CGS digital noise (first version)

CGS31 the CGS digital noise (ver 1.0) module is a very standard pseudo-random digital noise source with a few enhancements. Instead of running a fixed high frequency clock, a VCO is used instead, allowing for unusual sweeps, and for reducing the speed right down to a series of random pulses. The internal linear VCO can also be bypassed so an external source such as a 1V/Oct. VCO or LFO can be substituted. It has pink and white noise outputs, and two separate (unique) digital outputs for use in triggering other circuits.

While untested, the module should work on +/-12 volts.

How to use this module

Connect the CV inputs to a voltage source, or wind the speed input up to maximum. Take the required noise source from the appropriate output. The two LEDs indicate the activity of the two digital outputs. When an external CV source is used, the speed pot acts as an input attenuator. Adjust this pot for the desired effect. Alternately, feed an alternate clock pulse in to the external clock input.

A little on how it works

The schematic of the Digital Noise.

The circuit consists of several distinct blocks. The first is the internal VCO and its associated parts. From the left, there is a voltage follower then the VCO itself, which is part of a 4046 phase locked loop.

The second circuit block is the timing generator. At the left, there is a comparator based around one half of a TL072. This is there to convert any external clock signal into something suitable for the remainder of the circuitry. With the values given, the sensitivity is set at around 2V, allowing triggering from signals with a +/- 10 volt swing, or with a 0V to +10 volt swing, both of which are common in modular synths.

The output of this comparator is fed into the clock input of the shift register, a 4006.

The exclusive OR gates form a complex feedback path and are responsible for the generation of the pseudo-random pulse train. The 1M and 0.47 mfd capacitor are there to start the sequence.

Several outputs are taken from different points of the feedback path, two being buffered use as digital outputs, the remaining being filtered to produce both pink and white noise outputs.

There are many similarities between this design and the ETI 4600 noise source, as this was developed from my heavily modified ETI unit.

Construction

The component overlay. Connections can be determined from the circuit diagram.

Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges.

When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC sockets if used, then moving onto the taller components.

Take particular care with the orientation of the polarized components, the electrolytics, diodes, transistors and ICs.

When inserting the ICs in their sockets, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay.

It is a good idea to insert a 1k resistor between the +15V supply and the normalized connection on the CV jack. This is because +15 will momentarily be fed out of this jack when you plug something into it, due to the mechanical nature of the contacts.

Parts list

This is a guide only. Parts needed will vary with individual constructor's needs.

Part	Quantity
Capacitors
1n	1
2n2	2
10n	1
15n	1
100n	6
0.47 mfd 25V electro	1
10uF 25V	2
Resistors
1k	6
1k8	2
8k2	1
10k	8
15k	1
18k	1
33k	1
100k	3
1M	2
Semi's
1N4148	2
BC547 (or sim)	2
LM358 TL072	1
TL072	1
4006	1
4030	1
4046	1
LED	2
Misc
Ferrite Bead (or 10R resistor)	2
0.156 4 pin connector	1
CGS31 PCB	1

Notes

• The prototype PCB specifies an LM358 for the input. I have substituted a TL072 here as the LM358 appears to have an insufficient slew rate.
• A 10 to 22 ohm resistor can be used instead of the ferrite bead. If you don't care about power-rail noise, just use a link instead.

See also

• CGS digital noise (previous version)
• CGS digital noise for the current version
• The CGS modules
• CGS parts FAQ

References

• Digital Noise for music synthesizers. (archived) by Ken Stone, 2001, with permission of the author

External links

• CGS Synth discussion group, for discussion of locating parts, modifications and corrections etc.

Suppliers

• Possible sources of the 4006:
  • ABRA Electronics Corp.
  • Banzai Music GmbH
  • Circuit Specialists, Inc.