Rhythm generator

[Paul Maddox]

Bill,

>
>Hello,
>    I am currently building a synth with a built-in drum machine.
>So far I have 16 drum sounds that are triggered by a change from
>a logic 0 to a logic 1 (i.e. a change from 0 to 5 volts).
>

sounds cool...

>I'm now designing a rhythm generator to trigger these drum sounds.

uh oh....

>I don't want to simply program the rhythms beat-by-beat because I
>want some degree of "real time control" in the construction of my
>rhythms.
>

yep...

>The first idea I had was:
>Generate a square-wave (e.g. using a 555 timer) at frequency f.
>Put this through a counter to generate square-waves at frequencies
>f/2, f/4, f/8, f/16, etc.  8 bit counters are widely available, 

giving
>me all frequencies from f to f/128.
>Now for each drum sound, select some of these square-waves and
>perform a logic AND on them.  Use the result as the trigger for that
>drum sound.
>e.g.  I select f, f/4 and f/16 for the bass drum.  Take the logical
>AND of these three waves and use it for the trigger line of the bass
>drum (If you're having trouble following this, try drawing a graph of
>the function f AND f/4 AND f/16).  The bass drum triggers whenever
>this function rises from logic 0 to logic 1.
>

yeah, not bad, but limited (or complex) if you wanna do a fill or 
something, lots of discrete logic...

>The "real time control" I refered to earlier was that in this design
>I can flip the switches to modify the rhythm for each drum sound 

while
>it is playing.
>

cool, loads swithcs and LED's

>I thought this idea was pretty good... but:  I have 16 drum sounds
>and 8 frequencies.  This gives 16*8=128 switches to perform the
>selections.  This would make my front panel pretty crowded!
>

yeah, loadsa fun..

>My next idea was to replace the switches by latches (i.e.  chips
>that can each store one 8-bit number).  Then I would just need 8
>switches to select frequencies, and 16 "store" switches
>to update the latch for each drum sound.
>

yeah, possible, Have you thought about crosspoint switches? I did 
(little chips with a 4 by 4  (or bigger) matrix in)

>This is a lot better than before in one way:  the front panel looks a
>lot better!  Unfortunately, I now need 128 chips to store the 

"virtual"
>switch positions.
>

yep...

>Now I'm thinking:  why can't I replace the 128 latches by 1 static
>ram?  The problem with this is that with the latches, you can see all
>128*8 outputs all at the same time.  If you use 1 static ram, you can
>only see one latch output at a time.
>

hehe, getting close....

>Does anyone have any ideas that will help me simplify the design of
>this rhythm generator?
>

take a look at the shaltwerk (by doepfer)... I was planning to do a 
drum machine , only 8 channels, which had 16 steps per pattern, and a 
909/808 style of programming in real time, ie you select the sound you 
want and push the button for that step... it started off with 128 
switches and LEDs, two problems cost and power consumption of 128 
LEDS!!! erk...

I thought about latches and D-type flip flops, but I gave up, and 
started trying to use a uC (micro controller) , I was going to use an 
AVR, this had the advnatge of only needing 24 switches... 16 for the 
bottom and 8 for the channels, you select a channel(s) then program 
the step on the 16 switches at the bottom...
This had the davnatge of makeing it so I could later add pattern 
programming, RAM and so on... but I'm no good with programming...
if you fancy trying to program one I can help with the hardware 
design, but as for the software sorry Im cr*p at it.. I'm pinching a 
friends Midi G-Gate generator (16 channels) and I'll do the 
programming from pro24 on my ickle atari st.

>Thanks very much in advance,
>Bill.
>

hope this has helped a little.
Paul MAddox


 
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