Variable frame rate.

Forum for discussion of narrow-bandwidth mechanical television

Moderators: Steve Anderson, Dave Moll, Andrew Davie

Re: Another thought.

Postby Stephen » Fri Jun 29, 2007 11:21 pm

Viewmaster wrote:If the motion were of a type which rapidly stopped and started, (like an actor moving his face then not, then moving again), the poor old system would be hunting back and forth would it not, the Nipkow motor taking some milli secs to re adjust speed up and down....or am I misssing something?
And why do we need this additional complication?
As per a previous thread on NBTV signal compression, why do something neat if there is no need, or is it just the challenge? :)
You are right about the complication, Albert. I was thinking about some threshold rate of motion where frame rate and/or line rate might increase so it would not respond to small changes like moving faces to avoid "hunting" problems. In any case, scanning disc or drum systems would not be suitable due to significant mechanical inertia, but perhaps a variation of the "loudspeaker scanner" would be.

An additional problem, with respect to changing resolution, would be changing aperture size for the photosensor in the camera and the light source in the display. A camera lens diaphram assembly is a possibility, and John Logie Baird had a plethora of schemes for changing aperture size during scanning in a cyclic manner.

To do this all on the fly though is probably unrealistic and nothing more than a potential challenge. The results would probably not be worth the effort, in any case. What is very possible though is to have a universal camera with selectable frame rates to accomodate available bandwidth and a universal receiver that can automatically track the frame rate of the camera signal.

Viewmaster wrote:New thought.....Maybe a NBTV system to carry out real life animation frame by frame using the NBTV camera? Store one frame, arrange the animation, press a button on the NBTV camera and another frame is taken and so on.
Now that is a real challenge!
Last edited by Stephen on Fri Jun 29, 2007 11:42 pm, edited 1 time in total.
Stephen
User avatar
Stephen
Anyone have a spare straightjacket?
 
Posts: 427
Joined: Fri Jan 26, 2007 7:00 am

Re: Vibratory scanners.

Postby Stephen » Fri Jun 29, 2007 11:35 pm

Steve Anderson wrote:If a modulated laser was used as the light source the the mirror could be very small, say 1mm<sup>2</sup>. I think that the laser displays in 'light shows' use something very similar.
A mirror mounted at the focal point of the camera or display optical system path could be very small as well. The small mirror could mount to the stylus assembly of a "stereo" gramophone pick-up to allow independent vibration in the vertical and lateral directions. This scheme would have much lower moving mass and allow for faster scan rates.
Stephen
User avatar
Stephen
Anyone have a spare straightjacket?
 
Posts: 427
Joined: Fri Jan 26, 2007 7:00 am

Postby AncientBrit » Fri Jun 29, 2007 11:38 pm

Stephen,

The most practical solution would be some form of solid state frame store with a variable frame-rate read out.

Input sampling would be locked to the source (mechanical or electronic) frame rate.

One immediate application for such a system is the transmission of NBTV signals on radio.

Amateurs are restricted to a fairly narrow bandwidth and even using SSB it's a squeeze to fit in NBTV signal bandwidth of say 9kHz.

With a 6.25Hz frame rate the B/W would drop to 4.5kHz.

Of course a reciprocal store would be necessary at the receiver end.


Graham
AncientBrit
Green padded cells are quite homely.
 
Posts: 858
Joined: Mon Mar 26, 2007 10:15 pm
Location: Billericay, UK

Nullification.

Postby Stephen » Sat Jun 30, 2007 12:57 am

AncientBrit wrote:I was able to null the hum to quite an extent by using a secondary pickup cell outside the disc which gave an integrated pickup of scene lighting (mainly 50Hz) and then using a modulator with this signal fed into the one of the ports in anti-phase.
Very good, Graham. This would seem to be the solution to using slow scan rates with mains-powered lighting regardless of frame rate. The only difference would be that it would null moving bars in the case of arbitrary scan rates as opposed to stationary ones in the case of a 12.5 fps frame rate.
Stephen
User avatar
Stephen
Anyone have a spare straightjacket?
 
Posts: 427
Joined: Fri Jan 26, 2007 7:00 am

Postby Viewmaster » Sat Jun 30, 2007 3:13 am

Nulling hum reminds me of the 'hum dinger'used on the LT side of valve transformers.
A pot across the 6.3v supply and centre tap to earth....how many can remember those, and have used them?
Albert.
User avatar
Viewmaster
Frankenstein was my uncle.
 
Posts: 1290
Joined: Fri Apr 06, 2007 4:50 am
Location: UK Midlands

Postby Stephen » Sat Jun 30, 2007 3:23 am

Viewmaster wrote:Nulling hum reminds me of the 'um dinger'used on the LT side of valve transformers.
A pot across the 6.3v supply and centre tap to earth....how many can remember those, and have used them?
Albert.
Yes, the good old days-when the best loudspeakers had field coils that served as the HT supply choke and they had hum-bucking coils as well. Now that you have brought up the subject, there would be no need for an auxiliary photocell to cancel out hum. Simply combining the output of a LT transformer with the video signal, possibly with an adjustable R-C phase control network to adjust phasing and amplitude for best bar pattern reduction, would probably work.
Stephen
User avatar
Stephen
Anyone have a spare straightjacket?
 
Posts: 427
Joined: Fri Jan 26, 2007 7:00 am

Postby AncientBrit » Mon Jul 02, 2007 10:33 pm

Stephen,

re himdingers (yes I remember them!)

The incandescent lighting produced a 100Hz waveform, heating effect and all that.

So to balance this you would need to derive 100Hz from the transformer/rect/phase shifter.


Graham
AncientBrit
Green padded cells are quite homely.
 
Posts: 858
Joined: Mon Mar 26, 2007 10:15 pm
Location: Billericay, UK

Postby AncientBrit » Mon Jul 02, 2007 10:35 pm

I've just realised there's a typo in my last message.

Himdinging could be a whole new area to explore/avoid.....

Graham
AncientBrit
Green padded cells are quite homely.
 
Posts: 858
Joined: Mon Mar 26, 2007 10:15 pm
Location: Billericay, UK

Previous

Return to Mechanical NBTV

Who is online

Users browsing this forum: No registered users and 6 guests

cron