CMX608 datasheet(13/70 Pages) CMLMICRO

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RALCWI Vocoder

CMX608/CMX618/CMX638

 2014 CML Microsystems Plc

D/608_18_38/11

General Description

The CMX608/CMX618/CMX638 are a pin-compatible family of near toll quality voice encoders/decoders

(Vocoders). The voice encoding/decoding is performed by a new, low data rate algorithm which is based

on Robust Advanced Low Complexity Waveform Interpolation (RALCWI) technology. The data rate is

selectable between 2400bps or 2750bps (all without FEC) or 3600bps (with FEC). The frame length is

selectable from 20ms, 40ms, 60ms or 80ms (all without FEC) or from 60ms or 80ms (with FEC). The

integral Viterbi FEC decoder can optionally use "soft decision" metrics to improve its decoding ability if the

signal can be applied in a 4-bit digital representation of the received/demodulated analogue signal. The

FEC can also be used on its own, so that data can be decoded/error-corrected then re-encoded and

forwarded on. This allows use in a digital voice repeater.

RALCWI technology uses unique proprietary signal decomposition and parameter encoding methods,

ensuring high voice quality at high compression ratios. The voice quality of RALCWI-class Vocoders, as

estimated by independent listeners, is similar to that provided by standard Vocoders running at bit rates

above 4000 bps. The Mean Opinion Score (MOS) of voice quality for this Vocoder is about 3.5-3.6. This

value was determined by a paired comparison method, performing listening tests of developed and

standard voice Vocoders.

The RALCWI Vocoder ope

rates on a “frame-by-frame” basis. The 20ms source voice frame consists of

160 samples of linear 16-bit PCM sampled at 8kHz. The Voice Encoder performs voice analysis at the

high time resolution (8 times per frame) and forms a set of estimated parameters for each voice segment.

All of the estimated parameters are quantized to produce 48- or 55-bit frames, using Vector Quantization

(VQ) of different types. All of the vector quantizers were trained on a mixed multi-language voice base,

which contains voice samples in both Eastern and Western languages.

Voice parameters are coded by the Voice Encoder with high efficiency. Sensitivity to errors is estimated

for each output bit and a bit frame is split into two parts: “more sensitive to errors” and “less sensitive to

errors”. To protect the voice parameters against channel errors without reducing voice quality, the “more

sensitive” bits are protected. Such protection is provided by the FEC Encoder/Decoder mechanism. The

Voice Decoder de-quantizes the parameters and re-synthesizes the voice.

Control of the CMX608/CMX618/CMX638 is over a C-BUS serial port from the host µC.

On the

CMX618/CMX638 only, the CSEL pin allows the selection of an internal or external CODEC, by connecting

the CSEL pin to either IOVDD or VSS respectively. If the internal CODEC is selected, the SDI, STRB and

SCLK pins should be connected to Vss. Synchronisation to the host µC is by use of either the (rising-edge

active) SYNC input or by writing to the SYNC register ($02). All major data paths are buffered in each

direction with FIFO registers. The SSP port, which is used for the external ADC serial input and DAC serial

output, is buffered in this way, as is the data to and from the CODEC (CMX618/CMX638 only) and also the

vocoded data which is streamed into and out of the C-BUS interface. The management of these FIFOs is

handled automatically and does not require any user intervention, other than the supply or consumption of

data at the appropriate rate.

On the CMX618/CMX638 only, a voice CODEC is included, consisting of a microphone input to an ADC

and a DAC with an earpiece output. A variable gain stage is associated with each converter. The

differential input stage includes 20dB of switchable gain, for use as a microphone pre-amplifier. The input

gain blocks are provided to allow for inputs from different microphone or other audio devices. At the very

highest gain combinations, the input noise may start to be significant. In this case, users may wish to

consider an external low-noise preamplifier prior to the differential input stage. For most applications this

will not be necessary. The differential output stage includes 6dB of switchable gain. By using the output

stage in a differential configuration, a further 6dB of gain can be achieved. Both input and output stages

include a high-order digital channel filter, to constrain the input and output signals to an audio bandwidth of

4kHz. See Figure 20 and Figure 21 for further details. This avoids the necessity of adding external third

(or higher) order filters, thus saving external components. A small number of external components, as

shown in Figure 5, Figure 6 and Figure 7 is all that is required to implement the analogue interfaces to the

CMX618/CMX638 voice CODEC.

CMX608 数据表(PDF) 13 Page - CML Microcircuits

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