Testing The Dtmf Decoder - S-COM 6K Owner's Manual

5.

TESTING THE DTMF DECODER

a. Introduction
Your controller uses an 8880-type DTMF Transceiver IC. This crystal-controlled device encodes
and decodes all 16 DTMF digits. It has an extremely wide dynamic range (30 dB), and can detect
digits that have as much as -6 dB to +6 dB of twist. ("Twist" refers to the difference in amplitude
between the two individual tones that are summed to make up a DTMF digit. The more twist a
system gives to DTMF digits, the more difficult it is to detect the digits). Twist, in repeater systems,
can be caused by the user's transmitter.
The audio fed into the transmitter is pre-emphasized before it is applied to the limiter circuit. Pre-
emphasis causes high frequency tones to have more amplitude than low frequency tones, so the
high frequency tone of a DTMF digit can be severely clipped while the low frequency tone is not.
The result is twist, and the solution is the keep the DTMF pad audio at a reasonably low level. The
repeater receiver can also introduce twist if it has poor audio response, or if a poor impedance
match exists between the receiver and the controller.
b. Command Sources
The DTMF decoder can monitor all receiver inputs and the phone line for incoming digits, but it can
monitor only one device at a time. Therefore, a priority scheme is used to determine which source
feeds the decoder at any given time. If no source is active, the decoder's input is grounded.
c. Local Control
You can program the controller with a local DTMF keyboard, if desired. Connect the audio from the
keyboard to the Receiver #1 Audio Input, and connect a switch between the RX #1 COR input and
ground. Whenever the switch is ON (closed), the controller will accept commands from the
keyboard. Or, you can use the same scheme with Receiver #2 and leave the keyboard
permanently connected. Return the switch to the OFF (open) position when finished programming,
so that the repeater receiver (#1) can drive the DTMF decoder one again.
d. Testing
Generate an RF signal on the repeater's input frequency. While listening to the repeater output on
an extra receiver, send some DTMF digits to the repeater. You should hear a short burst of DTMF,
followed by silence, as the controller detects the digit and then mutes it.
If the digits are passed through to the transmitter with no muting taking place, then the DTMF
decoder is not recognizing the digits. There are several areas to check:
1. Check the audio level at the output of the Receiver #1 audio op amp; it should be 700 mV
rms (2 V p-p) as discussed in the installation section.
2. Check the operation of the Receiver #1 COR input. If the COR input isn't active, the DTMF
decoder will not be monitoring the repeater receiver audio.
3. Check the other receiver COR input and associated dipswitch. If a higher-priority input is
active, the DTMF decoder will be monitoring that input, not the repeater receiver.
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