The future: DC mains

[David]

Thanks to all for an interesting and educational thread!

Cheers

Dave R

[David]

"Even so, in Cambridge recently, the City Council was refused permission by the DSO (Distribution Service Operator) to install EV chargers, because the AC-DC converters were causing too much harmonic distortion"

Say what?

All home computers (AFAIK) and also data centres run on DC.

The railways? Trams? Trolley busses?

What is so special about some battery chargers in Cambridge that only they (and not the diverse nation wide infrastructure of AC/DC conversion) causes too much harmonic distortion?

Cheers

Dave R

[The Natural Philosopher]

Probably utterly crap design. Just about anything Cambridge does these days is utter crap

[Joe]

Half-wave rectification?

I used to know someone who had worked for one of the big TV rental companies when they were a thing. This company used a very large number of one model of TV, and were contacted and asked to do something about it. Half-wave rectification, not much power but in such numbers as to affect the general mains waveshape in some areas.

[Andy Bennet]

The frame rate was always locked to the mains - reason being that TV power supplies in the valve days were not that good and any mains ripple would show up as a modulation of the tube brightness. Hence why we had a

50 Hz interlaced frame rate and the USA had 60Hz.

[John Walliker]

They had certainly stopped locking to the mains by the early 1970s, probably much earlier.

John

[Ian Jackson]

Didn't they have to do this because of the complexity of the frequencies involved when transmitting colour?

[Andy Bennet]

I think that only applied to the 'new' 625 line system in the mid 60's to get ready for the PAL colour system. I believe mains locking carried on for the 405 system until it's death.

[Joe]

Possibly so. I was there in the mid-Seventies and as part of the training, was shown the atomic clocks in their rack. By that time, 405 output was standards-converted from the 625 line original signal, and could therefore be synchronised to anything.

I do remember seeing hum bars creeping slowly up the screen when I was younger. Presumably the picture was not then locked to mains.

[Joe]

Not so much that, as that colour was recovered in the receiver using a crystal, and there's only so far they can be pulled. That's a lot less than the mains varied by. My son-in-law recently mentioned to me that he had been keeping an eye on mains frequency (for some reason) and had seen it as low as 49Hz. I don't think that used to happen.

Before the days of (relatively) cheap synchronisers, i.e. when they were still several six-foot racks of electronics, it was necessary to pull distant TV studios into synchronisation with TV Centre by rather arcane means. The remote station would use a master oscillator which could be pulled far more than any crystal, and a comparator at TC would generate audio tones according to the difference. These tones were sent by telephone line to the remote station, where they pulled the oscillator frequency by the necessary amounts. The signal from the remote station was not broadcastable until lock was achieved, as it was outside the range for TV receivers to handle.

Or for Too Much Information:

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[John Walliker]

Not at all too much! I spent a gap year at Research Department in 1974 in the Storage and Recording section. I was in the lab where the BBC's first digital frame store was designed and built. This could be used, among other things, for standards conversion. It occupied two full height racks, one for each field. Memory was 1-kbit shift registers, chosen because, due to a special deal, they were much cheaper than the equivalent ram chips. Digital TV and audio recording to tape was by then already well established. Transmission of digital audio in line sync pulses was developed next door, initially as a way of simplifying outside broadcasts by avoiding the need for separate audio and video channels. The lab next door also built the first teletext encoder used for actual broadcasting at Crystal Palace, all built on hand wired veroboard because there wasn't enough time for pcbs to be made due to competition with the ITV labs.

John

[Brian D]

With solar power during the day it's not strictly true now, it's often slowed down at night to keep the average.

[Scott]

This would depend entirely on where you lived. Here it was BBC = channel , STV = channel 10.

My granny never accepted the term 'STV' and always referred to 'ITA'.

Heard of them. Did this degrade the picture compared to the BBC?

[Tim Streater]

Picture was rubbish enough anyway that you couldn't tell. Ours was a box that converted the frequency.

[Scott]

Should read BBC = channel 3, STV = channel 10

[wrights...]

ITV converters? Input: the local BIII channel. Output: the local BI channel. As a naughty boy I used to connect the output to our BI aerial. This wiped out BBC reception in the area. Bill

[Peter Able]

No, they just shifted the station's frequency down to a Band 1 frequency. Nothing was done to the signal content.

"If by "them" you mean an "external convertor box".

PA

[Peter Able]

Now that was naughty! I guess that you were also the boy who switched the Power Supply from DC to AC when the Chem Teacher was demonstrating Electrolysis of Water.

PA

[Harry Bloomfield Esq]

That may have been how it was originally, but not in later years - I know, because I wrote to the BBC's tech department, explaining I was considering designing a standard tied to theirs and asked them about it. I received an extensive and interesting personal reply, which explained it and the accuracy.

[Harry Bloomfield Esq]

+1