Moderators: Dave Moll, Andrew Davie, Steve Anderson
Steve Anderson wrote:Harry, here's the second part of the timebase circuits.......
Steve Anderson wrote:I forgot to mention, if you need a negative-going ramp, swap pins 2 & 4 on the DAC-08, though this could also be done in the deflection amps. Also an arrangement for swapping line/frame between vertical/horizontal. More later...though it is quite simple.
Steve Anderson wrote:Yep the two 4040s generating the ramps (IC201/203) will be reset using external sync detection circuitry via the MR input, pin 11. Pin 10 is the clock input from the precious circuit via the two switches. That won't change.
Now, I'm aware of the NBTVA standard sync arrangement
but all the other modes I have no idea about beyond the line and frame rates.
[/quote]How do you propose doing line and frame sync apart from the NBTVA standard 32-line, 12.5Hz system? As far as I know there are no standards for these other formats.
Steve A.
Steve Anderson wrote:Yes, both pin 10 & 11 will be required in both a camera and a monitor. The arrangement on pin 10 will be the same for both, though you could delete the switching if you want a single-standard arrangement.
Pin 11, the MR, will be needed in a monitor to synchronise the timebases to the incoming video - as usual. In a camera they need to be reset at switch-on and when you change standards to make sure that a vertical sync doesn't happen part way through a line. If this were a single-standard camera you would arrange the logic to ensure this could never happen.
If there is/are standards 'out there' for the other non-NBTVA rates, I'm unaware of them, more correctly those for 64/128/256 lines. 120/240 come from pre WWII times and used some rather odd sync arrangements.
I suggest for 64 and above lines adding a 'proper' vertical sync, not using the missing pulse system. You lose one line, but you don't really notice it. Plus adding front and back porches (blanking) to the signal to eliminate sync-width modulation by the video. The more resolution you have the more you notice it.
Steve A.
Steve Anderson wrote:Quite frankly you're just wasting your time if they're not HC, don't bother. you'll get a whole load of nonsensical outputs. Which it appears you have.
When all of the HC parts arrive you have a fighting chance. Quite simply MC/HEF cannot run at the frequencies required. End of subject.
Steve A.
ppppenguin wrote:They may be hidden but I can't see any decouplers on that board. Old, slow 4000 series CMOS will work more or less whatever you do but HC/HCT must have decent decoupling. I automatically fit a 100n ceramic cap right by each device. This is probably overkill but it give peace of mind. Plus an electrolyic or 2 on the board as a whole.
A little story. Over 30 years ago I designed a high resolution graphics framestore board. A monstrous thing with 128 of the then current 64kilobit DRAM chips. 100n across each chip as a matter of routine. I think I put 10uF at the end of each row and thank goodness I left space for a big capacitor on the board too. The way the design worked all those chips did their refresh cycle at the same time causing the supply to dip by too much. I think I needed a 4700uF decoupler to keep that within sensible limits. That was an extreme case but it shows what inadequate decoupling can do.
Return to The Tasmanian Devil VCR139....A for Andrew
Users browsing this forum: No registered users and 1 guest