Correct free running disc speed?

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Correct free running disc speed?

Postby Viewmaster » Mon May 21, 2007 10:44 pm

The Nipkow runs at 400 RPM when locked in sync. The free running speed is greater than this to allow the motor to be slowed down in order to get sync.
But I wonder what the best free running speed is? Too fast and the motor will be off more times. This may explain sync problems. The best free running speed will therefore be what? 420RPM? 450RPM? 500RPM?
I see no mention of this either on the NBTVA site nor in the handbook.
Or doesn't it matter at all? So, 5000RPM! :shock:

Albert.
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Postby AncientBrit » Mon May 21, 2007 10:56 pm

Unless regenerative braking is used the only way that the disc can slow down is through frictional losses.

I would suggest that the free running speed be set to 750rpm (12.5 fps x60) initially.

When lock is achieved, apply a finger briefly to the edge of the disc and then observe the response of the motor/disc.

It will hunt before settling. Tweak the mid range motor speed control to try to balance the over/under speed variations.

With regenerative breaking there will be more control over the slowdown and it should be possible to get improved damping.

Regards,

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Synchronisation.

Postby Stephen » Tue May 22, 2007 12:26 am

If you are really motivated, you could do the synchronisation and framing in a state-of-the-art 1920s style with Mr. Baird's phonic motor. See British Patent 336,655, filed 17 July 1929 in the Patents and Articles section of the forum.
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Postby Klaas Robers » Tue May 22, 2007 9:24 pm

The "phonic wheel" in the original Baird Televisors is a kind of hysteresis brake, that slows down the rotation when the teeth and the magnet coïncide at the moment of the sync pulse. I tried to use it once, but it is very difficult to keep synchronisation, and it is rather noisy!!! The disc should run slightly faster without synchronisation.

Peter Smith uses the wheel as a generator, which works as the electro magnet is always somewhat magnetised and can be used as a pick-up coil for pulses. So it does the same as the opto-bridge in our monitors.

The servo circuit with the 4046, if correctly working, will slow down and speed up the disc, so the disc should run on correct speed without synchronisation.
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Motor control.

Postby Stephen » Wed May 23, 2007 1:06 am

I have not built any television system from scratch, but if I were to do so, I would be tempted to use the line frequency signal embedded in the video signal to drive an AC motor. A notch filter would allow the line frequency, whether it be 375 or 400 Hz, to pass to a frequency converter circuit. The frequency converter circuit might just be a frequency divider that divides 400 Hz by 8 to output 50 Hz, or it might be the combination of a multiplier that multiplies 375 Hz by 2 followed by a divider that divides by 15 to output 50 Hz. The 50 Hz signal could then drive a 50 Hz synchronous AC motor directly or the 50 Hz signal could phase lock a 50 Hz power oscillator to do the same thing.

This scheme would take care of synchronising the motor driven scanning element to the line rate of the video signal. A motor stator rotation knob would allow framing. This approach seems to me to be more natural than overspeeding a DC motor and then using a feedback loop to slow it down.

I would imagine that others have done this in the past. Does anyone have any experience with this approach?
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Postby Klaas Robers » Wed May 23, 2007 5:52 am

Yes, the "Dutch Gang" did this using bike dynamos as a synchronous motor. In a Stereo Cassette Player one channel was recorded with the 50 Hz and the other with the video.
The remaining problem is/was that the picture was hunting, because there is no damping on the phase errors. Once it was hunting this remained so. The picture was swinging up and down about half the frame heigth with a frequency of about ½ Hz. (2 seconds swings).
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Baird's motor control and Phonovision

Postby Viewmaster » Thu May 24, 2007 8:28 pm

Stephen wrote:This scheme would take care of synchronising the motor driven scanning element to the line rate of the video signal. A motor stator rotation knob would allow framing. This approach seems to me to be more natural than overspeeding a DC motor and then using a feedback loop to slow it down.

I would imagine that others have done this in the past. Does anyone have any experience with this approach?


Here's a photo of the Baird Televisor showing the part circular gear on the coil assembly for rotation using the small gear.

This photo was taken from, "Television, today and tomorrow" by Mosely and Chapple, writen in 1934 it is entirely devoted to mechanical TV, and I recommend it.
I also include a shot of Baird's Phonovision...the transmitting end, as this has recently been mentioned.
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Re: Baird's motor control and Phonovision

Postby Stephen » Thu May 24, 2007 11:10 pm

Viewmaster wrote:Here's a photo of the Baird Televisor showing the part circular gear on the coil assembly for rotation using the small gear.

This photo was taken from, "Television, today and tomorrow" by Mosely and Chapple, writen in 1934 it is entirely devoted to mechanical TV, and I recommend it.
I also include a shot of Baird's Phonovision...the transmitting end, as this has recently been mentioned.
Thanks for the photos, Albert. They are very interesting. The phonic motor hardware seems to be very close to the figures shown in Mr. Baird's patent.

By the way, the picture of the recording apparatus is misleading. The apparatus in the foreground is obviously a reproducing apparatus, not a recording lathe, and it is playing a Phonovision recording. There is an electrical reproducer mounted on a tone arm that seems to be for an acoustical gramophone.

The cant of the electrical reproducer seems to be odd. Normally the stylus would tilt 10-25 degrees from vertical away from the tone arm pivot. In this case the stylus is tilting toward the pivot. That would suggest that the record groove was recorded from the inside out like a transcription disc.
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Postby Viewmaster » Fri May 25, 2007 12:53 am

It does say it is recording a dummy's head and there is a dummy shown on the left hand side. There seems to be an aperture in the rear wall.
The text doesn't offer any further expanation.
Phonovision is described in this book as, 'bottling up' pictures!

By the way, there are also photographs of both Baird's colour apparatus and his stereo too if any one wishes to see them.
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Postby Stephen » Fri May 25, 2007 1:20 am

I surely would like to see those additional pictures, Albert. I need to find a copy of that book as well!

Clearly that photograph shows the camera in the background for recording. I am wondering if there was a separate recording lathe, not shown, or if there was a lathe attachment that could be fitted over the illustrated turntable for recording.

The other possibility is that they were using pre-grooved recording discs that did not require a lathe. They typically comprised soft aluminium discs with shallow grooves embossed in them. One could place a blunt embossing stylus in a standard reproducer head and add a weight to the head so that the embossing stylus would create a deep modulated groove when the reproducer head was attached to a recording amplifier output.
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Postby Viewmaster » Fri May 25, 2007 2:30 am

Stephen wrote:I surely would like to see those additional pictures, Albert. I need to find a copy of that book as well!

The other possibility is that they were using pre-grooved recording discs that did not require a lathe. They typically comprised soft aluminium discs with shallow grooves embossed in them. One could place a blunt embossing stylus in a standard reproducer head and add a weight to the head so that the embossing stylus would create a deep modulated groove when the reproducer head was attached to a recording amplifier output.


It looks to be a shellac gramophone record on the turntable.

Re photos..... will take some more and post soon.

Re the book...... Abe books lists four copies (some pricey up to $200). UK book world list two (still pricey!)
I have the 4th edition (1934).
Later editions begin to bring in CRT TV.
Probably eBay is the best place to watch.

There is a preface by Baird in which he concludes,"I see no insurmountable barrier in the way of the advancement of television to a perfection rivalling the cinematograph."
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