Bandwidth? DC to light?

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Bandwidth? DC to light?

Postby Steve Anderson » Sat Jul 14, 2007 2:04 pm

Viewmaster wrote:This is always assuming that .1 to .25 msec std. NBTV pulses can ever be recorded on a cylinder at all. Doubtful. :cry: Albert.


The analogy I would draw here is that of a mechanical low-pass filter, that is there is a finite limit to the frequencies that can be recorded. With a mechanical process there are other factors thrown into the equation, mechanical resonances, non-lineararity and a certain degree of noise.

So, as a starting point, follows are three plots of a 125µs sync pulse passed through three filters, one each at 20kHz, 10kHz and 5kHz. These were all four-pole Sallen and Key Butterworth filters, Bessel would be better, Chebychev worse from a pulse shape point of view.

Which flavour of filter approximates a mechanical one is something that I have no idea about!

As is quite apparent, even with a 20kHz bandwidth things are not so good, at 5kHz it is marginal at best!

How this translates into a mechanical system is over to others...

Steve A.
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20 kHz 1.gif
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10 kHz 1.gif
10 kHz 1.gif (11.24 KiB) Viewed 4188 times
5 kHz 1.gif
5 kHz 1.gif (11.31 KiB) Viewed 4188 times
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Postby Viewmaster » Sat Jul 14, 2007 5:33 pm

Thanks for all that Steve. It's all rather gloomy in that on cannot really lengthen a sync pulse just to suit the limitations of cylinder recording as picture info would be destroyed even though one might shorten the pulse again on playback.
Running the cyl at faster speeds might work but the surface noise on either this or on a disc will always be a threat to that little pulse buried in it and struggling to show its little face. :)

Albert.
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Postby DrZarkov » Sat Jul 14, 2007 6:07 pm

How did they do it in the time of J.L. Baird, when NBTV was transmitted on shortwave? The bandwidth wasn't much more than 5 kHz in that time? If it works on shortwave without "tricks", why not on cylinders?
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Postby Steve Anderson » Sat Jul 14, 2007 6:48 pm

Viewmaster wrote:(It) will always be a threat to that little pulse buried in it and struggling to show its little face. :)

Albert.


Fear not Arbuthnot, the poor little sync pulse is not really needed in a mechanically-linked system. If there were a defined number per rotation of the cylinder or disk, then none would actually be needed.

Attached is a sketch of a Nipkow disc (equally applicable to a cylinder) showing the outer rings of display holes (black), then the synchronous video track in blue, then lastly the audio in red.

At 750RPM there is going to be plenty of bandwidth, and sync pulses are not required! It's inherent. The track spacing/pitch for a reasonable duration recording is another matter.

Steve A.
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Postby Steve Anderson » Sat Jul 14, 2007 6:53 pm

DrZarkov wrote:If it works on shortwave without "tricks", why not on cylinders?


There is absolutely no reason at all. It can and surely has been done.

Steve A.
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Postby Viewmaster » Sat Jul 14, 2007 7:11 pm

Steve Anderson wrote:
Viewmaster wrote:(It) will always be a threat to that little pulse buried in it and struggling to show its little face. :)

Albert.


Fear not Arbuthnot, the poor little sync pulse is not really needed in a mechanically-linked system. Steve A.


A seperate electrically linked cyl machine is attractive to anyone who already has an existing NBTV machine...it's just an experimental add on.

As it now seems that sync pulses are unrecordable on a cyl I shall use my rotating opto fork sync wheel on a seperate machine to sync it with the seperate NBTV.

Re Baird transmission on short wave.....did he use sync pulses?
But if he did then there wasn't any recording surface noise in which the pulses might get lost as there will be on cylinder recordings.

By the way, all this surface noise will give a great firework display on the NBTV picture....I just can't wait to see it! :lol:
Albert.
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Postby Steve Anderson » Sat Jul 14, 2007 7:19 pm

Viewmaster wrote:Re Baird transmission on short wave.....did he use sync pulses? Albert.


Nope, there were no sync pulses in that era. There was simply a period of black which is where I was going with my posting "Sync pulses, do we really need them?". In my opinion, no, we don't need them.

Steve A.
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Postby Klaas Robers » Mon Jul 16, 2007 12:35 am

When transmitting on short wave you CAN use a higher bandwidth than 5 kHz. Yes, you are causing interference to the neighbouring stations, but if you select a period of time when the neigbouring stations are asleep....

Even on the short wave amateur bands it is possible to do these experiments. For FM modulation on the VHF and UHF bands one could use 3 channels of the 12½ kHz grid. There are enough channels available....

On the other hand I have the impression that short wave AM stations use more than 5 kHz of bandwidth. Inside that bandwidth the LF-bandwidth will be 2,5 kHz, which is very narrow. So they transmit somewhat more and hope thet the neighbouring stations are geographically far away.
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Your crackers my Lord.

Postby Steve Anderson » Mon Jul 16, 2007 1:03 pm

When I lived in the UK I took and passed the Amatuer Radio Licence requirements and ended up with the callsign G1CLW. I was living in Plymouth at the time, in the west-country of the UK. I was a member of the local radio amateur club and all of them were encouraging me to learn Morse and get a 'full' licence* so that I could use the bands below 30MHz.

But no matter how much I explained that I wasn't interested in below 30MHz, they simply thought I was crackers in staying VHF/UHF only.

My interest was not 2m, nor 70cm, but 23cm where there is so much bandwidth available that you have to try really hard to cause interference!

Then I moved to Asia and let my UK licence lapse. But not being a Thai national (even though I am now a permanant resident), I cannot hold a Thai Amateur licence.

So I have to confine myself to frequencies that as yet have not been allocated, basically above 400GHz (I.R. & light!).....no-one looking? Then I'm going to transmit!

Steve A.

*I believe that the requirement for Morse has recently been lifted, but I'm not sure about that.
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