Sync separator

Forum for discussion of narrow-bandwidth mechanical television

Moderators: Steve Anderson, Dave Moll, Andrew Davie

Sync separator

Postby Klaas Robers » Thu Apr 12, 2007 7:51 pm

Andrew,

have you already looked at the sync pulses that come from the sync separator PCB? When you connect your oscilloscope to the line sync output while running a video signal from one of the club CDs, you should find very stable sync pulses of 400Hz (2,5 msec). First see that the contrast pot is in its optimal position, i.e. 1,4 volt peak tot peak video going to the LED-driver board. Then adjust the 1k trimming pot to the centre of stable sync pulses. This should not be critical.
Then connect it to the frame pulse output and you will see slower pulses, 12,5 per second. If this is Ok the PCB is working fine.

Then make the connection from the line pulses to the clamp pulse input of the LED-driver. After having done this you might connect the select clamp input of that board to +12. I have a small switch connected to be able to select both active clamp or DC-restoring. Simply switching from ground to +12.

DC-restoring works too when no sync pulses are present, also when the contrast setting is not Ok, with unknown video signals. But with standard and clean CD-signals the active clamping is working better.

The line pulses are also needed for the synchronisation circuit of the disc motor. This is the reason for you to look into this now.
User avatar
Klaas Robers
Frankenstein was my uncle.
 
Posts: 1465
Joined: Wed Jan 24, 2007 8:42 pm
Location: Valkenswaard, the Netherlands

Re: Sync separator

Postby Andrew Davie » Thu Apr 12, 2007 10:24 pm

Klaas Robers wrote:Andrew,

have you already looked at the sync pulses that come from the sync separator PCB? When you connect your oscilloscope to the line sync output while running a video signal from one of the club CDs, you should find very stable sync pulses of 400Hz (2,5 msec). First see that the contrast pot is in its optimal position, i.e. 1,4 volt peak tot peak video going to the LED-driver board. Then adjust the 1k trimming pot to the centre of stable sync pulses. This should not be critical.
Then connect it to the frame pulse output and you will see slower pulses, 12,5 per second. If this is Ok the PCB is working fine.

Then make the connection from the line pulses to the clamp pulse input of the LED-driver. After having done this you might connect the select clamp input of that board to +12. I have a small switch connected to be able to select both active clamp or DC-restoring. Simply switching from ground to +12.


Thank you for the above. I had not looked in detail, but I have now.

The 400Hz stable synch pulses are there, and I make them 0.2V each. Just appear as a single dot with a *barely* perceptible rise/fall on my oscilloscope. The 12.5Hz frame pulse is there too -- took me a while to find how to display it, so I could measure, though. Eventually I switched to AC coupling, and I was able to see a much clearer rise/fall instead of just a single fast-moving series of single lines. I switched to a time/div of 0.1s, giving 1 second across the screen... that let me accurately count 12.5/second. I also measured this pulse at 0.6V.

I have connected the synch select to 12V as suggested, and the monitor still works OK. So now, on to the motor driver circuit :)
User avatar
Andrew Davie
"Gomez!", "Oh Morticia."
 
Posts: 1511
Joined: Wed Jan 24, 2007 4:42 pm
Location: Hobart, Tasmania, Australia

Postby Klaas Robers » Thu Apr 12, 2007 11:39 pm

Uhh.?

Both pulses should have a heighth of 12 volts and indeed without noticable rise and fall times. The line pulses should have a width of about 50 usec and the frame pulse a width of 1,2 msec. This is according to the description in A5 format. Did you misread the oscilloscope?

By the way, you can use the line pulses or the frame pulse as a so called external trigger pulse of your oscilloscope. There is an extra input for it, in the neighbourhood of the trigger controls and you can switch triggering to that input. Eventually you can also use channel 2 to input the sync pulse and trigger on channel 2. Then your oscilloscope is always correctly triggered and you can look to all different type of signals, with or without sync pulses and all types of amplitudes. You might want to have a second probe for this task, but you can also use your coax cable with crocodile clips.

Now you can also see the missing sync pulse. Trigger the oscilloscope with the frame pulse and look at the line pulses. After 31 line pulses one is missing.....
User avatar
Klaas Robers
Frankenstein was my uncle.
 
Posts: 1465
Joined: Wed Jan 24, 2007 8:42 pm
Location: Valkenswaard, the Netherlands

Postby Andrew Davie » Fri Apr 13, 2007 1:21 am

Klaas Robers wrote:Uhh.?

Both pulses should have a heighth of 12 volts and indeed without noticable rise and fall times. The line pulses should have a width of about 50 usec and the frame pulse a width of 1,2 msec. This is according to the description in A5 format. Did you misread the oscilloscope?

...

Now you can also see the missing sync pulse. Trigger the oscilloscope with the frame pulse and look at the line pulses. After 31 line pulses one is missing.....


Perhaps a combination of inexperience, misreading the oscilloscope, changing from the 10x probe to the 1x alligator clips mid-flight, and more importantly loose connections is/was the problem -- there are wires and alligator clips everywhere -- time for me to start permanently soldering things, I think.

It took me about half an hour to find and diagnose the problems. I can confirm, though, that the pulses are indeed 12V high. According to my precise measurements, the frame pulse was 0.7223411ms wide :wink: and the line pulse 62us. Not quite what you quoted.

It's incredibly difficult for me to have things running, avoid getting snared in the Nipkow disk, and hold all the pieces in the right place... AND manipulate the oscilloscope. Tomorrow I'll start wiring things such that that is not necessary anymore.

I did try the triggering via the external trigger input, but I couldn't get that to work. I had black clip to ground, red clip to frame pulse... and switched the A triggering to external input. Didn't seem to make any difference, and behaviour was basically the same when I was viewing onscreen. The image quality deteriorated significantly, though, when I had the alligator probe hooked up (that is, red to frame pulse, black to ground). Looked like vertical stripes of white added to the image.
User avatar
Andrew Davie
"Gomez!", "Oh Morticia."
 
Posts: 1511
Joined: Wed Jan 24, 2007 4:42 pm
Location: Hobart, Tasmania, Australia


Return to Mechanical NBTV

Who is online

Users browsing this forum: No registered users and 5 guests

cron