05-04-2017, 08:07 AM
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.
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.
