Detecting the presence of a signal?

[Coper]

I have a possibly beginner's question.
ADC I sample the signal, using digital filters I select one frequency from it.
For example, I have 128 signal samples and I need to decide whether there is a signal in this section or not.
Maybe RMS
XRMS=Sqert( (x1^2+x2^2+x3^2+xn^2)/n)

but maybe it's even easier somehow
simpler, I mean with faster math than 128 square rooting and addition


Or can you recommend a book that doesn't deal so much with theory, but with practical examples of digital signal processing?

 

[KlausST]

Hi,

please define what you mean with "signal".
And please define how you decide whether it´s present or not.

A signal can be anything (usually anything but noise)
But your RMS calculation also includes noise.

Does your "signal" have any dedicated frequency and/or waveform. Or is it something like "dual frequency" (DTMF).
.. or digital data...or a carrier (for AM, FM...)

****
You tell you use digital filters to select one frequency. Is it a single frequency you are after?
Maybe a DFT or an FFT (frequency analysis) is what you are looking for.

But still they need to take every single sample into calculation.

***
If you are looking for one single (sine) frequency. Then after your filter you should have sine. Thus there is a fixed relation between RMS and peak or peak_to_peak.
A sine with 1V amplitude
* has a peak of 1V
* has a peak to peak of 2V
* has an RMS of 0.707V
So maybe it´s easier to look for peaks instead of doing ful RMS math.

***
Please give more detailed informations.

Klaus

 

[danadakk]

Goertzel method of tone detection :

Single tone detection with the Goertzel algorithm - Embedded.com

Presented here is a discussion of an infinite impulse response (IIR) filter structure used to perform spectrum analysis in the detection and measurement

www.embedded.com

https://www.embedded.com/detecting-ctcss-tones-with-goertzels-algorithm/

Tone Detection with the Goertzel Algorithm - Discussions - SigmaDSP Processors & SigmaStudio Dev. Tool - EngineerZone

The Goertzel algorithm detects the presence of a chosen frequency range in an audio stream. It's often used for tone decoding, such as in DTMF (tone dialing) and

ez.analog.com

GitHub - CyCoreSystems/goertzel: Tone detection and goertzel library for Go

Tone detection and goertzel library for Go. Contribute to CyCoreSystems/goertzel development by creating an account on GitHub.

github.com

Regards, Dana.

 

[Coper]

please define what you mean with "signal".
And please define how you decide whether it´s present or not.

Sorry i forgot about that.
the signal is a sine wave
if the maximum of the signal is -1 +1, then the minimum is -0.2 +0.2 (anything below 0.2 is noise)
at the same time, the signal should be detected as True if it is present in at least half of the processed sample.

 

[barry]

Are trying to say : 'A signal detection is true if there is a sinewave with an amplitude greater than 0.2 for half the sample time'?

Can it be any frequency, or a specific frequency?

Maybe you can just do an FFT of the signal.

 

[FvM]

What's the signal period in samples?

 

[Coper]

Sampled is more than 4 and less than 5.5 samples per period.

 

[FvM]

According to the given information, rms calculation is the simplest way. By the way, you calculate square root once, not 128 times.

--- Updated Oct 24, 2022 ---

Less effort, not sure if it fulfills your requirements, calculate average rectified value, (sum abs(x))/n.

 

[KlausST]

Hi,

squaring is rater simple compared to squareroot.
You may optimize a lot.

Usually you have to
* square 128 values
* add 128 values
* divide by 128
* take the root
Compare with threshold

But you may do instead:
* calculate the new thresbold at compile time to: threshold^2 × 128

The during run time you just need to:
* square 128 values
* add the values up
... no divide by 128
.... no sqare root
* compare with new threshold

Klaus