Double sideband TV's ...

[colly0410]

Been thinking (stop laughing at the back, I can think) Alexandra Palace transmitted double sideband video on channel 1 until Crystal Palace replaced it with vestigial sideband also on channel 1 in 1957. How did the older double sideband tellies cope with the change?  Also did the Eiffel Tower 441 line transmitter use double sideband video? Just my random thoughts...

[Kalee20]

I'm interested! I can imagine a slight change to contrast level, plus a slight worsening of definition - all other things being equal.

I can imagine also the older sets, for double sideband (plus carrier) being slightly more susceptible to interference.

Would be good to know if this is correct!

[PerdioPal]

I think good old Shannon's law creeps in here, VSB (to the first approximation) has half the power and half the bandwidth. DSB has twice the power and twice the bandwidth, signal to noise would be the same(ish).

[Murphyv310]

Hi.
I believe some TRF's were not that happy but realigning was the cure, certainly not from experience but from articles I read in the past.

[ppppenguin]

By 1956 I wonder how many prewar sets were still in use.

[Panrock]

Hi.
I believe some TRF's were not that happy but realigning was the cure, certainly not from experience but from articles I read in the past.

Not TRFs, but I suspect this would have been a problem with the second generation pre-war EMI sets like the HMV 904, one of whose vision IFs is tuned to the upper sideband. 

Left alone, after 1956 there would have been a blurrier picture. The expected drop in gain/contrast would probably have been mitigated by the greatly increased e.r.p of Crystal Palace. I can just imagine how re-tuning the coil-in-question to the lower sideband (even if this were possible) would soon run into problems with sound-on-vision.

Steve

[colly0410]

I've just twigged why there was such a big gap between channels 1 & 2. Presume the channel planners thought when Holm Moss fired up in 1951 there was a chance during a lift of HM & AP causing co-channel interference to each other so channel 2 was moved up a bit, & when Crystal palace fired up on CH 1 with vestigial sideband they just kept the gap so sets already in use would work OK... Why did I only twig this now & not years ago? I can be a durr brain sometimes, lol...

[PerdioPal]

VSB on a DSB receiver will give twice the output up to the VSB cut off frequency, that is why VSB sets have a slope in the response around the carrier. Much easier than SSB (with carrier) at the transmitter end, the extra bit of the sidebands was merrily sent to a dummy load. OK this did reduce efficiency a bit but most of the signal was low frequency and did reduce bandwidth.