TCM8000 Datasheet, PDF (5/19 Pages) Texas Instruments – CELLULAR TELEPHONE AUDIO PROCESSOR

TCM8000
CELLULAR TELEPHONE AUDIO PROCESSOR
SCTS041A – D4086, AUGUST 1992 – REVISED APRIL 1993
detailed description – receive path
input conditioning
The received demodulated signal is applied to RXIN and passes to a gain-adjust block that provides ± 4 dB of
adjustment in sixteen 0.5-dB steps. At this point, the receive signal path is split into the voice path, the data path,
and the supervisory audio-tone (SAT) path.
receive voice filter
The received voice signal is processed in the band-pass RXBP filter, whose response has been designed to
meet the requirements of AMPS/TACS and NMT systems. The response incorporates the required 6-dB/octave
de-emphasis. Frequency response templates and a typical response are shown in Figures 9 and 10.
expander
The expander implements a 2:1 dynamic range expansion of the signal at its input, EXIN. It is designed to
produce 0-dB gain at 100 mVrms. Envelope attack and decay times are determined by a capacitor attached
to ETC, with a value of 220 nF producing attack time of 3 ms and decay times of approximately 13 ms. The
expander can be bypassed under control of the serial interface.
receive summing circuit and volume control
A 3-input summing circuit is provided, which takes as input the voice signal from the expander, audio tones from
the TDI input, and a call-tone input applied to CTI. Each input can be enabled or muted under control of the serial
interface. The output from this block is passed to the volume control, which provides ± 15 dB of output level
control in sixteen 2-dB steps.
output buffers
Two output buffer circuits are provided. The buffer that drives HFO is capable of driving loads down to 10-kΩ
impedance and is intended for use with an external speaker driver. The HSO buffer features a high-drive bipolar
output stage that allows impedances of 500 Ω to be driven directly with low distortion. The HSO output path also
incorporates a summing circuit to facilitate side-tone injection from the ST input and a limiter circuit, which allows
maximum sound pressure levels to be determined. The limit level is controlled by the voltage applied to RXLIM.
Selection between the HFO and HSO outputs is achieved via the serial interface.
receive data path
A switchable response filter is provided to condition receive data before it is output on RXDO. Frequency
response templates and typical responses are shown in Figures 11 and 12 for AMPS/TACS and NMT,
respectively.
receive SAT path
Received supervisory audio tones (SAT) are separated from the voice signal in a switchable response
narrow-band filter. This filter has a center frequency of 6 kHz in the AMPS/TACS mode and 4 kHz in the NMT
mode. The filter output is passed to a level-adjust circuit, which provides eight 1-dB steps of adjustment. The
processed SAT signal is output on PSO.
miscellaneous
midsupply
The analog midsupply, effectively signal ground, is derived from a resistive divider connected between V_CC and
GND to produce V_CC /2 at REF. This voltage is buffered by a pair of low output impedance amplifiers to produce
midrail supplies for the receive and transmit sections at B_BRX and V_BTX, respectively, with separate midrails used
to minimize coupling between the receive and transmit circuits. REF, V_BTX, and V_BRX should be decoupled to
ground with capacitors physically mounted as close to the device as possible. V_BTX and V_BRX directly control
the 0-dB point of the compressor and expander, respectively, so it is important that a stable and well-regulated
supply is provided to V_CC.
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