Stupid Yamaha DJX-IIB tricks

[Grant Richter]

Yamaha has very cleverly issued the CS1X synth engine in a custom format for
US$219.

A Cakewalk Studioware panel for the unit can be downloaded at
http://www.abstractreality.com/djx

The DJX-IIB unit is targeted at the toy market, but the sound quality is as
good as a CS1X. All my DJ friends are highly offended by this unit, comments
like "That's cheating", "they ripped off every DJ from the last 20 years"
and "I want to puke" are typical reactions from "serious" DJs.

This wholesale appropriation of folk technology by a corporate giant demands
revenge by hacking into the unit and turning into something "they" never
intended, thus following the Cyberpunk maxim "that the street finds it own
use for things".

Opening of the unit and modifying it will surely void your warranty!

The unit consists of an enclosure (about the size of a turntable) containing
a battery powered stereo amplifier and two speakers. The sound is pretty
good for a dinky unit, speakers are ported for reasonable bass response. The
stereo power amplifiers are on a separate PCB subassembly and have an input
impedance of 5K.

The front panel PCB subassembly contains a stereo volume pot and stereo 3
band analog isolation filters. A dual op-amp summer would allow inserting
external audio after the isolation filters, thus allowing the unit to be
used as a stereo practice amp for other battery powered equipment. Handy for
people "off the grid".

The front panel also contains what appears to be a small microprocessor
powered from +5 volts. The built in effects section panel controls are 0-5
volt signals from the effects "tweek" and "wet-dry balance" potentiometers.
Also the effects on-off switch can be paralleled by a 4016 powered from +5
volts. The pattern select switches can be controlled in a similar way. The
"turntable" is an encoder, more difficult to interface a voltage to.

The actual XG synth engine is a DSP powered from 3.3 volts. The filter
resonance and frequency controls appear as 0-3.3 volt signals at pins 1 and
2 of the CD4051 A/D multiplexer. The "Pattern/Scratch" balance control is a
0-3.3 volt signal that appears directly at a DSP pin near resistor R101.
The DSP functions from a 8.64 megaHertz crystal. An external clock from a
HC4046 could be injected and take over clocking control. How far from the
8.64 Mhz target you could deviate from has not been tested.

In summary, the Yamaha DJX-IIB "DJ Box" can be modified to act as a voltage
controlled synth module by the use of appropriately scaled external control
voltages. The internal control voltages can be intercepted at the
potentiometer wipers and mixed, using op-amp summers, with external control
voltages. The summing amp outputs must be hard limited to the appropriate
0-5 volt or 0-3.3 volt range or the DSP and/or micro may be killed.

The following controls could be modified to accept external CV inputs.

Filter frequency and resonance
Effects tweek and wet/dry balance
Pattern/Scratch balance
Effects on/off and pattern selects
The flashing display dot on the 1 beat can be brought out for
synchronization
The entire DSP clock could possibly be put under voltage control
External audio inputs may be added