Low cost, low bit rate vocoder ICs

5 September 2007 Analogue, Mixed Signal, LSI

The CMX618 from CML Microcircuits is a new RALCWI (robust advanced low complexity waveform interpolation) vocoder IC developed in cooperation with SPIRIT DSP.

The IC is suited to both wireless and wireline voice applications such as voice scramblers, alarm systems, military field applications and digital professional/leisure two-way radios (DMR/dPMR) to name but a few examples.

RALCWI technology uses proprietary signal decomposition and parameter encoding methods, ensuring high speech quality at high compression rates. The CMX618 offers near toll quality voice performance at very low bit rates, with independent MOS scores indicating that the device generates sound quality comparable with a standard codec running at bit rates above 4000 bps.

The CMX618 includes an integrated voice audio codec, offering a complete analog voice to low bit rate vocoded data function. There are three programmable vocoding rates of 2050 bps, 2400 bps and 2700 bps which, when combined with the embedded FEC (forward error correction), provide an overall bit rate of 3600 bps. Frame length is selectable from 20, 40 or 60 ms without FEC, while 60 or 80 ms can be selected with FEC protected frames. The selectable integral Viterbi decoder can use soft-decision metrics to improve its decoding ability. The FEC can also be used separately, so that data can be decoded/error-corrected, then re-encoded and forwarded on. This feature is particularly useful in voice repeaters, providing optimal voice performance and maximum range.

CML also offers designers the CMX608, a vocoder option without the integrated audio codec. This enables the use of an external audio codec solution. Both the CMX618 and CMX608 are free from any licence and royalty costs, fully meeting the requirements of high performance, low bit rate digital voice applications. They are available in small, low power 48-pin VQFN and LQFP packages. An evaluation kit is also available (EV6180), facilitating quick and easy evaluation. Connections are provided for audio input and output, power supply and a serial bus (C-BUS) header for connection to a host microcontroller.

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