The A-129/5 is a detector which can recognise voiced and unvoiced sections in the speech signal (by analyzing the frequency content), and switching the carrier signal accordingly.
The incoming speech signal is processed through a pre-amplifier with adjustable gain and a treble boost unit (which improves the vocoder effect). The voiced/unvoiced recognition system controls a voltage controlled switch (like an A-150) which is used to switch between the voiced and unvoiced carrier signal (e.g. VCO and Noise). Additionally the voiced/unvoiced information is available as a gate signal. An LED displays the unvoiced state. The A-129/5 can also be used as a frequency selective switch (switch frequency is 1.5KHz) and as a pre-amp.