Single-channel refers to a type of universal constructor discovered and developed by Simon Ekström and others starting in December 2015. The initial elbow operation toolkit was near-minimal, with just one push, one pull, and one output glider of each color (see color of a glider). Later searches produced a much larger and more efficient library.
Single-channel recipes consist of a stream of gliders on a single lane and aimed at a construction elbow, usually separated from each other by at least 90 ticks. In spite of these strict limitations, single-channel recipes can be made to do surprising things. For example, it is possible to build a Snark directly on the construction lane of an active construction arm, starting from a single elbow block. This can allow the arm to reach efficiently around complex obstructions by bending itself through multiple lossless elbows. Known recipes can also remove an elbow when it is no longer needed, by controlled demolition of the Snark.
As of the end of 2017, almost all single-channel recipes are made up of singletons and synchronized pairs of gliders, but no synchronized triplets or larger groups. This is not an inherent limitation of single-channel construction, but rather a limitation in the search program used to find currently known single-channel toolkits.
A useful byproduct of this limitation is that single-channel recipes can be trivially adjusted to allow them to safely cross perpendicular data streams, including other single-channel recipes (or earlier parts of the same recipe). To avoid collisions with a crossing stream, each singleton glider or glider pair can safely be delayed by any even number of ticks, or technically by any multiple of the period of the current intermediate target. The final result of the construction will not be affected.