Why did 405 look so good?

[ppppenguin]

I have a Prowest PM14/1A dual standard video monitor. This is a very high quality device. Both 625 and 405 pictures look excellent on it, though obviously the line structure is more visible on 405. There is a brightness change as you switch standards, necessitating adjusting the brightness control. I don't remember which standard is brighter. The EHT is stabilised and black level clamping is likely to be very accurate. So perhaps it is due to writing speed. I'd need to investigate further.

[Refugee]

I was thinking of writing speed way back.
I am pretty sure there was something in the sales blurb for CRT scopes about fast response phosphor. It would have been in the early 1980s or late 1970s or some time like that.

[Mark Hennessy]

Are my posts invisible?

[colly0410]

Interest comments & thanks for them..... I presume that most dual standard sets used mean level AGC on both 405 & 625, & when single standard 625 sets came on stream in 1969 that they used sync tip AGC & that would look better with a stable'ish black level than the DS sets. Also did DS sets use AC coupling on the video gubbins without bothering with DC restoration? Just some other random thoughts I've had..

[Murphyv310]

Hi.
Well many 405 line sets had no AGC whatsoever!!!! Contrary to common belief people weren't thin in the days gone by, by the lack of AGC and alleged constant contrast adjustments.
The Pye V4 and some of its successors had gated AGC and a proper black level clamp, flywheel sync etc so these refinements were not just for 625 and negative modulation.
There also were many 405 line sets with no DC restoration or even a jot of of DC coupling and the pictures never had the milky look on BBC2 prior to duplication.
The Philips 210 had proper line gated AGC on both systems, had a decent IF that would resolve 5.25Mhz and still had this phenomenon of milkiness.
David Boynes also agrees with me on this that for some time BBC2 was poor then all of a sudden it cleared. I too have heard many engineers complain of the same issues and it's sudden improvement, also in a previous post as to the reason cited by Henry Chmiel from British Relay........ I rest my case....... Perhaps someone that actually was involved with BBC engineering at the time could clarify the situation

[Geordie McBoyne]

I do remember in 1967 just a few months before the introduction of colour the picture quality of the 625 line picture dramatically improved. From the previous milky looking pictures we were watching on BBC2 the picture quality became absolutely superb only to deteriorate very slightly when colour was introduced.
Never had any complaints from customers about the quality of the 405 line pictures.
I have a 1960 Pye 405 line TV which has a particularly good 23" CRT. The lines really are noticeable on the American 23SP4 CRT. Some sort of spot wobble would be a good idea. In 1963 KB introduced a line eliminator circuit which elongated the spot in the vertical direction. An adjustable negative voltage was supplied to the focus electrode of the CRT. Might try this circuit in the Pye sometime.

Geordie McBoyne.

[Murphyv310]

I do remember in 1967 just a few months before the introduction of colour the picture quality of the 625 line picture dramatically improved. From the previous milky looking pictures we were watching on BBC2 the picture quality became absolutely superb only to deteriorate very slightly when colour was introduced.
Never had any complaints from customers about the quality of the 405 line pictures.
I have a 1960 Pye 405 line TV which has a particularly good 23" CRT. The lines really are noticeable on the American 23SP4 CRT.  Some sort of spot wobble would be a good idea.  In 1963 KB introduced a line eliminator circuit which elongated the spot in the vertical direction. An adjustable negative voltage was supplied to the focus electrode of the CRT.  Might try this circuit in the Pye sometime.

Geordie McBoyne.

Hi.
Certainly was later up here with no improvement seen till Black Hill duplicated.

[colly0410]

My random thoughts again... I'm wondering if some of the perceived deficiencies of 625 were because it was at UHF, & the signal wasn't as strong as 405 was on VHF, so the AGC would be working flat out. Would we have noticed such a big difference if 625 was at VHF like Ireland? IIRC mean level AGC on positive modulation would result in the highlights not being bright enough, the blacks not being dark enough & the screen being mid grey when when it should be all black. What effect did mean level AGC have on a negative modulation picture? Was it highlights too bright & blacks too dark? I just can't remember.... What effect did AC coupling have?

[Murphyv310]

Hi.
The effect of no DC coupling would be that "black" would appear to have scan in it so not black in the proper sense also the picture would always appear to have light in it so dark scenes would appear to be too bright.
Constant brightness correction would be required as the average video level would be rising and falling dependent on video content. Now this would have been seen on 405 line sets no matter if they were AGC controlled or not, the lack of a DC path from the detector to the CRT cathode is not the same as mean level AGC although it does give a similar effect. What can be done though it to add a DC restoration diode and this does go some way to make matters better and is a better solution to sets with mean level AGC.

I've mentioned the Philips 210 & Thorn 1400 that were indeed very good performing dual standard sets, other sets like the Pye 11U were no doubt the worst, many sound issues, limited bandwidth on 625 and unreliability made a lot of customers hate 625, there were others too that were pretty awful sets but like I say there were the good ones too, another good set was the Bush TV135 with the transistorised IF stage, these worked well until engineers not used to semiconductors got their mucky paws in them.

This debate has been discussed before on other fora and the overall thought is 405 displayed a good black and white picture with strong whites and good blacks (on well designed sets) and due to this customers were more than happy with 405. The use of Band 1 & 3 for 405 had a distinct advantage after duplication as the transmitter network was established, well over 90% of the country could receive at least one 405 line channel, 625/UHF though was way down in comparison at that time and many who could receive it needed a masthead amp and a set with a decent tuner. Many held onto 405 into the mid 70's if they lived in fringe areas and it was only the roll out of relay and fill in sites that the situation improved.

Undoubtedly we did have a problem here in the UK early on with picture quality on BBC2, some say it was resolved just prior to colour starting and certainly up here it was later, this reminds me of 1969, during the school holidays I worked a Wednesday and Saturday morning in the Co-op TV service dept in Irvine, I used to fir mains droppers, tidy the workshop and tune in new sets and test them prior to delivery, one reason why I walked into a job in January I think. Anyway the Co-op took delivery of some Alba single standard sets (Rebadged Philips 230 sets) and I remember the engineers comment "Who'd want to watch a picture like that, its a flat as a pancake" so we certainly did have an issue up here that mysteriously went away a few weeks later.

[colly0410]

Just spotted the closed thread on UKVRR site from 1910, I must have missed it as I can't remember it. .... I've got my late Mothers Elizabethan T12 B/W single standard portable she got free in 1971/72 that still works, the brightness/ black level was/still is all over the place so presume it has AC coupling..