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ML145505

PCM Codec–Filter Mono–Circuit

LANSDALE Semiconductor Inc. 

ML145502, ML145503, ML145505

LANSDALE Semiconductor, Inc.

DEVICE DESCRIPTIONS

A codec–filter is a device which is used for digitizing and recon-

structing the human voice. These devices were developed primarily

for the telephone network to facilitate voice switching and trans-

mission. Once the voice is digitized, it may be switched by digital

switching methods or transmitted long distance (T1, microwave,

satellites, etc.) without degradation. The name codec is an acronym

from “Coder” for the A/D used to digitize voice, and “Decoder” for

the D/A used for reconstructing voice. A codec is a single device

that does both the A/D and D/A conversions.

To digitize intelligible voice requires a signal to distortion of

about 30 dB for a dynamic range of about 40 dB. This may be

accomplished with a linear 13–bit A/D and D/A, but will far

exceed the required signal to distortion at amplitudes greater than

40 dB below the peak amplitude. This excess performance is at the

expense of data per sample. Two methods of data reduction are

implemented by compressing the 13–bit linear scheme to compand-

ed 8–bit schemes. These companding schemes follow a segmented

or “piecewise–linear”curve formatted as sign bit, three chord bits,

and four stepbits. For a given chord, all 16 of the steps have the

same voltage weighting. As the voltage of the analog input increas-

es, the four step bits increment and carry to the three chord bits

which increment. With the chord bits incremented, the step bits

double their voltage weighting. This results in an effective resolu-

tion of 6–bits (sign + chord + four step bits) across a 42 dB dynam-

ic range (7 chords above zero, by 6 dB per chord). There are two

companding schemes used; Mu–255 Law specifically in North

America, and A–Law specifically in Europe. These companding

schemes are accepted worldwide. The tables show the linear quanti-

zation levels to PCM words for the two companding schemes.

In a sampling environment, Nyquist theory says that to properly

sample a continuous signal, it must be sampled at a frequency high-

er than twice the signal’s highest frequency component. Voice con-

tains spectral energy above 3 kHz, but its absence is not detrimental

to intelligibility. To reduce the digital data rate, which is proportion-

al to the sampling rate, a sample rate of 8 kHz was adopted, consis-

tent with a band-width of 3 kHz. This sampling requires a low–pass

filter to limit the high frequency energy above 3 kHz from distort-

ing the inband signal. The telephone line is also subject to 50/60 Hz

power line coupling which must be attenuated from the signal by a

high–pass filter before the A/D converter. The D/A process recon-

structs a staircase version of the desired inband signal which has

spectral images of the in-band signal modulated about the sample

frequency and its harmonics. These spectral images are called alias-

ing components which need to be attenuated to obtain the desired

signal. The low–pass filter used to attenuate filter aliasing compo-

nents is typically called a reconstruction or smoothing filter.

The ML1455XX series PCM Codec–Filters have the codec, both

presampling and reconstruction filters, a precision voltage refer-

ence on chip, and require no external components. There are three

distinct versions of the Lansdale ML1455XX Series.

ML145502

The ML145502 PCM Codec–Filter is the full feature 22–pin

device. It is intended for use in applications requiring maximum

flexibility. The ML145502 is intended for bit interleaved or byte

interleaved applications with data clock frequencies which are non-

standard or time varying. One of the five standard frequencies (see

ML145503 below) is applied to the CCI input, and the data clock

inputs can be any frequency between 64 kHz and 4.096 MHz. The

Vref pin allows for use of an external shared reference or selection

of the internal reference. The RxG pin accommodates gain adjust-

ments for the inverted analog output. All three pins of the input

gain–setting operational amplifier are present, providing maximum

flexibility for the analog interface.

ML145503

The ML145503 PCM Codec–Filter is intended for standard byte

interleaved synchronous or asynchronous applications. TDC can be

one of five discrete frequencies. These are 128 kHz (40 to 60%

duty cycle), 1.536, 1.544, 2.048, or 2.56 MHz. (For other data

clock frequencies, see ML145502 or ML145505.) The internal ref-

erence is set for 3.15 V peak full scale, and the full scale input level

at Txl and output level at RxO is 6.3 V peak–to–peak. This is the +

3 dBm0 level of the PCM Codec–Filter. The +Tx and –Tx inputs

provide maximum flexibility for analog interface. All other func-

tions are described in the pin description.

ML145505

The ML145505 PCM Codec–Filter is intended for byte inter-

leaved synchronous applications. The ML145505 has all the fea-

tures of the ML145503 but internally connects TDC and RDC (see

pin description) to the DC pin. One of the five standard frequencies

(listed above) should be applied to CCI. The data clock input

(DC) can be any frequency between 64 kHz and 4.096 MHz.

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