the deafest old UHF tuners can be given enough signal for them to perform well. Unlike the old days where you relied on whatver came down the aerial lead.

[Murphyv310]

As far as I can remember the 405 tv's were not so good in daylight, especially with direct sunlight on them, but they were excellent at night.
Mike

That's down to the tube Mike. Later ones had a much darker screen so were less effected by daylight.

[Synchrodyne]

Surely the write speed to the tube face was faster with 625 so there was less time for the phosphor to take the electron beam current and light up. The difference in write speed is close to 50%.
Try it with a CRT scope and you will see an extreme example as you change the time base speed. You need to turn the brightness up.

Let’s say that the respective 405 and 625 line scans take about the same time for a complete frame, assume a constant video level somewhere towards peak white, and that the energy available from the electron beam is thus more-or-less fixed on a per time basis.  Then the energy delivered to the screen phosphor per frame would I think be more-or-less the same for both cases.  Would there then be a difference in perceived screen brightness?  I suspect not much if any.  Or is there a flaw in this argument?

 
 
Cheers,
 
Steve

[Synchrodyne]

Apart from poor UHF performance of some dual standard sets there's a psychological or perceptional reason. If you can see the lines sharply defined you tend to think the picture is sharp. Even if it isn't.

In the 1960s, most of the overseas TV programmes that came into NZ were, as I understand it, on film.  Most of those were probably originated on film, but allegedly some of the British programmes were film transfers from 405-line videotape.  I imagine that the transfers were done with spot wobble to ensure flatness-of-field and to avoid lininess, but the net result was visibly poor vertical definition.  This evidently applied to one popular Granada-origin serial-type programme, and its poor picture quality attracted adverse attention, with the above explanation (405 videotape-to-film transfer) for it being published in one of the daily newspapers.
 
That I think would be consistent with the notion that lininess disguised the relatively poor vertical definition.  And maybe that’s why spot-wobble did not become mainstream – it got rid of the lininess but showed the poor vertical definition.
 
I don’t think that many American export programmes were transfers from videotape to film, but a few were, and with those the black-fringing propensity of the image orthicon camera tubes that were widely used back then.
 
Also, the British and American TV programmes had differences in their soundtracks, as well.  In the American case, recording levels were high enough for there to be audible peak clipping now and again.  In the British case, peak clipping was avoided but hiss was more evident.
 
 
Cheers,
 
Steve

[Hartley118]

Nowadays even the deafest old UHF tuners can be given enough signal for them to perform well. Unlike the old days where you relied on whatver came down the aerial lead.

My recollection of our first dual standard set around 1965 was that it struggled to produce enough video level on 625, so it compared badly with 405. It was a decent 23-inch Murphy, but it's valve UHF tuner was short of gain.  I recall dodging up a little grounded base transistor UHF preamp which certainly helped contrast considerably, suggesting that the problem was simply front end gain.

I had the impression that it wasn't until the UHF-only sets of the late 1960s that designs were properly sorted.

Martin

Martin.

[Murphyv310]

Hi.
Some interesting comments here.
Steve I've read your reply with interest. Although the frame rate between 405 & 625 are the same the "writing" speed between the two standards are significantly different. Refugee has indeed a very good point with the scope and the very same can be reproduced on a TV screen so long as the line scan and EHT are not linked as varying line speeds effect EHT and thus brightness. If you observe a frame collapse thus one line on lower line speeds the result is brighter and on fast line rates the line is duller. This has to do with the writing speed, the phosphors have less time to "charge" on a faster line speed with a obviously faster drawing time.
As I previously remarked the "overall" brightness between 405 & 625 will be little different due to "perceived" brightness, although the writing speed is faster on 625 more of the phosphors will be"activated" and the eye will adjust to this even though each line will be slightly less bright.
I recently made up a little board with video amp, inverted, syncs etc to use on the scope in Z mod, It will resolve 180 lines to 819 lines, now Refugees comment would mean that brightness would be poor on 819 but bright on 180, in practice there is only a marginal difference as on 819 as we are using considerably more phosphors than on 180.

The comment about valve tuners on UHF doesn't stand up. by the mid 60's many sets had transistorised tuners but still we had the milky pictures on 625, In fact before I started working we had a British Relay set and BBC 2 was always poor with no depth of contrast and no punch, like I say come duplication all of a sudden 625 livened up giving a big improvement.
I also remember my Philips T-Vette I had and it too did the same just after duplication with 625 very much giving 405 a run for it's money and it had only one valve..... the EHT rectifier.
OK Valve tuners were pretty crap but I can assure you they were not the only reason for UHF's ills.

[Murphyv310]

Hi.
One other thing I have just remembered was a discussion I had with Henry Chmiel (British Relay area Engineer) around 1971-2. I remember asking him about the poor BBC2 pictures before we duplicated and his answer (rather long winded if I remember) was that it was down to the vision level on 625. The peak white was only 80% that of the VIT signal and black level was around 5% above what it should have been.
In the 60's British Relay used a UHF all valve receiver in the stations (MR754) and this initially was thought to have been the problem. The designers in Croft Street along with Murphy came up with a fully transistorised receiver with synchronous detection PLL & AFC the MR756 and the result was the same. Later on after the 405 line receivers were ditched after duplication all stations used the all valve receiver in the BBC2 position and the only issue's with them was a slight increase of noise and reduced top end response with the 2T pulse being about 7% down.

[Mark Hennessy]

Let's think about what we need to test this hypothesis:

For 625, the active line period is 52us, so set your scope to 5us/div

For 405, the active line period is 80.3us, so set your scope to 10us/div

Not perfect, but enough to make the point. It's just one click of the timebase switch.

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.

It's also instructive to play around with a CRT PC monitor if you still have one. I've retained my 17" ADI multiscan unit, which covers a massive range of H and V frequencies, and has a wonderfully sharp Sony CRT. You can display just about anything you like on this - though I doubt it'll go down to 10kHz - and simply switching between 640x480 and 1280x1024 goes some way towards answering the question. Or keep the resolution the same but alter the frame rate... It also is a useful tool for assessing the quality of your graphics card - back in the 1990s when I bought it, a lot of "generic" graphics cards had significant HF rolloff.

[ppppenguin]

Having used scopes to view video (mainly 625) on 20us/div, 10us/div and 5us/div over many years and more times than I've likely had hot dinners I can't say I've noticed any difference in trace brightness between those settings. There may have been differences which I didn't notice but there's certainly nothing gross. Similarly viewing whole fields of video at 2ms/div isn't very different in brightness either. The time things get dim are when the duty cycle goes down, such as strobing a single line out of 625.

Specialist TV waveform monitors usually automatically increase the brightness when strobing out single lines. Even then it's still nowhere near as bright as viewing all the lines.

But this is a very different question to the 405/625 conundrum.