Hi John,

     In the past I have used the low-pass filter described below, but=20
I'm not sure how to make it a high-pass:

----------------------------

A single pole (first order) high pass is pretty easy.
The basic building block for a lot of simple filters is a single pole=20
low pass:

FILT <--- FILT + FF(NEW - FILT)

This algorithm is performed once for each new input sample.
NEW is the new input value for that iteration and FILT is the filtered=20
output value.
FF is something I call the filter fraction. It adjusts how "heavy" the=20
filter is.
FF =3D 0 is infinitely heavy since the filter output never changes.
FF =3D 1 just passes the input directly to the output without any=20
filtering applied.
Obviously useful values are in between.
On some hardware it can be useful to make FF =3D 2**-N so that the=20
multiply by FF becomes just a right shift by N bits.

You can make a high pass filter by subtracting the low pass filtered=20
signal from the original.

----------------------------


     Does it mean that (NEW - FILT) is the high-pass filter output?

     Thanks.
     Mark Jordan



On 24-Jun-12 17:48, John Ferrell wrote:
> It is easier than it sounds.
> Sample the audio above the highest expected audio. Any signal above 4khz
> in a radio with a 3khz bandpass is white noise. Logically speaking,
> (White noise) =3D NOT (Audio). Be careful of Patents though. I know there
> was a patent issued in the mid-1980's for selecting the best usable comm
> channel of a redundant set of channels.
>
> Don't let this discourage software/DSP discussion. I am really
> struggling with especially the DSP aspect.
>
> On 6/24/2012 1:20 PM, enkitec@gmail.com wrote:
>>        Hi,
>>
>>        I want to build a FM radio squelch using a microprocessor with an=
 ADC.
>>        The ADC will be sampling the FM radio audio output and turn on a
>> LED when there is only noise present. The LED will be off when someone
>> is using the channel (no noise, just voice).
>>
>>        What is the best way of doing this in software?
>>
>>        Thanks for any idea.
>>        Mark Jordan
>>
>>
>>
>>
>>


--=20
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