statistical properties of a signal are used to distinguish between a voice signal and an &# 34 ; on hook &# 34 ; indication on a low bandwidth digital data link . noise is intentionally added to the voice signal to guarantee a minimum power level , while not disrupting normal demodulation . a digital signal , having a known maximum power level upon demodulation , is used to represent the &# 34 ; on hook &# 34 ; condition . upon demodulation , the minimum power level of the noise - colored voice signal is greater than the maximum power level of the &# 34 ; on - hook &# 34 ; signal , and the two signals can be distinguished by a power level threshold detector . this apparatus and technique is particularly well - suited to under - sampled , such as cvsd , data links , which are usually noisy and distorted . it alleviates the need for direct access to the digital link and simplifies the required hardware and retrofit modifications .