Hi Doug, >Could anyone point me in the direction of a simple circuit which will >facilitate the muting or suppression of a 1750 hz tone on the received >audio.
Interesting question! Simple? Maybe not. But it can be done (we design and manufacture base station controllers that are compatible with 2175 Hz and 2300 Hz tone keying formats.) To "mute" a tone burst, you must detect it and then turn off the audio for the duration of the burst. The detection part depends on your application. If enabled all of the time, a tone mute needs a narrow detection bandwidth and a timer on its output to ensure that so-many milliseconds of tone are present before triggering the mute. That's because 1750 Hz is well inside the voice band and the detector will otherwise be "falsed" a lot. (Witness how often DTMF decoders are falsed in radio applications unless they're slowed down from 40 mS, the phone company standard -- and they require TWO tones to be present, at relatively similar amplitudes, and within 1.5% of the correct frequencies. The phone company doesn't have much problem with this because the DTMF decoders are removed from the circuit after the number is dialed.) By the way, you'll still hear the beginning of the 1750 Hz burst due to the detector's latency unless you have an audio delay in the line. If you're talking specifically about the initial tone burst that activates a repeater, then the detector filter doesn't have to be as sharp because you can make up for the falsing problem by requiring the tone to be present for a longer time (the weather service alert tone is a very long tone, partly for this reason). But you'll need a longer audio delay to remove the first part of the burst. If you disable the detector once the repeater is up, then you won't have to worry about falsing causing dropouts in the audio. Or, you could simply wait for the burst to end before bringing up the repeater, in which case no burst is heard. To "suppress" a tone, you'll need either a lowpass filter or a notch filter. We worked with VHF-AM aviation radios equipped with audio lowpass filters that removed 2175 Hz keying tones, and the resulting audio sounded pretty bad. A lowpass is probably out. A notch filter needs to be fairly narrow if you don't want a noticeable hole in your audio passband. Switched-capacitor filter ICs do a good job for detectors and notches (our notches are 50 dB deep and <50 Hz wide). But be aware that a narrow filter puts an accuracy requirement on the tone source. If the users' 1750 Hz encoders are off by a few dozen Hz, you'll hear the tone. Maybe this will stimulate some more discussion... 73, Bob, WA9FBO S-COM LLC ________________________________________________________________________ AOL now offers free email to everyone. Find out more about what's free from AOL at AOL.com.
