# Coming up with a squelch value

I am building a scanning-type system. I have samples in $\textrm{dB}$ and I want to determine when to allow them through. I am trying to decide between allowing them through when they reach a percentage over their normal value (or of the noise floor), or if I should use a static $\textrm{dB}$ value. An example of the first way would be to let a sample through when it is $50\%$ over the normal value. An example of the second would be to let it through when it is one $\textrm{dB}$ over the floor.

Which way is standard, and what are standard values I might use? I understand it depends on the application, but I'm just trying to get a rough idea.

• As you expected, both methods are used, depending on the application. Think of an analog radio scanner with a squelch knob; this is used to set an absolute squelch level. More sophisticated systems that can't rely upon a user setting a squelch knob will automatically select a (likely time-varying) squelch level instead. – Jason R Jul 13 '16 at 18:22
• @JasonR - is there a sort of standard for 'x dB over limit' that I can use? Or any kind of guideline to get a handle on how high above the noise floor average I should look for? – horse hair Jul 13 '16 at 18:26
• @horsehair not really, that all depends on things like the actual noise floor, and the characteristics of your signal. – Marcus Müller Jul 13 '16 at 19:32
• @horsehair: You will always have the tradeoff between probability of detection and probability of false alarm. – Jason R Jul 13 '16 at 19:38