Category: Uncategorized

sk540-ramps

Some calculations

time to charge a capacitor:
t = -ln(1-Uc/Ucharge)*RC

Uc = 1V
Ucharge = 5V

so we have t = 0.22314355 * RC

horizontal ramp: 64uS -> 0.0000064 = 0.22314355 * RC
RC = 0.00028681088753
47nF, 6.1 kOhms
vertical ramp: 20 ms (1 field) -> 0.02 = 0.22314355 * RC
RC = 0.08962840235449
10uF , 8.9 kOhms -> 10k pot

does not work: ATTENTION the 4053 can have up to 2k5 Ohms ON resistance. we have to discharge the cap in max 9uS – length of h-sync + back porch
0.000009 = 0.22314355 * 2500 *C -> 16nF max
4n7/61k

also: the v-ramp we want to charge only on h-sync, so that’s 313 pulses of 10.4 uS, total charge time of 3.255 ms. Discharge in max 0.5ms.
discharge: 0.0005 = 0.22314355 * 2500 *C -> max 890nF
charge: 0.003255 = 0.22314355 * R * 100nF, R = 145.8k

Linear Technologies 1251 and 1228

This monday I just got some samples of the LT1251 – 40MHz Video Fader and DC Gain Controlled Amplifier and of the LT1228 – 100MHz Current Feedback Amplifier with DC Gain Control and I got really excited, because that’s probably two IC’s we’ve been looking for for a while. Although we have our own discrete VCA module that seems to work quite nicely, an integrated video capable VCA could open a lot of new possibilities… Building a 6×4 matrix mixer would take 24 sk10 modules, for example. I would also love a voltage controlled version of the sk30 gamma module, and for VCF’s an integrated VCA seems almost always necessary. We tried our hands on the Rohm BA7655A Video VCAs, they are nice but need to be AC coupled, which defeats the purpose to be able to interchange CV and video signals. Our current 6-channel mixer built on these chips also seems to have some serious phase shifting issues (which could also stem from the really wild pcb layout I did, but anyway…) I really start to dislike the rohm vcas – to really be able to fade to black you need to put both in series, which means another ac-coupling stage, and the whole mute-if-cv-is-1V shit means a lot of control circuitry around it to prevent them to go to white, because when they’re muted they don’t output black but white! who wants something like this, anyway?