Here is the schematic for the colour overlay prototype I have been breadboarding. The next logical step would be to design a proper circuit board. However I’m not happy with it for a number of reasons:
- Too many ICs, too complex
- Not able to support both video standards at the same time (need to select either a PAL or NTSC crystal)
- Doesn’t work reliably with PAL, in any case because the PAL colourburst changes phase on each line and it is a 50/50 chance that the AD724 will change phase in the opposite direction to the incoming signal
- The MC44144 is an obsolete part
I want to explore other alternatives. I have a few ideas. However for the time being I’m taking a break from analog video. My son needs an alarm clock so I’m going to build him one!
3 thoughts on “Schematic”
Hi, I know this post is 3+ years old but I’m kinda desperate (haha). I’ve been trying to get the MC44144 working on a prototype board and it doesn’t seem to work no matter what I try. Your schematic above looks similar to what I tried with a few key differences:
a) no 75 ohm resistor on the input. the datasheet says to use one but it looks like you aren’t. does one not actually need one?
b) 47 pF capacitor for C13 (I’m squinting) instead of 470pF like the data sheet says. is the datasheet incorrect? I’m willing to consider anything at this point.
In short, I’m wondering if you actually tested this circuit or did you not get that far? I’m really scratching my head about what I’m doing wrong.
Hi there Matt,
It’s been a long time since I looked at this project, so I can’t really comment on the datasheet. I might have mistyped the value of the 47pf capacitor. I can confirm I did test the circuit and had it working on the bench. The MC44144 did not always oscillate reliably, that might be related to differences with the datasheet recommendations or not and it would make sense if I really did use 47pf instead of 470pf. My advice would be to experiment!
Just a question, what are you intending to make?
I’m working on a very niche project to help preserve a specific set of old 80’s arcade games. I’ve been interested in preserving my childhood memories relating to laserdisc arcade games for 20+ years and have a few mature projects related to this: http://www.daphne-emu.com and http://www.laserdisc-replacement.com . Most of the 80’s laserdisc arcade games were designed so that the game would retrieve sync from the laserdisc player. However, three of the arcade games were designed so that the game would generate sync and the laserdisc player would sync up to the sync provided by the game. My laserdisc replacement solution must generate its own sync, so I am trying to modify the game hardware so that instead of generating its own sync, it is getting the sync from the laserdisc player (via composite NTSC).
Now you may say “Why do you need the subcarrier clock? Why not just use an LM1881 and be done with it?” This may be the route I eventually have to go if I can’t get the MC44144 working, but I spent some time reverse engineering the behavior of the original sync generator chip. It’s a Sony CX 773A. I was able to deduce that internally it operates off of a 14.3 MHz clock and its sync output is basically just some simple counters and other logic gates. So in order to make a “perfect” replacement for this sync generator chip, I need a 14.3 MHz clock that is in sync with the incoming NTSC signal. I’m hoping that by using both a LM1881 and MC44144 together that I can re-create the “exact” sync pattern from the original Sony chip. It may not be necessary to get the game working with a replacement laserdisc player, but it would give me a sense of completeness that I made a “perfect” solution. This naturally relies on the MC44144’s 14.3 MHz output being stable. If it’s not, then I probably can’t use it.
You can find my reverse engineering notes here: https://www.daphne-emu.com:9443/mediawiki/index.php/BegasBattleNotes#Sync_Generation
And you can find my current prototype schematic and board layout here: https://forum.allaboutcircuits.com/threads/how-to-sync-to-ntsc-color-burst-3-58mhz-sine-wave-that-only-lasts-for-9-cycles-every-63-microseconds.187231/