TCM8000

CELLULAR TELEPHONE AUDIO PROCESSOR

SCTS041A – D4086, AUGUST 1992 – REVISED APRIL 1993

POST OFFICE BOX 655303

5

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-k

Ω

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 0-dB point of the compressor and expander, respectively, so it is important that a stable and well-regulated