Camera/monitor motor control

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Camera/monitor motor control

Postby Viewmaster » Mon Jun 11, 2007 4:49 am

Most of the mechanics and electronics for my combined camera/monitor is now complete. I have one last decision to make and maybe someone here can help me make it, please..........

I am using a DC motor rather than a bicycle dynamo locked to the mains frequency mentioned in the NBTVA handbook because I wish to play back material from other sources too which will have their own sync pulses, and so must control my motor speed, rather than the mains frequency.

For using the camera for recording I have a 400 hertz sq wave generator built into the circuiits which will be used to lock the motor to 12.5 revs per sec for recording by using the 4064 PLL. This will require a continuous 32 holes in the disc to give a similar 400 hertz as the other input to the 4064.

But I need that missing hole in the disc as you know, for playing back other material in order to get sync on all playback.

So, I can either have the missing disc frame sync hole offset at another radius and use a 2nd opto fork in parallel with the first one (can one strap opto forks in parallel I wonder?).

This will give me the 'continuous' 400 hertz signal for recording.....one opto fork picking up 31 holes and the 2nd parallel opto picking up the missing 32nd hole.

....or I can have 2 seperate sets of sync holes in the disc at different radii, one consisting of 32 holes for recording and the other for playback with the missing frame hole. One opto looking at one set of 32 holes for recording and the other opto looking at the other set of 31 holes for playing back.

Any comments please?
Albert.
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Postby gary » Mon Jun 11, 2007 10:51 am

If you have just one offset frame sync hole and two detectors the outputs can be logically OR'd together using TTL to provide the 32 pulses. This requires extra circuitry but it is simple and the pulses should probably be cleaned up anyway.
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Syncrhronisation apertures.

Postby Stephen » Mon Jun 11, 2007 12:50 pm

As I have related on the forum, I have not been in the position of constructing my own NBTV camera or display to date. However, if I were to start such a project, it seems to me that the ordinary scanning aperture holes may serve for synchronisation as well, including the missing pulse. Imagine a a linear IR source on the opposite side of the scanning disc that has a width about that of the scanning aperture spiral. Light from the linear IR source may pass through each scanning aperture momentarily at the start or end of each line, and a photodetector may respond to these optical pulses for synchronisation. Since the scanning apertures will be 180 degrees out of phase on the other side of the disc, a simple mask mounted between the IR source and the photodetector that blocks IR transmission from a single middle aperture secures the missing pulse at the start of each frame. This arrangement seems to be simpler than drilling an extra set of 32 apertures.
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Re: Camera/monitor motor control

Postby Steve Anderson » Mon Jun 11, 2007 1:08 pm

Viewmaster wrote:....or I can have 2 seperate sets of sync holes in the disc at different radii, one consisting of 32 holes for recording and the other for playback with the missing frame hole. One opto looking at one set of 32 holes for recording and the other opto looking at the other set of 31 holes for playing back.

Any comments please?
Albert.


You could simply tie the phototransistors in paralell as shown below as they are 'in the dark' most of the time and on different radii. The addition of SW1 allows you to select either continuous 400Hz pulses (SW1 closed) or the missing sync flavour (SW1 open).

R1 has been adjusted to take account of the sdditional LED and R2 is just a representation of the rest of the circuit, the 47k pot, Klaas's Schmitt trigger gates etc..

As Gary rightly points out, the pulses should be cleaned up and Klaas's gates are one way of doing it.

Steve A.

Sorry Klaas, I spelt your name wrong...I've amended the above, the 'aa' doesn't appear in English...
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Re: Syncrhronisation apertures.

Postby Steve Anderson » Mon Jun 11, 2007 1:30 pm

Stephen wrote:Light from the linear IR source may pass through each scanning aperture momentarily at the start or end of each line, and a photodetector may respond to these optical pulses for synchronisation.


Yep, there's no reason why this couldn't be done. On one side of the disc/drum a line of LED emitters behind a slit-type mask(perhaps with a diffuser) on the other side an IR detector, maybe more than one at some distance to cope with the perceived angular position of each line. These would have to be discrete components, not an opto-fork arrangement.

The concept is shown in the attached sketch. The heavy dotted outline is where the normal visible display LEDs would be located. This is looking from behind the disc, the sync emitters/detectors could be either side depending on construction arrangements.

Steve A.

Thinking about it further, the sync arrangement would have to be rotated around the axis of the disc by 5.625 degrees, as shown this would generate a sync pulse right in the middle of a display line...not quite what we want!
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Postby AncientBrit » Mon Jun 11, 2007 5:28 pm

You can use a mono to delay/phase the detected pulses.

I used such a circuit on my disc camera.

In fact I went further.
I used a phase locked loop to generate 32 pulses from a single frame hole phased by the mono.

This worked well.

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Postby Steve Anderson » Mon Jun 11, 2007 5:50 pm

AncientBrit wrote:You can use a mono to delay/phase the detected pulses.

I used a phase locked loop to generate 32 pulses from a single frame hole phased by the mono.

GL


Yes, that's more than one way of doing it, assuming the disc speed is rock solid, which I would imagine they aren't unless driven by some form of syncronous motor. A DC brushless motor being the ideal, direct drive, no rubber bands or knicker elastic, spot on exactly where you want it, when you want it.

The bicycle dynamo is a 'poor mans' alternative.

But good to hear you had success with this arrangement.

Steve A.

Beyond pressing old VCR head-drum motors into this service, I have yet been able to find a source of DC brushless motors without the drive electronics, just the raw motor and its windings (with or without Hall-effect detectors) that would be suitable for our use. I'm sure that we could get some made in China or Korea, but with a minimum order quantity of 10,000!
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Postby AncientBrit » Mon Jun 11, 2007 6:34 pm

There was nothing special about the motor, just the standard club cassette type motor and a simple speed lock cct using a 4046 and a darlington NPN.

I presume the inertia of the disc helped. The files titled ..dsc that I posted a few weeks ago were all shot using this camera and there is no hunting or warping of the image.

Perhaps I was lucky.

I generally steer clear of large servo motors.
You need a lot of dc power to lock them and to hold them in synch.

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Postby Viewmaster » Mon Jun 11, 2007 6:50 pm

Well, thanks to all of you for your very useful positive replies and the circuit too.......now where's my soldering iron? :lol:

As an precaution I will have a strobe disc mounted on the Nipkow disc to check speed before recording.... using two small ultra bright LEDs in series with an R and a diode across the 12v AC transformer to get the mains 'flicker' . These are mounted out of sight but one can see the LED illuminated strobe disc from the front of the machine.

Thanks again, much appreciated.

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Postby Steve Anderson » Mon Jun 11, 2007 6:54 pm

AncientBrit wrote:I generally steer clear of large servo motors.
You need a lot of dc power to lock them and to hold them in synch.

GL


Ah, I'm not referring to servo motors, the DC brushless motor I have used here is the motor from the head-drum assembly of a domestic VCR. When running at 750RPM it uses 300mA at 12V and in a NBTV frame time of 80mS it's rotational variation is less than 10uS on its own, with no flywheel effect from an attached disc/drum (i.e. with no attached mass). It doesn't wander, it doesn't drift, it stays right where it should be.

The variation I suspect is from the accuracy of the rotating magnets field and/or the Hall-effect sensor. I intend (one day) to confirm this with an opto-sensor arrangement.

What this means is that sync pulses could be generated electronically, you know where the disc is at any instant in time, no need to detect its position.

In other words, the disc (camera or display) follows our instructions (timing), we don't follow it. Who's the boss here?

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Postby gary » Mon Jun 11, 2007 10:01 pm

AncientBrit wrote:There was nothing special about the motor, just the standard club cassette type motor and a simple speed lock cct using a 4046 and a darlington NPN.

I presume the inertia of the disc helped. The files titled ..dsc that I posted a few weeks ago were all shot using this camera and there is no hunting or warping of the image.

Perhaps I was lucky.

I generally steer clear of large servo motors.
You need a lot of dc power to lock them and to hold them in synch.

GL


Graham, are you referring to dsc_Graham1.PS7 (etc)? or are there some files with a .dsc extension somewhere?
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Postby AncientBrit » Mon Jun 11, 2007 10:30 pm

Gary,


I'm refering to dsc_Graham1.PS7 etc where I've incorporated the dsc in the title.

These were originally recorded using the parallel port of the PC as an interface.

(I don't think I've actually created any .dsc file extensions , only
.PS7 and .001 etc.)

I also wrote a whole suite of NBTV editing and painting programs in VB6 about 6 years ago.

But they all use the "native" format of 64x32 pixels per frame and are not linked to any CD sample rates.


Regards,


Graham
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Postby gary » Mon Jun 11, 2007 11:03 pm

AncientBrit wrote:Gary,


I'm refering to dsc_Graham1.PS7 etc where I've incorporated the dsc in the title.

These were originally recorded using the parallel port of the PC as an interface.

(I don't think I've actually created any .dsc file extensions , only
.PS7 and .001 etc.)

I also wrote a whole suite of NBTV editing and painting programs in VB6 about 6 years ago.

But they all use the "native" format of 64x32 pixels per frame and are not linked to any CD sample rates.


Regards,


Graham


Thanks, Graham. Yes, you were kind enough to send me a copy of them a couple of years ago and they have survived several machine upgrades in that time so I was just looking over them again. I hadn't realised, though, that the sample videos were from your *disk* camera - they really are of superb quality. I hadn't realised that people were getting that kind of quality, shows what missing out on the conventions means to us antipodeans, I must get to one, one day, sigh.
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Postby AncientBrit » Mon Jun 11, 2007 11:14 pm

Thanks Gary for the kind words,


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Convention venue.

Postby Steve Anderson » Tue Jun 12, 2007 1:00 pm

gary wrote:Shows what missing out on the conventions means to us antipodeans, I must get to one, one day, sigh.


We could comprimise and have the convention half-way between the UK and Australia. I'm in Bangkok which is about half-way. If held in January or February it would allow those from the Northern hemispere to get away from their winter for a while.

Steve A.

No, I'm not being serious! But it would really suit me.
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