[sdiy] Sequencers anyone?

[Tom Wiltshire]

On 13 Jul 2008, at 04:06, Paul Perry wrote:

> The key is, PCB mounted faders.
> Or, if you use pots, mount them in rows along the edges of PCBs
> You can even do this with vero.
> (otherwise use snap-in type PCB mounting pots).
> Preserves your sanity & more reliable as well.

I absolutely agree. If you try and wire individual pots to the board,  
you'll spend far to much time on it to ever be able to offer it for  
the sort of money you'd like to. To keep it cheap, you really need to  
minimise the construction as far as possible - PCB mounted pots and  
jacks are the way to go.

Otherwise, go for it.

Someone mentioned the 10bit ADC on PIC/AVR chips. Although this might  
seem like a limitation, you can't actually set a standard pot more  
accurately than that anyway. Just consider; the scale on a typical  
knob might be 8cm long, so 10bit resolution means 128 steps per cm,  
or a resolution of nearly 13 steps per millimetre. I've yet to meet  
someone who can set a pot to better than 0.1mm just by turning it.  
Even if you did manage, you'd have to hold your breath, hope your cat  
didn't sneeze, and hope no trains were going by locally, since any  
vibrations will shift the value at least that much.
Remember that the Prophet 5 used 7-bit control resolution, and  
Sequential still had to use software hysteresis to eliminate unwanted  
drift in the control values being picked up by the uP as an edit.  
This problem gets worse as the resolution goes up.

Looked at another way, let's assume that the inputs are reading a  
0-5V CV, representing 1V/Oct. This gives us 1024/5 readings per  
octave, or 1024/60 readings per semitone. This gives us an accuracy  
of 17 steps per semitone, which is about 6 cents, the limit of  
perceivable pitch change. I don't know that I'd regard this as quite  
good enough (I'd want a margin of error in there) so I might choose a  
scheme that allowed the knobs to set pitch over a range of an octave  
or two (thereby improving the resolution per semitone) combined with  
some kind of 16' / 8' / 4' range selector.

Hope this helps,

T.