Voice Channel Unit (VCU)

The VCU Card is a plug-in card that facilitates voice communication between sites. The terrestrial telephony interface supports 2- or 4-wire E&M, FXS, and FXO analog voice interfaces.

For voice communications, the channel unit provides the telephone interface to a 2- or 4-wire circuit with interchangeable plug-in daughtercards for other types of interfaces. The analog telephone channel is then digitized utilizing PCM coding. A Digital Signal Processor (DSP) onboard the channel unit provides echo cancellation and voice compression functions. The DSP also controls the voice activation feature of the channel unit by detecting the presence of speech in the channel.

Once the voice has been compressed, the signal enters the transmission modem portion of the circuitry where Forward Error Correction (FEC) is applied and removed and the signal is QPSK- or BPSK-modulated for transmission. The modem supports QPSK and BPSK modulation at information rates of 32, 16, or 9.6 kbps with rate 1/2 or 3/4 coding. Each channel unit incorporates a voice activation feature. This feature provides end and beginning of speech messages, which communicate the presence or absence of speech to the modem at the other end of the conversation. When the far end receives an end of speech message it removes its carrier and holds the receiving modem at its last frequency so that reacquisition can be accomplished quickly. When the end of speech message is received, the outgoing telephone channel is muted. When voice activity returns a beginning of speech header is transmitted to the far end in order to allow lock up of the receive modem and to initialize the FEC decoder. Once those two functions have been accomplished the channel is again opened for voice traffic.