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CMX639

Consumer / Commercial CVSD Digital Voice CODEC

MX-COM Inc  

Consumer/Commercial CVSD Digital Voice CODEC

Group Delay Distortion

(1000Hz-2600Hz)

(600Hz-2800Hz)

(500Hz-3000Hz)

Xtal/clock Frequency

8

Notes

5

10, 11

Min.

0.500

CMX639 Preliminary Information

Typ.

Max.

Units

1.024

450

750

1500

2.048

µs

µs

µs

MHz

Notes:

1. All logic inputs except Encoder and Decoder Data clocks.

2. The source impedance driving the coupling capacitor should be less than 1kΩ. A lower driving source

impedance will reduce encoder output channel noise levels.

3. For an Encoder/Decoder combination.

4. See Figure 5.

5. Group Delay Distortion for the full codec is relative to the delay with an 820Hz, -20dB signal at the

encoder input

6. Not including any current drawn from the device by external circuits.

7. Recommended values

8. dBmOp units are measured after the application of a psophometrically weighted filter that is commonly

used in voice communication applications per CCITT Recommendation G.223.

9. Forced idle encode/decode control not available on D4 (16 pin SOIC) package.

10. Some applications may benefit from the use of an Xtal/clock frequency other than 1.024MHz.

Note: CODEC time constants and filter response curves are effectively proportional to Xtal/clock

frequency and so will shift with the use of Xtal/clock frequencies other than 1.024MHz. For example, the

specified Encoder Decoder (Full Codec) Passband of 300Hz min. to 3400Hz max. for a 1.024MHz

Xtal/clock will shift to 600Hz min. to 6800Hz max. when the device is operated from a 2.048MHz

Xtal/clock. For this reason, all CMX639 CODECs involved in the same communications link should

usually be operated from the same Xtal/clock frequency.

Example 1: A design saves the cost of a 1.024MHz Xtal or clock generator by making use of an already

existing clock source of a frequency other than 1.024MHz.

Example 2: Best noise performance is achieved when the CMX639 CODEC data clock is internally

generated. If a CODEC bit rate other than 16kbps, 32kbps or 64kbps is desired then an Xtal/clock

different than 1.024Mhz can be used to proportionately shift the available set of internally generated clock

rates, as needed.

Example 3: To increase the CODEC high frequency response and audio bandwidth a faster Xtal/Clock

speed can be used. Other designs may prefer the proportionately higher CODEC bandwidths and data

rates that can be supported with faster clock speeds.

11. In general, optimum CODEC performance is achieved when both encoder and decoder Xtal/Clock signals

are synchronized. While this is practical in many telecom applications it may not be in others such as

wireless data links. The CMX639 decoder can generally deliver best performance when its data clock is

recovered/derived from the received data stream and applied as an external data clock to the decoder per

the decoder timing depicted in Figure 4. Nonetheless, some Xtal/Clock frequency and data rate

combinations are better served by the use of internal clocks. Experimentation with each specific design

may provide the best guidance for making this design choice.

2000 MX-COM, Inc.

www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054

Doc. # 20480209.006

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All Trademarks and service marks are held by their respective companies.

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