Baseband Signal Processor

FX829

© 1997 Consumer Microcircuits Limited

8

D/829/4

1.5

General Description

The FX829 consists of five main sections: the audio filter section, the programmable attenuators, the

DTMF encoder, the FFSK transmitter and the FFSK receiver. All these sections are controlled via a

serial ("C-BUS") interface. The four sections are described below.

Audio Filtering

This consists of an input amplifier and a common audio filter section, which may be switched between

Rx and Tx. The filter section comprises an anti-alias filter followed by low-pass and high-pass filtering

with an amplitude limiter to set the maximum deviation. Three variable attenuation blocks may be

used to set the volume (in Rx) or the modulation level (in Tx). Pre- and de-emphasis can be added

externally using resistors and capacitors around AMP1, AMP2 and the microphone amplifiers, see

Figure 7. The anti-alias filter is designed to reduce aliasing effects above 50kHz which is

approximately half the internal filter's sample rate.

The filtering is designed to meet the ETS 300 086 specification.

Various powersave modes are incorporated.

MOD1 and MOD2 Attenuators

The MOD1 input can be connected directly to SUM OUT, so that the MOD1 and MOD2 outputs can

then be used for two point modulation. Alternatively, the MOD1 attenuator can be used for auxiliary

gain adjustment, in which case the input signal must be ac coupled with a suitable capacitor.

DTMF Encoder

This generates the standard DTMF tones according to the CONTROL 2 Register settings. It also has

a powersave mode.

FFSK Tx

The Tx function of the FFSK modem operates continuously in a free format mode, which means that

the preamble and frame sync have to be programmed like normal data bytes. However, a 2-byte

checksum may be generated automatically by simply marking the beginning and end of the data to be

used. Any number of whole bytes may be used to generate the checksum.

After the last byte has been transmitted one additional "hang bit" is automatically added to the end. All

Tx operations are programmed from the "C-BUS" via an 8-bit buffer. The Tx part of the FFSK modem

has a powersave mode.

The modulation output is one cycle of 1200Hz for a "1" and one and a half cycles of 1800Hz for a "0"

at 1200 baud, or one half cycle of 1200Hz for a "1" and one cycle of 2400Hz for a "0" at 2400 baud.

FFSK Rx

In Rx, the modem automatically achieves bit sync and then recognises the previously selected SYNC

and/or SYNT word of the MPT1327, ETS 300 230 or PAA1382 specifications. At the same time as

the above, it can also recognise a user programmed 16-bit RX SYNC WORD.

On reception of the SYNC, SYNT or RX SYNC WORD, the device will automatically (or manually at

any time) start checking the data and checksum. It provides a 1-bit correct/incorrect result every byte,

so that any number of bytes can be checked.

The Rx part of the FFSK modem operates at 1200 or 2400 baud and has a powersave mode. Both

FFSK Rx and Tx work in half duplex mode.