sk00 power supply
Here’s the schematics and the board for the power supply. I have not built it, yet, so I have no idea if it works as intended. Originally, we wanted to supply up to 2 Amps per channel, but when I started to order the parts, I found out that the LM78xx series of voltage regulators can only supply 1A. For starters this is way enough, but someday we’d maybe want to change the design, using a LM350 or something? I also didn’t include HF-coils at the moment, maybe critical modules should have their own coils ? Or should we straight things out more in the power supply ? We’ll see how this develops when there are more modules connected…
Some final words
We all have to move to more serious stuff again, so that’s it for the moment:
sk01-sync-splitter, sk02-resyncer, synkie-frontpanel
We’ll certainly continue at the “Makers Forschungswoche” in Fribourg in April. Some things have become clear already:
Change the bus layout
Instead of a monolithic bus, where most of the signals are used by few modules, we’ll move to three busses:
- Power: -5V | GND | +5V | VCC
- Sync: H-Sync | V-Sync | Backporch | Odd/Even
- Digital: A four wire digital bus, the specs are to be determined.
Interconnects will be standard 4 x 1 pinheaders with ribbon cables between bus and modules. The sk00-supply module already reflects these changes.
The video-sync signal (genlock) should be treated like any video signal and be carried out on a front cinch connector, so we can fool around with it too, or feed it to other devices like mixers or surveillance cameras as a genlock signal.
LM1881 signals
As you can see in the waveforms below,we’ve got two problems with the output of the LM1881 sync separator chip that we have to clean up:
There’s a small gap between the H-sync and the backporch pulses, that gives us spikes in both the picture and the sync signal. The H-sync comes a bit late, we’ll can’t do anything about that, but the second problem is that the backporch impulse is way too short.
H-Sync and backporch combined
Here we can see the backporch before it is fed to the NAND gates. They seem to trigger immediately. We’ll have to slow the ramp or maybe have a look at the LM1881 datasheet again…
LM1881 Backporch output
Black level
I’m still not really sure where we want the black level. Right now it seems that black is at 0V, with the sync pulses going below 0. We also had to turn the potentiometer for the black level on the sk01 module all the way down to zero – it doesn’t serve a purpose anymore. We’ve seen also the back level move with the brightness of the input picture from our camera. The signal seems to be ac-coupled but we sure want to have a clear reference. Next thing I will build a module around the Maxim MAX7450 chip, a “Video-Signal Conditioner with AGC and Back-Porch Clamp”, which we happen to have lying around. Let’s see if we can get this straight with a simple additional module.
I’m impatient to continue theses explorations. Thanks as well to Max who did all the work, while I was mostly blogging ;-), and to Flo for all the helpful inputs.
