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MC145503 Scheda tecnica(PDF) 8 Page - Motorola, Inc

Il numero della parte MC145503
Spiegazioni elettronici  PCM CODEC-FILTER MONO- CIRCUIT
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Produttore elettronici  MOTOROLA [Motorola, Inc]
Homepage  http://www.freescale.com
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MC145500
•MC145501•MC145502•MC145503•MC145505
MOTOROLA
8
DEVICE DESCRIPTIONS
A codec–filter is a device which is used for digitizing and
reconstructing the human voice. These devices were devel-
oped primarily for the telephone network to facilitate voice
switching and transmission. Once the voice is digitized, it
may be switched by digital switching methods or transmitted
long distance (T1, microwave, satellites, etc.) without degra-
dation. 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 perform-
ance is at the expense of data per sample. Two methods of
data reduction are implemented by compressing the 13–bit
linear scheme to companded 8–bit schemes. These com-
panding schemes follow a segmented or “piecewise–linear”
curve formatted as sign bit, three chord bits, and four step
bits. For a given chord, all 16 of the steps have the same volt-
age weighting. As the voltage of the analog input increases,
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 effec-
tive resolution of 6–bits (sign + chord + four step bits) across
a 42 dB dynamic 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 world
wide. The tables show the linear quantization 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 higher than twice the signal’s highest frequency
component. Voice contains spectral energy above 3 kHz, but
its absence is not detrimental to intelligibility. To reduce the
digital data rate, which is proportional to the sampling rate, a
sample rate of 8 kHz was adopted, consistent with a band-
width of 3 kHz. This sampling requires a low–pass filter to
limit the high frequency energy above 3 kHz from distorting
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 reconstructs 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 aliasing com-
ponents which need to be attenuated to obtain the desired
signal. The low–pass filter used to attenuate filter aliasing
components is typically called a reconstruction or smoothing
filter.
The MC145500 series PCM Codec–Filters have the co-
dec, both presampling and reconstruction filters, a precision
voltage reference on chip, and require no external compo-
nents. There are five distinct versions of the Motorola
MC145500 Series.
MC145500
The MC145500 PCM Codec–Filter is intended for stan-
dard byte interleaved synchronous and asynchronous appli-
cations. The TDC pin on this device is the input to both the
TDC and CCI functions in the pin description. Consequently,
for MSI = 8 kHz, 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
MC145502 or MC145505.) The internal reference 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. All other functions
are described in the pin description.
MC145501
The MC145501 PCM Codec–Filter offers the same fea-
tures and is for the same application as the MC145500, but
offers two additional pins and features. The reference select
input allows the full scale level of the device to be set at
2.5 Vp, 3.15 Vp, or 3.78 Vp. The –Tx pin allows for external
transmit gain adjust and simplifies the interface to the
MC3419 SLIC. Otherwise, it is identical to MC145500.
MC145502
The MC145502 PCM Codec–Filter is the full feature
22–pin device. It is intended for use in applications requiring
maximum flexibility. The MC145502 contains all the features
of the MC145500 and MC145501. The MC145502 is in-
tended for bit interleaved or byte interleaved applications
with data clock frequencies which are nonstandard or time
varying. One of the five standard frequencies (listed above)
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 ad-
justments for the inverted analog output. All three pins of the
input gain–setting operational amplifier are present, provid-
ing maximum flexibility for the analog interface.
MC145503
The MC145503 PCM Codec–Filter is intended for stan-
dard byte interleaved synchronous or asynchronous applica-
tions. 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 MC145502
or MC145505.) The internal reference 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 max-
imum flexibility for analog interface. All other functions are
described in the pin description.
MC145505
The MC145505 PCM Codec–Filter is intended for byte in-
terleaved synchronous applications. The MC145505 has all
the features of the MC145503 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


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