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Decoding Translator ID's

B

BobOnTheJob

Translators often ID via FSK (I think that's the mode), which is inaudible to the listener. Anyone have a lead on an FSK decoder that can be fed from an FM tuner's discriminator output? It would be handy to be able to verify that these things are indeed working as well as being a plus to the FM DX community.
 
The Morse code transmission speed of the FSK identifiers of most translators I've installed is very slow, perhaps 2 or 3 WPM, so you will need a tuner with a DC-coupled discriminator output. Try connecting a scope (if the tuner has AFC, makes sure it's defeated) and look for the entire baseband audio waveform to shift up or down. Or find an old tuner with a center tuning meter and watch the pointer move to the side.

Instead of FSK, some translators send a "modulated CW" ID tone of approximately 1 kHz via ~ 30% amplitude modulation of the RF amplifier. FM receivers will usually limit this out if the input signal is strong enough for full quieting. However, you could probably copy it with a multimode receiver set for AM.
 
Older tuners like the Pioneer TX series have a "multipath" or scope output that's demodulated AM. Have to back the RF input down far enough to get below the AGC/clipper to make it work. I've actually verified AM noise (bad screen supply caps) in an FM transmitter that way.
 
boiseengineer said:
Older tuners like the Pioneer TX series have a "multipath" or scope output that's demodulated AM. Have to back the RF input down far enough to get below the AGC/clipper to make it work. I've actually verified AM noise (bad screen supply caps) in an FM transmitter that way.
Click to expand...
That's my kind of engineering! And I do own a TX 9500 II tuner with the jacks you mention.
 
Play Freebird said:
The Morse code transmission speed of the FSK identifiers of most translators I've installed is very slow, perhaps 2 or 3 WPM, so you will need a tuner with a DC-coupled discriminator output. Try connecting a scope (if the tuner has AFC, makes sure it's defeated) and look for the entire baseband audio waveform to shift up or down. Or find an old tuner with a center tuning meter and watch the pointer move to the side.

Instead of FSK, some translators send a "modulated CW" ID tone of approximately 1 kHz via ~ 30% amplitude modulation of the RF amplifier. FM receivers will usually limit this out if the input signal is strong enough for full quieting. However, you could probably copy it with a multimode receiver set for AM.
Click to expand...
The one I'm trying to decode is an FSK system. I'm told that with a dead carrier, you can hear a very faint "unmodulated code" sound. Your idea about looking at the tuning meter got me thinking...if one hooked a voltage controlled audio oscillator to the tuning meter (which I checked...mine rests around 7 volts & goes up & down by about a volt as you tune across a strong signal) and that meter did indeed shift, the ID should be revealed in audible form. Anyone have a lead on a cheap or simple (or both) voltage controlled audio oscillator for this experiment?
 
I checked with Crown to see what their FSK ID specs were & they swing the carrier in 1 direction by 5-10khz at 5 words per minute in morse code. That should be adequate to hear a tone change with that 555 circuit. Off to Radio Shack with my shopping list...
 
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