CVSD Codec
CMX639
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
from 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 so for 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 as
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.
ã 2000 Consumer Microcircuits Limited
11
CMX639/2
CMLMICRO [ CML MICROCIRCUITS ]
