A demux, short for demultiplexer, is a digital circuit that takes a single input signal and routes it to one of several output lines. Think of it like a train track switch: one track comes in, and the switch directs the train down one of many possible tracks. Which output receives the signal depends on a set of control signals, often called selection inputs, that tell the demux where to send the data.
Demultiplexers are foundational building blocks in digital electronics, showing up in everything from computer processors to telecommunications equipment. They’re sometimes called “data distributors” because their entire job is distributing one signal to the correct destination.
How a Demux Works
A demux has three types of connections: one data input, a set of selection (control) lines, and multiple outputs. The selection lines are binary signals, meaning they’re either 0 or 1. The combination of values on these selection lines determines which output is active at any given moment. Only one output receives the data at a time; all the others remain inactive.
The number of outputs a demux can have follows a simple formula: 2 raised to the power of the number of selection lines. So a demux with 2 selection lines has 4 possible outputs (2² = 4). One with 3 selection lines has 8 outputs. One with 4 has 16. This scaling pattern means even a small number of control lines can address a large number of outputs.
For example, imagine a 1-to-4 demux with 2 selection lines. If both selection lines are set to 0, the input data goes to output 0. If the selection lines are set to 1 and 0, the data goes to output 2. Each unique combination of selection values activates exactly one output line.
Demux vs. Multiplexer
A demux performs the exact opposite function of a multiplexer (mux). Where a mux takes many input signals and funnels them into a single output, a demux takes a single input and fans it out to one of many outputs. The multiplexer is a “many-to-one” device; the demultiplexer is “one-to-many.”
These two circuits frequently work as a pair. In time-division multiplexing, a common technique for sending multiple data streams over a single communication line, the multiplexer sits at the transmitting end and combines several signals into one. At the receiving end, a demux separates that combined signal back into the original individual streams. The mux is the data selector; the demux is the data distributor.
Where Demultiplexers Are Used
Communication systems are one of the most common applications. When multiple data streams are combined for transmission over a single channel (to save bandwidth or reduce wiring), the receiving side needs a demux to split them apart again. This applies to everything from telephone networks to fiber-optic links.
Inside computers, demultiplexers help with tasks like memory address decoding. A processor needs to select one specific memory location out of thousands or millions. A demux can take the address signal and activate the correct memory chip or register, ensuring data gets read from or written to the right place.
Digital signal processing relies on demuxes to separate multiplexed signals into individual data streams for further analysis or conversion. Any system that consolidates signals for efficiency and then needs to separate them again at the other end uses some form of demultiplexing.
Digital vs. Analog Demultiplexers
Most explanations of demuxes focus on digital signals, where the data is a stream of 1s and 0s. But analog demultiplexers also exist. These use analog switches, typically built with CMOS technology, to route continuously varying voltage signals rather than just binary data. Analog mux/demux chips can transfer both analog and digital signals, since their internal switches pass signals bidirectionally. This makes them versatile in applications like audio routing, sensor data collection, and test equipment where you need to switch between real-world signals rather than digital data.
Dedicated integrated circuit chips serve as off-the-shelf demultiplexers. Texas Instruments, for instance, manufactures the CD74HC4051, a widely used analog multiplexer/demultiplexer available in standard, automotive, and military-grade versions. The 74-series logic family includes several common demux chips that engineers can drop directly into circuit designs without building the logic from individual gates.
How Size Is Described
Demuxes are typically described by their input-to-output ratio. A “1-to-4 demux” has 1 data input and 4 outputs, controlled by 2 selection lines. A “1-to-8 demux” has 1 input and 8 outputs with 3 selection lines. Larger demultiplexers can be built by cascading smaller ones together, so a pair of 1-to-8 demuxes can create a 1-to-16 configuration.
The selection lines always follow the 2ⁿ rule. If you know how many outputs you need, count up in powers of 2 to find the required number of selection lines. Need 16 outputs? That’s 2⁴, so you need 4 selection lines. Need 32? That’s 2⁵, requiring 5 selection lines.