An encoder includes a signal detection circuit and a signal encoding circuit. The signal encoding circuit is configured to encode the Nth-frame downmixed signal when the signal detection circuit detects that an Nth-frame downmixed signal includes a speech signal, or when the signal detection circuit detects that the Nth-frame downmixed signal does not include a speech signal, encode the Nth-frame downmixed signal when the signal detection circuit determines that the Nth-frame downmixed signal satisfies a preset audio frame encoding condition, or skip encoding the Nth-frame downmixed signal when the signal detection circuit determines that the Nth-frame downmixed signal does not satisfy a preset audio frame encoding condition.