[sdiy] sample and (infinite) hold

[Richard Wentk]

At 21:29 05/01/2005, Scott Juskiw wrote:

>I'm using a polypropylene at the momen., I've tried polycarbonate, 
>polystyrene, they all droop eventually. The S&H output is fed to a 
>quantizer which sets the root key of a repeating sequence. After a minute 
>or so the output will droop so that the quantizer will knock the sequence 
>down a semitone. I'd like the sequence to stay in key for 20 minutes, or 
>20 days, or the full length of a Klaus Schulze piece.

You can do 20 minutes with analogue. You can't do 20 days, or the full 
length of a Klaus Schulze piece. :-)

One thing to consider is circuit board leakage. Some designs put a guard 
track around the hold cap which helps keep the charge in.

If the droop seems more or less constant, the easiest way to compensate for 
it is to add a charge top-up circuit that injects a very small current into 
the cap to make up for any losses. This is an approach that's prone to all 
sorts of errors - it's impossible to build a circuit that will be accurate 
across a full range of temperatures - but it's better than a passive hold.

The digital alternative with an ADC/DAC combination is the only solution 
that can provide infinite hold. But done properly it's certainly not cheap. 
You could probably get away with an 8-bit ADC/DAC combination for key 
voltages.

However - if you have a quantizer you already have this circuit. Can you 
not lock the quantizer output so it only changes within a short window 
triggered by a key on?

Richard