DUAL_TONE_DECODING

Published:2009/7/6 20:26:00 Author:May | From:SeekIC

Two integrated tone decoders, XR-567 units, can be connected (as shown in Fig.1A) to permit decoding of simultaneous or sequential tones. Both units must be on before an output is given. RIC1 and R'1C'1 are chosen, respectively, for Tones 1 and 2. If sequential tones (1 followed by 2) are to be decoded, then C3 is made very large to delay turn-off of Unit Luntil Unit 2 has turned on and the NOR gate is activated. Note that the wrong sequence (2 followed by 1) will not provide an output since Unit 2 will turn off before Unit 1 comes on. Figure 1B shows a circuit variation which eliminates the NOR gate. The output is taken from Unit 2, but the Unit 2 output stage is biased off by R2 and CR1 until activated by Tone 1. A further variation js given in Fig. IC.Here, Unit 2 is turned on by the Unit 1 output wh.en Tone 1 appears, reducing the standby power to half. Thus, when Unit 2 is on, Tone 1 is or was present. If Tone 2 is now present, Unit 2 comes on also and an output is given. Since a transient output pulse may appear at Unit 1 turn-on, even if Tone 2 is not present, the load must be slow in response to avoid a false output due to Tone 1 alone. The XR-267 Dual Tone Decoder can replace two integrated tone decoders in this application.



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