Multiplexing and synths

In your DIY synthsizer endeavours the topic of multipliexers will come up.
They are basically multiple input & single output switches.
(or single input, multi out ).
They are useful in synths, as you can control them with voltages.
On a macro level, a sequential switch module  is a good example of a multipliexer.
Below is a Doepfer 151 which is such a module. 

It allows you to route one source to one of many destinations
or route one of many sources to a single destination.

You can use the 151 to route the 3 eight step sequencers into different combinations.
Setting the toggle switch to 2, 3 or 4 steps will change the sequence length.

On a micro / integrated circuit level there are a number of decent ICs which are good to keep in your box of tricks. They are often referred to as a Mux or a demux (demultiliexer).
They can be analog, in which case it's very similar to an analog switch; but they can also be entirely digital, built from logic gates.

 A mux is a bit like the mechanical rotary switch above except it's digitally controlled.

Like standard switches, manufacturers often refer to the number of poles & throws when classifying them
EG: The 74HC4067; 74HCT4067 is a single-pole 16-throw analog switch (SP16T) 

 It has 4 inputs (s0, S1, S2, S3).
It has 16 outputs (Y01 to Y15).

A demux is  simply the reverse of a mux. It takes a single input and selects one of multiple outputs. A multiplexer is often used with a complementary demultiplexer on the receiving end.
Chaining multipliexers is commonly done in electronics to construct large multipliexers from smaller ones. These switches have a limited current carrying capacity when you compare them to standard switches, but they have the advantage that you can open & close them with digital logic voltages.


List of Multipliexer ICs
74157     Quad 2:1 mux.     Output same as input given
74158     Quad 2:1 mux.     Output is inverted input
74153     Dual 4:1 mux.     Output same as input
74352     Dual 4:1 mux.     Output is inverted input
74151A     8:1 mux.     Both outputs available (i.e., complementary outputs)
74051     8:1 mux.     Output is inverted input (CD 4051)
74052   Dual 4 channel (two 4 way switches)
74053  Three 2 way switches
x4066  four logic controlled single-pole, single-throw switches in one intergated circuit.
x4067    Single 16-Channel Multiplexer/Demultiplexer (74HC4067)
74150     16:1 mux.     Output is inverted input
74HC4067    16:1 mux
DG 506B     16-channel single-ended analog multiplexer (See NLC 32:1)
DG 408  (DG stands for Double-Gate .... its a CMOS chip)
                8 channel single-ended analog multiplexer. (NLC Statues - build notes)
DG 508    Multiplexer Switch IC.  8:1 CMOS Mid Voltage MUX (NLC Statues - build notes)

The CD405x, CDx4HC405x and CDx4HCT405x devices are digitally controlled analog switches that use silicon gate CMOS technology.
They are bidirectional switches that allow any analog input to be used as an output and vice versa.
My favourite multiplex ICs are the CD4051, CD4052, & CD4053 . You can do a lot with just these.

The 4051 is a single eight-way multiplexer, the 4052 has two four-way multiplexers, and the 4053 has three two-way switches. Below are some examples I've found used in the synth world.

4051 -  (CD4051, 74HC4051, HCF4051...)
            Eight channel multiplexer having three binary control inputs, A, B, and C, and an inhibit
            input. The three binary signals select 1 of 8 channels to be turned on,and 
            connect one of the 8 inputs to the output.
            Fairlight CMI IIx - Switched resistor filter
            Ciat-Lonbarde - Tetrazzi 
            Ciat Lonbarde - Paper Circuit -Terazzi organus
            Buchla 312 Gating module

4052 -  (HEF4052, CD4052, 74HC4052, MC14052, MC4052...)
            Dual 4 channel multiplexer, having two binary control inputs, A and B, and an inhibit input.
            The two binary input signals select 1 of 4 pairs of channels to be turned on and connect 
            the analog inputs to the outputs.
                                                    (NLC 4SEQ) 
                                                       (Null A)
                                                      Bindubba sequencer
                                                      Vectrex Video Game 
                                                     Ciat Lonbarde - Master Brando - Paper Circuit 

4053 -  (HEF4053, CD4053, MC14053, 74HC4053...
                The CD4053B is a triple 2-Channel multiplexer having three separate digital 
                control inputs, A, B, and C, and an inhibit input. Each control input selects one 
               of a pair of channels which are connected in a single-pole, double-throw configuration

List of De-muxiplexers
74139         Dual 1:4 demux.     Output is inverted input
74156         Dual 1:4 demux.     Output is open collector
74138         1:8 demux.     Output is inverted input
74238         1:8 demux.    
74154         1:16 demux.     Output is inverted input
74159     CD4514/15     1:16 demux.     Output is open collector and same as input

Buchla 312

NLC 32:1

Driscoll

Links
+ CMOS & Lunetta synths
+ DIY index
+ Multiplexing for beginners (Youtube)
+ nONLinearCircuits 32:1 - build notes
+ NLC. Statues - build notes
+ NLC Router
+ NLC Chopper

If you find any mistakes, or omissions please let me know.
I'll update the article accordingly. Happy New Year.