TCM8000 Datasheet, PDF (4/19 Pages) Texas Instruments – CELLULAR TELEPHONE AUDIO PROCESSOR

TCM8000
CELLULAR TELEPHONE AUDIO PROCESSOR
SCTS041A – D4086, AUGUST 1992 – REVISED APRIL 1993
Terminal Functions (continued)
PIN
NAME
NO.
I/O
FN FR
DESCRIPTION
VBTX
VCC
XT0, XT1
44
1
21, 20
5
6
26, 25
Buffered midsupply for all transmit sections, nominal output VCC /2
Most positive supply voltage, 5 V ± 5%
Crystal oscillator terminals for connection to 3.58-MHz crystal resonator
detailed description – transmit path
input conditioning
A pair of uncommitted operational amplifiers is provided at the main audio signal input. These components can
be configured using external resistors to adjust the gain as required to suit particular microphones and external
preamplifiers. The TCM8000 has been designed to provide 100 mVrms at the outputs of the preamplifier at pins
M1O and M2O. The switch MICSW allows the selection of either amplifier input for hands-free or handset
microphones. A digitally controlled gain/attenuation block, MICTRIM, allows adjustment of the signal level into
the compressor in sixteen 0.5-dB steps. A digitally controlled switch, TDSW, allows alternative signals from other
sources such as audio or DTMF tones to be injected into the transmit signal path from TDI. The output of TDSW
is ac coupled at TXVO through an external capacitor into the compressor.
compressor
The compressor provides a 2:1 dynamic range compression of its input signal, converting an input range of
60 dB to 30 dB at its output. The test circuit diagram (Figure 2) shows the external components required to set
time constants and to limit gain under idle channel conditions. The envelope time constant is determined by
the value of a capacitor attached to CTC with 220 nF providing attack time of 3 ms approximately and decay
times of approximately 13 ms. The compressor can be bypassed under control of the serial interface. The output
from this stage, CO, is externally ac coupled into the transmit filter at TXVI and into the compressor rectifier input
at CMPR.
TX filtering and limiter
The transmit band limiter is located between a pair of band-pass filters. Filter TXF1 is located before the limiter
and provides 6 dB per octave preemphasis; filter TXF2, located after the limiter, features a phase-equalized
response to control overshoot. The frequency responses of TXF1 and TXF2 are switchable between 3 kHz and
3.4 kHz cut off to meet the requirements of AMPS/TACS and NMT systems, respectively. Frequency response
templates and typical responses are shown in Figures 3 through 6.
The limiter is provided to meet maximum deviation specifications, and the limit level is controlled by the dc
voltage applied to TXLIM. The output signal from POUT is ac coupled into the limiter input to eliminate
asymmetrical limiting that could arise as a result of dc offsets.
output conditioning
The voice output signal from TXLP2 is passed to a three-input summing circuit, whose other inputs come from
the TX data path and from TXSUM. Each input to the summing circuit can be enabled or muted under control
of the serial bus. The overall deviation level is controlled by a digitally controlled gain/attenuation block that
provides sixteen 0.5-dB steps of output level adjustment. The transmit supervisory audio tone (SAT) is applied
to TXSUM. TXSUM can also be used as an input for transmit data, allowing implementation of alternative data
filtering and conditioning to that implemented on the TCM8000.
transmit data
Transmit data, which is voice-band FSK coded for NMT and wide-band Manchester coded for AMPS and TACS
(8K-baud TACS, 10K-baud AMPS), is applied to TXDA. The signal is filtered by the TX DATA filter, whose
response can be switched from a band-pass to a low-pass configuration to meet the requirements of NMT and
AMPS/TACS, respectively. Frequency templates and typical responses are shown in Figures 7 and 8.
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