[sdiy] TR-7x7? (was Re: Proms)

[Dan Gendreau]

> > There is a simple R/C analog envelope generator for each drum
> tone except
> > for CC/RC. CC/RC are output directly from a 6bit DAC with no
> envelope, which
> > explains why the 707 CC/RC sound kinda cheap. All other drum sounds are
> > multiplexed through an 8bit DAC.

Another correction here:
I looked at the schematic again more carefully, the 707 RC and CC DO have an
envelope, I missed on the first look.


> 	This is *great* stuff!  I haven't used my TR-707 in months,
> specifically because of the difficulty in using the sounds in a mix...
> I'm a total novice when it comes to digital circuitry tho; what is the
> significance of the 8bit or 6bit DAC - can they be easily converted to a
> higher-quality DAC, if the pinouts are similar, or is the circuit
> dependant on the output of that particular DAC?

Well, the CC/RC use a simple resistor array as a DAC. There are only 6 bits
available. However, there is no reason why it couldnt be upgraded to 8bit.
The ROM is 8bit and the Latch after the ROM is also 8bit. It would just be a
matter of putting in an 8bit DAC or an 8 resistor array(a DAC is more likely
these days).

> I know of the
> modification to add pot-control of the decay on the open/closed hats,
> but are you saying that *all* the sounds ('cept the CC/RC) use
> individual VCA's?

Well, not VCAs per-se... There is an R/C envelope generator for each drum
and you could probably control the decay rate of each. The drum samples are
multpilexed through a single 8bit DAC into Seek/Hold buffers, but its a
multiplying DAC, so the envelopes are also multiplexed into the DAC... So in
a way, the DAC acts as a VCA for each drum sound.

Its an odd arrangement, but it works. After the S/H the sound is then
lowpass filtered and sent to the individual or mix output.


> > Ideally, It would be cool to substitute in a larger rom with
> many banks to
> > chose from. Better yet, use a uC to download sounds to SRAMs
> via Sysex! But
> > alas, I havent had the free time to tackle any of that yet. :)
>
> 	This is *really* an exciting idea!!!  Is something like this even
> possible?!?  If this could be created as a kit, I'm certain many folks
> would be interested, regardless of price!!  I'm afraid my level of skill
> doesn't really offer much towards a project of this complexity, but I'd
> love to be a part of it...

Yes, its technically possible. All it would take is an AVR chip and 3 SRAMs
on a board along with a bit of glue logic. The microcontroller would listen
to the incoming midi data for certain sysex codes and when it recognizes
some proprietary sysex code it could dump the sysex bytes into one of the 3
SRAMS. It would take about about 48 seconds to download  a complete rom via
MIDI. The same SRAMs could be set up to mimic the TR707 voice ROMs.

I dont have time to start another project,
-Dan G.