MC145483 데이터 시트보기 (PDF) - Motorola => Freescale
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MC145483 Datasheet PDF : 19 Pages
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RO –
PI
PO –
PO +
VDD
VSS
VAG Ref
VAG
TG
TI –
TI +
HB
FREQ
–
+
–1
1
VDD
R*
R*
VSS
RECEIVE
SHIFT
REGISTER
DAC
SHARED
DAC
0.886 V
REF
SEQUENCE
AND
CONTROL
–
+
ADC
FREQ
TRANSMIT
SHIFT
REGISTER
Figure 1. MC145483 13–Bit Linear PCM Codec–Filter Block Diagram
DR
FSR
BCLKR
PDI
MCLK
BCLKT
FST
DT
DEVICE DESCRIPTION
A PCM Codec–Filter is used for digitizing and reconstruct-
ing the human voice. These devices are used 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 analog–to–digital
converter (ADC) used to digitize voice, and ‘‘DECoder’’ for
the digital–to–analog converter (DAC) used for reconstruct-
ing voice. A codec is a single device that does both the ADC
and DAC conversions.
To digitize intelligible voice requires a signal–to–distortion
ratio of about 30 dB over a dynamic range of about 40 dB.
This may be accomplished with a linear 13–bit ADC and
DAC. The MC145483 satisfies these requirements and may
be used as the analog front–end for voice coders using DSP
technology to further compress the digital data stream.
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 in–band 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 analog–to–
digital converter. The MC145483 includes a high–pass filter
for compatibility with existing telephone applications, but it
may be removed from the analog input signal path by the
high–pass bypass pin.
The digital–to–analog conversion process reconstructs a
staircase version of the desired in–band signal, which has
spectral images of the in–band signal modulated about the
sample frequency and its harmonics. These spectral images
are called aliasing components, which need to be attenuated
to obtain the desired signal. The low–pass filter used to at-
tenuate these aliasing components is typically called a re-
construction or smoothing filter.
The MC145483 PCM Codec–Filter has the codec, both
presampling and reconstruction filters, and a precision volt-
age reference on–chip.
MC145483
2
MOTOROLA
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