Behind the scenes UK experimental colour transmission 1957

[colorfixer]

It's more than power supply filtering. Watching 50Hz video under 60Hz non incandescent lighting can be bothersome due to differing flicker rates. In my office, when I watch a 50Hz/Pal feed, the screen noticeably flickers when I light the room with fluorescent lighting. With good ol' inefficient incandescent lighting, it isn't nearly as bad. I'd suspect it's probably not as bad the other way around.

I won't even touch the fact that to some 60Hz viewers, a 50Hz video source flickers no matter what, until they eventually become used to it.

[ChrisW6ATV]

Originally Posted by colorfixer

I won't even touch the fact that to some 60Hz viewers, a 50Hz video source flickers no matter what, until they eventually become used to it.

That drove me nuts when I visited the UK and Ireland in 2000. I ended up "watching" Teletext most of the time the TV was on. Now, THAT was cool.

[NewVista]

Originally Posted by colorfixer

It's more than power supply filtering. Watching 50Hz video under 60Hz non incandescent lighting can be bothersome due to differing flicker rates. In my office, when I watch a 50Hz/Pal feed, the screen noticeably flickers when I light the room with fluorescent lighting. .

My objections to these 'reasons', that you hear now and then, for 50hz countries using 25fps TV are as follows:

1. TV's (even old ones) shouldn't have AC power artifacts. If you see this, I suggest you book the set in for repair.

2. When GB designed 405/50, fluorescent lighting wasn't available*. So it's a non-sequitur to claim "50 hz was chosen to sync flicker with fluorescent lights".

3. Some countries happily enjoy 50hz pwr & 60hz TV (Chile, Japan).

4. Why is there no 12hz beat with my 72hz computer & Fluoro lights ?



*not available commercially until 1939

[ppppenguin]

Originally Posted by NewVista

1. TV's (even old ones) shouldn't have AC power artifacts. If you see this, I suggest you book the set in for repair.

Keeping hum out of the entire broadcast chain wasn't easy. Hence it's a natural choice to lock field to mains.

I have gone back to the source here. Blumlein et al, The MArconi-EMI tlevision System - The Transmitted Waveform, Journal IEE 1938 p761

"Similarly with interlaced scanning the spot traverses the frame 50 times per sec, thus raising the flicker above that perceptible to the human eye......A frame frequency of the order of 50 was necessary to overcome flicker and the exact value of 50 was chosen on account of the supply frequency of this country...."

I can't be bothered to retype any more of the article but but it goes on to say that hum effects at 37.5Hz were intolerable.

Clearly at the time the decision to use 50Hz was taken it was regarded as essentially flicker free. Especially as the competition was using 25Hz. the question was could they save bandwidth by going lower than 50Hz and the answer was clearly no.

[NewVista]

Originally Posted by ppppenguin

Keeping hum out of the entire broadcast chain wasn't easy. Hence it's a natural choice to lock field to mains.

.

I can see now why they were compelled to do that in those days

[tubesrule]

Originally Posted by NewVista

My objections to these 'reasons', that you hear now and then, for 50hz countries using 25fps TV are as follows:

1. TV's (even old ones) shouldn't have AC power artifacts. If you see this, I suggest you book the set in for repair.

This is not so straightforward as Jeff and the Blumlein article have pointed out. Firstly, removing ripple from the power supply to an adequate level is VERY difficult to do cost effectively using 1930/40's technology. The required amount of iron and capacitance would make the set very heavy and very expensive. The power supply on the 1936 RCA RR359 prototype sets weighs almost 100lbs by itself due to the size and number of chokes.

The problem is compounded by interlace scanning. Only a very small amount of magnetic interference is required to shift a line enough to ruin the interlace. Just having a power transformer in the set will create enough magnetic interference to show ripple in the image. Magnetically shielded transformers could be used but again are very expensive and large.

Case in point, I have a Sony 9-90UB solid state 405 line set from the 1970's and when I run it here on 60Hz power, feeding it with a 405/50i signal, the picture shows pronounced ripple and line bunching. Even when powered from a battery to eliminate the internal transformer, there is still some noticeable ripple in the image depending on where the set is placed due to stray magnetic fields at 60Hz.

Edit: I see Steve got in there while I was typing This is why the power supply on the RR359 is so big and heavy. They have large chokes and capacitors in the mains derived EHT to try to filter out the ripple. Hardly something that would be practical on a commercial set.

[andy]

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[ppppenguin]

2 video monitors side by side, one locked, the other without a signal. I've seen intereference between them on many occasions. Not with top quality kit but not everything is top quality.