Level Indication and Metering
Some amplifiers and the more sophisticated mixers are fitted with a ‘signal-present’ indicator that illuminates to give reassurance that a channel is receiving a signal and doing something with it. The level at which it triggers must be well above the noise floor, but also well below the peak indication or clipping levels. Signal-present indicators are usually provided for each channel, and are commonly set up to illuminate when the channel output level exceeds a threshold 20 dB below the nominal signal level.
A vital design consideration is that since it may be active most of the time, the operation of the signal-present detector must not introduce distortion into the signal ...