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gary wrote:Harry, the serrated disk is a "light chopper" which more or less modulates the signal to a higher, and more easily managed frequency. It seemed to solve two problems - that of poor low frequency amplification, and the slow response time of his selenium cells. Note that an electronic version of this method has been used by NBTVA members, with varying degrees of success, in improving the recording of NBTV on tape.
WOW i see what you mean this is something to come up with back then ..this sounds a bit like a mechanical PWM i wonder if thats another invention of hes .
I have never heard of the electronics being explained i just was thinking it was a simple amp ...it sounds like he used a wide band Am radio ...i think.
I have not heard of a this mechanical idea at all thanks for explanation i was hoping someone knew .Don't forget that the 83 Hz is then multiplied by the number of serrations to get the actually modulation frequency. Baird has stated this gave him a modulation frequency of between 10,000 and 500,000 Hz. The latter value, whilst being theoretically possible, would probably have been difficult to achieve in practice
This i didn't understand as well i was thinking why perhaps 83 hz per line and why isn't it used today ,Now this information is really interesting and helpful you get a big Star of this post !
Pauline hanson is enlightened:wink:I have never discovered the exact number of serrations he ultimately used. There are 2 related patents relating to this technique posted on this site 235619 and 270222
harry dalek wrote:WOW i see what you mean this is something to come up with back then ..this sounds a bit like a mechanical PWM i wonder if thats another invention of hes .
harry dalek wrote:I have never heard of the electronics being explained i just was thinking it was a simple amp
harry dalek wrote:I was trying to count them i see one small one looks around 40 50 ish i have seen a early version large white one which looks double or triple that and this one in the above drawing could count that one in the better down load copy sounds like he was playing around with the bandwidth
harry dalek wrote:Did you say before you moved house you had a made a falkirk transmitter just wondering if so what did you try if you had on that disk .
Don Tomkins wrote:Steve, Would it be possible for to scan and post pages 347/348 from the chapter on Magnetic Amplifiers you recently posted, many thanks. Don
gary wrote:No it's not PWM. It's AM modulation. The base band signal spectrum is shifted up in frequency by the frequency of the carrier which, in this case, is the frequency of the light being chopped by the serrated disk. Look up AM Modulation esp. chopper modulators.
Its a very clever idea he was a man with a goal alright...i will google the chopper modulators learn a bit more .Well it was a simple amp (by today's standards) but even modern amplifiers have a response that falls off at very low frequencies. Shifting the baseband up brings it into the "sweet spot" of the amplifiers response curve - look up a typical amplifier frequency response curve - you will see what I mean.
Are there a circuits of hes work any where Gary its sad if he didn't do schematics ? The amplifier and such camera monitor idea as you say must of got changed with every new idea .\Harry, I just realised that we have been over this before here:
http://www.taswegian.com/NBTV/forum/vie ... highlight=
Oh and even the same drawing but do have a high quality version above !
have to go over them if the curved line disk at the monitor end is talked about.I have seen and photographed the Falkirk transmitter (it's controversial as to it's originality) when it was in the British Museum - I have lost those photos unfortunately - when I went to see it again it had been moved. I was told it had gone to the Museum Of the Moving Image in London and I have photos of that but I am sure it is just a replica - on that machine the number of slits on that machine is exactly - wait for it - 32!
Pretty sure the old photos its over 32 slits i would like to see that photo of baird standing next to it in the Museum and check the later photos .....the nipkow for the monitor is gone or is that just a later camera .....Unfortunately I am pretty sure that the Falkirk system was never shown or photographed in it's working state as Baird wanted to maintain secrecy on the details so I am not sure you will ever be able to ascertain the exact number of slits he used - and I would easily lodge a bet that he used several versions each with a higher number of slits as better engineering skills became available to him. It wouldn't be long before he ditched the concept completely as better light sensors became available to him.
It does look like he changed them the one with the lower number looks improved over the large number white one ...i will check any photos from now on i see of this .Not quite, I did say that my early attempts at building a camera/monitor were based on the Falkirk system - because that was all I had to go on at the time - this was in the late sixties or very early seventies and all trace of it is now long gone - and - like Baird - I never documented. I did have a single nice photo but that somehow got all scrunched up and torn and eventually lost or thrown away.
I recall you talking about it but at the time had no idea what type of he's work it was .
Tend to know better these days not to chuck stuff away i find you always come back to it or want it after its gone....
Pity theres no photo ! i have nothing to show of my first go either but a memory .The number of slits I used would have been a bit of a guess as my knowledge of AM modulation at the time would have been rudimentary - I was only 16 at the time - but, of course, the EXACT number is pretty irrelevant - ideally you want the number (and speed) as high as possible and at least enough to give a chopping frequency higher than the highest frequency in the signal but any number will shift the signal spectrum upwards. You certainly, though, do not want so many that the frequency extends beyond the frequency response of the transmission medium (including amplifier).
Sort of answers a question to wide a bandwidth is just as bad as to little ,i see with the Carbon Arc oscillator could get to a mhz range i want to try the strobe chopper idea first .
What i am wondering as i expect drifting and what effect that would have have on a magnetic amplifier ...since its also controlling the voltage so bandwidth swing and voltage swing...if at the end its a bit like watching a dx tv signal i am fine with that ..
A magnetic amp bistable flip flop encoder motor control is a bit beyond me yet .
Baird did NOT invent this technique but was the first to apply it to this purpose, and he claims, at the high frequencies required.
AncientBrit wrote:
Or am I misunderstanding the description on p154 of Wireless World and Radio Review?
AncientBrit wrote:After scanning the image with the normal Nipkow disc the resulting optical 'signal' is then commutated (or as we might say analogue sampled) at a high frequency.
This is then presented to the sluggish selenium cell.
Surely the cell will respond to the mean signal?
AncientBrit wrote:It will still have a slow rise and fall time.
You won't have improved the frequency response by chopping the optical signal, just reduced its amplitude by the duty cycle of the interruptor disc.
AncientBrit wrote:If you want to amplitude modualte the signal and move the spectrum upwards you need to interrupt the actual electrical signal, not the optical signal.
Graham
AncientBrit wrote:If the response time of the cell were negligible then I agree the two processes would be interchangeable.
Graham
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