A Forward Error Correction (FEC) engine provides optimum performance in real-life applications, with the RALCWI vocoder being comprised of four independent functions, which are selectable by the host: Voice Encoder, FEC Encoder, Voice Decoder, FEC Decoder.
The CMX608 and CMX618 are half-duplex ICs, with the new CMX638 offering full-duplex capability. The CMX618 and CMX638 include an integrated voice CODEC, for complete analogue voice to low bit-rate vocoded data function (with integrated channel filters, removing the need for external components). The CMX608 is offered without the CODEC, for customers wishing to use a different CODEC solution.
Encoding: The voice encoder uses a 20ms voice frame size, with 3 programmable bit-rates (2050bps, 2400bps or 2750bps), The optional FEC encoder performs channel coding of the encoded voice (at the rate selected) and forms an encoded, interleaved bit-stream of 3600bps (216 bits per 60ms packet or 288 bits per 80ms packet). The FEC operation utilises a packet of either 3 or 4 x 20ms vocoder frames for optimum error correction performance.
Decoding: The optional FEC decoder performs de-interleaving and channel decoding of the coded bit-stream (216 bits per 60ms packet or 288 bits per 80ms packet) and forms an error-corrected bit-stream of encoded voice at 2050bps, 2400bps or 2750bps, depending on the mode selected. The FEC decoder can optionally use ‘soft-decision’ metrics to enhance its decoding ability. The voice decoder then converts the error-corrected bit-stream back to a digitised voice signal.
For optimum voice performance the following functions are included:
– Soft Decision Decoding (SDD)
– Discontinuous Transmission Detection (DTX)
– Voice Activity Detection (VAD)
– Comfort Noise Generation (CNG)
– Single (STD) and Dual (DTMF) Tones (can be detected, sent separately in the coded bit-stream, then regenerated at the far-end)
Vocoder applications include: Voices storage and playback devices, VoIP, Voice pagers, Digital WLL, Digital PMR/LMR, Voice Radio, Digital Trunking, Regenerative digital voice repeaters, digital voice scrambling and encryption and much more.