Baird's medium wave transmissions

[kareno]

The club handbook mentions that Baird's transmissions were restricted in bandwidth to 4.5kHz by the BBC. Is this the case? The figure of 4.5kHz suggests the 9kHz channel spacing which only came into effect in the 1980s in Britain. Were there any recognised spacings/bandwidths at that time? Might the BBC have widened the filters for Baird's transmissions?

4.5kHz would have had a drastic effect on vertical resolution.

[Lowtone]

Was it only for the tests or for the programms ?

[kareno]

Well, that's another interesting question - was the bandwidth different for the test transmissions and public broadcasts?

I'm sure someone knows the answers to these

[Steve Anderson]

Somewhere I've seen reference to 10kHz...it was related to filtering the vision signal, a form of pre-emphasis. If I remember correct it was used to get around the HF attenuation of the landlines between the studio and transmitter.

This may not have been a BBC/Baird arrangement, perhaps in the US.

What it means is that they were trying to squeeze 10kHz out of the system, so I think the intention was there, whether it was achieved or not I don't know.

Steve A.

[Lawnboy]

i had read somewhere (cant remember where) that the BBC transmitter had 4.5KHz sidebands, giving an absolute maximum bandwidth of 9KHz. Don McLean also mentions it in his book Restoring Baird's Image:
"The achievement of practical television in the 1920s pre-dated the development of the high bandwidth broadcast transmitter. As a result, television had to use the existing broadcast radio infrastructure."
"30 lines in the television picture were about as much as the medium wave [transmitters] could stand. Though there was never any intention of having equal detail horizontally and vertically, such an approach would have needed a transmission bandwidth of 13 kHz. As such, the public would have to suffer slightly fuzzy pictures." (p. 176-177)

[kareno]

Actually, Lawnboy, it was that reference that started me thinking on this. I'm disturbed by McLean's comment that 4.5kHz sidebands allow 9kHz bandwidth - for AM transmissions the bandwidth is the sideband width i.e. 4.5kHz. I don't think they has vestigial sideband back then.

[Steve Anderson]

As a follow-up, on page 12 of Harry's posting in 'Resources', the recent addition of "The Baird Televisor" there is mention of ...

..."makes the most efficient use of the detail permitted by the 10-kilocycle band, which is all that can be transmitted through the BBC stations at present." (My italics). The inference being this was 10kHz of modulation bandwidth, i.e. a 20kHz width DSB AM signal.

Steve A.

P.S. DSB = Double Side-Band, not Digital Satellite Broadcasting!

[Steve Anderson]

Also of note, even then they were having the same dreaded video polarity problem. See page 17 of the same document mentioned above, "Negative Pictures" and "Turning a Negative Picture into a Positive Picture".

Steve A.

[Klaas Robers]

As far as I know, the Medium Wave practice was that there was no guarantee of neighbor channel reception. So if you are transmitting on a certain frequency, the neighbouring channels are not occupied by near by stations. So with a spacing of 10 kHz the AF bandwidth could be 9.5 kHz and the frequency band occupied 19 kHz (two side bands). I don't know in how far the BBC transmitters had that bandwidth available.

That is also the reason that the Baird TV transmissions were after midnight, when most MW stations had closed down. And yes, indeed the AF bandwidth of the Baird signal should have been about 13 kHz, so the dot resolution (vertical) was less than optimal.

Receivers in 1930 were mostly straight receivers, no superhets. So the bandwidth could be small or wide, all influenceable with the antenna coupling and the regeneration control. So square formed MF-filters with steep fall off edges were not common practice