Construction Diary -- Part 2, Spinning the Nipkow Disc

Original build of a televisor by a complete novice.

Moderators: Steve Anderson, Dave Moll, Andrew Davie

Postby magicanim8r » Sat May 26, 2007 10:29 am

Andrew,
Congratulations on all of your hard work! You have inspired me to try again with my motor control circuit. I plan to look back through your posts and see what I can do to hopefully get my circuit to work.

Best regards,

Al
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Postby Andrew Davie » Sat May 26, 2007 5:26 pm

I'm not happy with the new 'high brightness, high contrast' display. It's very harsh, and I'm going to re-do it with absolutely correct resistor values for the gamma circuit. The images below are from the latest display -- as you can see, the faces tend to appear quite clearly, but there's something about them that is disturbing and unsatifying -- it just doesn't look right to my eye.

The images aren't very well framed by my masking window -- this was a bit of a hack job with the photos, sorry :)

Edit: I just realised something interesting. I was wondering how I could see some of the images outside the masked area (because the LEDs are entirely behind the masked window). What is happening is that the LEDs are so bright that the image formed outside of the masked inner area is the incidental (escaping) light reflecting off the back of the Nipkow disk, back off the front of the mask/light box (matt black plastic) and then back through the Nipkow holes. Awesome!
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highcontrast.jpg
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Images are OK to me.

Postby Steve Anderson » Sat May 26, 2007 6:34 pm

Andrew,

What you have achieved thus far is remarkable, those images to me would seem perfectly acceptable with the exception of the 'streaking' caused by mechanical (presumably hole-drilling/punching) limitations. That's the area that I feel needs attacking next. It's always been a problem with home construction to get things as accurate as we might like them.

Over to the mechanical guys here....(not my forte)...

Steve A.
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Postby Andrew Davie » Sat May 26, 2007 7:56 pm

These two pictures give some idea of image brightness under fairly comfortable indoor lighting conditions. About the same as a fairly bright CRT, I'd say.

The third shows the brightness difference from my first working images, to the current 40 LED version.
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afar1.jpg
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afar2.jpg
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change.jpg
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Postby Andrew Davie » Sun May 27, 2007 12:05 am

Today I worked on the gamma resistors. I carefully calculated the best values for each parallel piggyback resistor, and installed piggyback resistors on all three of the originals (150, 82 and 56 ohms). My original addition to the 150 ohm resitor 'converted' that to 27 ohms, so I took R to be 9 ohms (since this resistor was 3R). That left me needing values of 9*1.6 = 14.4 ohms and 9 * 1.3 = 11.7 ohms for the other two.

Since the first had 82 ohms, I calculated 1/14.4 = 1/82 + 1/? (ie: ? = 17.5 ohms). Best I had was a 16 ohm resistor, which I used. Back-calculating, I got 13.4 ohms (vs. 14.4 -- close enough) - a multiple of 1.5R instead of 1.6

The second had 56 ohms, I calculated 1/11.7 = 1/56 + 1/? (ie: ? = 14.8 ohms). Best I had, again, was 16 ohms -- so I used this. Back-calculating I get 12.4 ohms (vs 11.7 -- close enough) - a multiple of 1.37R instead of 1.3

Having done the above, I modified my diffusion filter to be dog food can plastic covers (2) separated by a 5mm air gap. Now there is absolutely no sign of LED spot brightening -- complete uniformity over the view area. This is awesome! I could afford to use the two plastic covers because my brightness is so amazingly high -- they had little effect on the final result as I had to turn my brightness down anyway.

Now I don't get that feeling of 'wrongness' in the images any more -- they seem to have a much softer feeling in the highlights. This is probably because the gamma resistors are now correct.

It was at this point, after I'd redone the gamma (note: all three resistors this time, not just the original two) and installed the new filter arrangement, that I noticed that Graham on track 46 of CD#1 is smoking a pipe. Well, i think he is :) This tended to point towards improved clarity and image definition.

Attached image shows the single-board with the recently installed motor control section sitting at the bottom of the board. The blank area bottom-right is probably going to hold the 'negative video handler'.
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withmotorcontrol.jpg
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Postby Andrew Davie » Sun May 27, 2007 10:50 pm

I wasn't happy with those black lines between scanlines, so I took to the Nipkow disk with a 'hand file' -- in fact, a very small drill bit held in my fingers, and used to enlarge the holes by moving the drill bit back and forth manually.

I spun the disk, holding the drill bit positioned where the worst 'black line' was, then stopped the disk. The hole that the bit was positioned over was then easy to find -- and I manually 'filed' that hole so it was a bit bigger -- then repeated the process. I slowly managed to even out all the holes such that there are no black lines anymore. In fact, some of the holes I've made a bit too big, but I don't mind that so much -- it's not noticeable when viewing with the eye.

The improvement in picture quality is amazing. This picture shows the improved contrast now that I have the correct gamma resistors installed, along with the removal of the black lines between scanlines.
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handfiled.jpg
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who.jpg
Who could this be?
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who2.jpg
Approximate actual size
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Last edited by Andrew Davie on Mon May 28, 2007 1:26 pm, edited 3 times in total.
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Hand-made disc.

Postby Stephen » Sun May 27, 2007 10:57 pm

This is a fantastic improvement, Andrew. These are amongst the best NBTV pictures that I have seen. Your "hand-crafted" scanning disc is superb.
Stephen
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Postby DrZarkov » Mon May 28, 2007 6:14 pm

Wow, this is really great! That are indeed the bext pictures I've ever seen.

BTW: The first film ever I've converted with Video2NBTV was an episode of Dr. Who (the android invasion with Tom Baker to be exactly), too.
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Postby Andrew Davie » Mon Mar 24, 2008 11:02 pm

For roughly 6 months my televisor sat on the bench in a partially complete state (wires twisted together, not soldered), bits that had 'fallen off' just heaped together. I regret that now. I've spent the last few days fixing up the things that were easy, and have a basically functioning machine EXCEPT for the motor synchronisation.

Yes, again, this circuit is proving my nemesis. This time I seem to have a problem with 'noise' on the signal for the IR sensor. I had to totally replace BOTH IR widgets. I think the 'spark' from a couple of weeks back must have fried them, because I couldn't get either working. I checked with my handy old video camera and confirmed no transmit (I also put a replacement transmitter in parallel and it lit up). So replaced the xmit IR LED and measured the signal coming out of the receiver. Nothing. Replaced it with another, and now I'm getting a signal.

So I fire it all up and play a track and I notice that I have an underlying signal (looks like a sine wave with spikes) coming through the IR sensor receiver. It's definitely related to the motor. If I don't spin the motor, the IR signal is clean. If I spin it, then there's a helluv a lot of noise on the signal.

So, the first thing I'd like to do is clean this up. My first thoughts are that I can put zener diodes on the two leads leading to the motor. But really this is only a guess and I'd really appreciate some comments on 'noise' coming from my motor and the most effective solution to isolate the IR receiver signal from the motor noise on the circuitry.

I plan to take this baby into work for show and tell sometime this week.
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Postby Steve Anderson » Tue Mar 25, 2008 1:17 am

Andrew Davie wrote:So, the first thing I'd like to do is clean this up. My first thoughts are that I can put zener diodes on the two leads leading to the motor. But really this is only a guess and I'd really appreciate some comments on 'noise' coming from my motor and the most effective solution to isolate the IR receiver signal from the motor noise on the circuitry.


Make sure all the electronics has thourough de-coupling across the supply rails, 100n disc ceramic caps from supply to ground/0V as close to the chips as possible, maybe on the underside of PCBs.

Use screened leads for all low-level/high impedance signals and put something like a 100n cap directly across the motor terminals.

This is all on the presumption you haven't already.

Steve A.
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