http://www.stanford.edu/~dgi/university/
"The idea surfaced in 1997 when I first hooked up a static RAM to a
digital to analog converter. Could I make a record player out of it? I
wanted to put audio into memory and then play it back like a record
player by turning a disc. The problem is, static RAM is expensive and I
needed a lot of it.
I thought of using a magnetic tape, but that was daunting mechanically.
The idea resurfaced last year while I was building some stuff using
microcontrollers, but the bandwidth was just not there.
A digital signal processor seemed to be the answer but the cost of the
development kit was $3000. Fortunately, thanks to a generous "donation"
from "corporate sponsors" I obtained the development kit and a DSP
board. I was finally going to finally make something. Although I had
never used a DSP before, never burned an EPROM, never written any
assembly code, I knew that something interesting would come out of it.
Hacked the assembly code out of a digital delay routine. Made up an
encoder circuit and hooked it to some general purpose IO pins. The
kernel of the interpolation algorithm was a little tricky but I first
simulated it in Mathematica and then listened to the sound of each
iteration on the code till it was smooth. Replaced the board's ROM to
make it believe it was a record player.
Made a first prototype out of laser cut MDF. Encoder wheel laser printed
and glued on. Stepper motor for precise speed control. But I found that
the phase was locked - couldn't be pushed - with a stepper, and torque
was lacking. I needed a variable speed feedback controlled DC motor and
I needed it fast. I'm leaving school and won't have access to my shop
next week.
Ebay came through with a couple of beat up DJ turntables. (Not DC it
turned out - brushless.) I lazer cut a carrier plate for the DSP board,
fitting it inside the turntable enclosure, a control panel of laser cut
polycarbonate went where the tonearm had been, and I etched a couple of
PC boards for encoders and control panels. The platters had to be cut. I
turned them on a lathe, removing their indicating flanges, for my
encoder to sit right under the platter edge, looking up toward the
record, held to the chassis in a CNC machined Delrin optical shroud.
The record itself was first made of ABS, then styrene was tested. But a
laser cut poster (left over from a lecture series announcement) worked
best. The paper's wax coating is lubricious, making it slide well over
the slipmat and giving good finger sticktion. Also it is light enough to
slide, stiff enough to maintain encoder spacing and floppy enough not to
bind on the spindle, although the graphics annoy me.
The input and output are line level analog stereo audio, like that from
a CD player or mixer. The codec is 48 Khz, times 16 bits times 2
channels (stereo) times 20.2 seconds = one million 32-bit words in
memory. But my next one could do 20 minutes with a bit more SDRAM. All
the electronics could be made for about $100 in quantity 100.
The DJs were pretty excited. I talked to the DJs who frequent my machine
shop, Eto and Daniel, and brought the thing over to KZSU to play with it
where Johnny Afro took it for a spin. The response was 1 I want one 2
You should patent it 3 You're going to make a lot of money.
Well, the patent is already taken by a Swiss guy in 1997- coincidentally
the year that I also worked out the idea. Coincidentally, this man chose
the same encoder line pitch as I did, and put the optical encoder in the
same place as I did - without knowing of his design.
Altough patented, it seems that this configuration of
sampler-looper-turntable does not yet exist. There are samplers with
turntables but no motor. There are turntables with motors and encoders
but they require a separate laptop computer. Maybe I should make more of
these. What will happen next?"
---------------------------------------------------------------------
To unsubscribe, e-mail: idm-unsubscribe@hyperreal.org
For additional commands, e-mail: idm-help@hyperreal.org