demultiplexer

A demultiplexer is a device or circuit that takes a single input signal and routes it to one of several output lines based on a control signal. It performs the inverse operation of a multiplexer. Where a multiplexer combines many signals into one, a demultiplexer separates one signal back into many. This is essential in telecommunications, data transmission, and instrumentation where multiple channels must share a single transmission line.

In digital electronics, a demultiplexer typically has one data input, several control inputs (called select lines), and multiple outputs. For example, a 1-to-4 demultiplexer has one data input and two select lines, allowing the input signal to be routed to one of four output lines. The select lines work as binary addresses: with two select lines, you can address 22 = 4 outputs. With three select lines, you can address 8 outputs, and so on. At any given moment, only one output line carries the input signal; the others remain inactive.

Demultiplexers appear in several forms. In analog signal processing, an analog demultiplexer uses switching circuits to route a continuous signal to one of several output channels. In digital systems, demultiplexers are constructed from logic gates and are often built into larger integrated circuits. Software demultiplexers separate interleaved digital streams in protocols like time-division multiplexing (TDM), where multiple channels are encoded sequentially into a single data stream.

Common applications and failure modes

Demultiplexers are standard in serial-to-parallel conversion, audio routing, video switching, and telecommunications network equipment. They allow a single high-speed line to feed multiple lower-speed devices. In practice, problems arise from signal timing errors, where the input changes while a select line transition occurs, or from crosstalk when output lines are not properly isolated. Propagation delay through the demultiplexer, measured in nanoseconds for digital types, can cause timing violations in fast systems.

The term reflects its function: the prefix 'de-' indicates reversal, and 'multiplex' means many signals over one channel. Demultiplexers are inverse complements to multiplexers and are almost always used in paired systems; wherever signals are multiplexed for transmission, they must be demultiplexed at the receiving end.