bread & butter sequencer+quantizer revisited (was: Re: [sdiy] using a 4051+counter as a sequencer core)

[anthony]

OK first off I must state that a requirement of this particular project is 
that it cost ZERO dollars for me to build - i.e. I am building it with parts 
I have at hand. I wouldn't begin such a project as building a sequencer with 
quantizer unless I had a pretty good feeling that I could do it without much 
fuss so here goes:

I have 5 coaxial 50kOhm pots (for a total of 10 pots) that have these really 
awesome solid aluminium knobs. 5 is sort of a wonky number to have, but a 
10-step sequencer is perfectly fine by me. (If I'd had 8 of these pots I'd 
have probably gone for a straight-up clone of the Oberheim Minisequencer, 
but this may end up being more fun & wilder...).

Oh and the knobs also have a DPDT pull-switch, which seems like it'd come in 
very handy to swap CV outs for switching sequence length and the like. I 
think the most ergonomic/aesthetic arrangement would be to have them in a 
circle (pentagon...).

My plan for the CV controls is to have 10 indipendent CV sources (ala Ken 
Stone's joystick controller or something similar) that would be available 
full-time for something else if need be. These outputs would also be mixed 
together for a combined CV output but being one at a time via the sequencer. 
So I'd need some analog switches here. I have all kinds and all manner of 
such beasts and this part will work itself out. (Ken Stone's Sequential 
Switch circuit has provided me with a lot of fresh ideas.)

*** MAIN QUESTION FOLLOWS ***

RIGHT NOW the beast I am working on is the quantizer. I started with Scott 
Stite's cool design for a quantizer using a counter and a 12-bit DAC and 
some LF398 S&H circuits, but naturally I didn't just have an AD7245 lying 
around & no 4516's either. But I did have 2 4520's and 2 AD7524's and 2 
AD7528's. I haven't quite wrapped my brain around the necessity for a 12-bit 
DAC when not all of the bits are used. Something about the precision? I also 
have not wrapped my brain around how to hook 2 8-bit AD7524's up to get a 
16-bit DAC. Would that get me the precision I need? Do I just connect the 
RFB pin on the first DAC to the REF pin on the second DAC?I thought about 
trying just a 4520 and only one AD7524 and see how that worked for me. Also 
would it be pointless to use synchronous 4520 sections cascaded in 
ripple-mode (ie would I really be that much better off using 4516's?). It's 
really pointless to worry about that at this point because I am going to use 
them nonetheless. They do seem like a good candidate even though they're 
up-only - I only need one unit as well while with the 4516 I'd need 2. The 
datasheet shows a way cascade them synchronously, negative-edge triggered, 
but the parts count would be increased by 2: a 4012A of all things and a 
4071. I have several 4071's and I suppose I could make half (all I need) a 
4012 with a 4011 I think. I have the room on the protoboard that I'm using 
so if it'd make that much diffrence. But really it seems a bit of a hassle.

The S&H circuits would be a variation on a few of Ray Wilson's awesome 
designs using FET's and op-amps (I have a lot of TL082ACP & TL084ACN which 
would be good in the offset department). I'm really not too worried about 
troubles popping up here. I eventually plan to have channels for each stage 
of the sequencer - on other boards. I have even toyed with the idea of each 
VCF and VCO have its own channel on the quantizer - just using the same 
stairstep. I'd even like to be able to indiviually vary how much the 
quantizer influences each part by changing the gain at the S&H stage or 
something. I'll probably invest in a lot of LF398's for that part though 
(but I am not at the stage yet!)

I should add that this is for my guitar synth which has largely been 
inspired by a free VST soft-synth plug-in that e-phonic makes called 
Lander - but with a lot more features and controls added. When Juergen 
Haible's Matrix FX hit the scene that became a huge influence as well. My 
intention is for to to simultaneously control onboard effects via normal 
audio guitar P2V and envelope follower inputs and also provide some basic 
accompaniment. The sequencer may do bass-lines or be the basis for 
fluorishes ranging from mild to wild.

Now if I can fit it all into a 19"x16" rack space then YAY!


aa