2.1 Channel
A nominal configuration consisting of two main channels (left and right) and a low-frequency effects (LFE) channel; in practice, systems that combine stereo audio without a dedicated LFE channel with a subwoofer are also often referred to as 2.1.
Explanation
2.1 声道 In a strict program format, this refers to two main channels (left and right) plus a low-frequency effects (LFE) channel. The `.1` after the decimal point indicates a bandwidth-limited effects channel, rather than counting the subwoofer as one-tenth of a channel.
Consumer devices also commonly refer to two main speakers plus a subwoofer as 2.1, even if the input is only 2.0. In this case, the crossover extracts low frequencies from the left and right channels and sends them to the subwoofer; this is a form of bass management and does not generate independent LFE content out of thin air. Program channels, amplifier outputs, and the number of physical speakers must be described separately.
A true 2.1 stream can preserve dedicated low-frequency effects in the LFE channel; when downmixed to 2.0, whether and to what extent the LFE is included is determined by the format, metadata, and device policy. In music releases, “2.1” is sometimes used to denote a mix with a dedicated low-frequency channel, but its purpose and crossover cannot be determined by the number alone.