US Patent 11437057 Detection of TV state using sub-audible signal

A computer-implemented method includes receiving, at a microphone of a voice-controlled device, a speech input, generating an electrical signal having a first gain level that is below a gain threshold for audible detection by a user, transmitting the electrical signal to the speaker and detecting, by the microphone, an audio signal that includes a combination of ambient noise and a probe audio signal, wherein the probe audio signal is output by the speaker based on the electrical signal. The method further includes determining a power level of the probe audio signal and determining a state of the display based on the power level of the probe audio signal.