On 3 Mar, 01:55, "Philip Martel" <pomar...@comcast.net> wrote:
> "Rune Allnor" <all...@tele.ntnu.no> wrote in message
>
> news:1172739385.584905.114880@m58g2000cwm.googlegroups.com...
>
>
>
> > On 1 Mar, 02:42, "Philip Martel" <pomar...@comcast.net> wrote:
> >> "Rune Allnor" <all...@tele.ntnu.no> wrote in message
>
> >>news:1172597519.134222.261390@m58g2000cwm.googlegroups.com...> On 27 Feb,
> >> 18:17, Jerry Avins <j...@ieee.org> wrote:
> >> >> jecottrel...@gmail.com wrote:
>
> >> >>    ...
>
> >> >> > Harmonics are generally: 18, 36, 55, 70, 90, 115, 178, 227Hz.
>
> >> >> Don't the specific numbers depend on engine speed? Is that necessarily
> >> >> constant?
>
> >> > I think helicopter rotors are designed to rotate at a fixed speed.
> >> > Engine power varies, though, depending on lift and load.
>
> >> > Rune
>
> >> That's true, but the Doppler effect will cause the frequencies heard by a
> >> fixed microphone to vary.  I put a couple of  aircraft spectrograms taken
> >> by
> >> my company on a web
> >> page.http://home.comcast.net/~pomartel/signatures.htmlFirst image is a
> >> small
> >> plane passby, second is a helicopter.
>
> > Nice images! Haven't seen that sort of stuff for a long time.
>
> > What type of plane was it? P51? One or two gazelles?
>
> > Rune
>
> I couldn't find what type of plane it was, but my boss remembered (or
> deduced from the gram) that the prop had 3 blades.  

He deduced it from the gram. As I said, I haven't seen those sorts of
things for a long time; I got it wrong.

Rune

"Rune Allnor" <allnor@tele.ntnu.no> wrote in message 
news:1172739385.584905.114880@m58g2000cwm.googlegroups.com...
> On 1 Mar, 02:42, "Philip Martel" <pomar...@comcast.net> wrote:
>> "Rune Allnor" <all...@tele.ntnu.no> wrote in message
>>
>> news:1172597519.134222.261390@m58g2000cwm.googlegroups.com...> On 27 Feb, 
>> 18:17, Jerry Avins <j...@ieee.org> wrote:
>> >> jecottrel...@gmail.com wrote:
>>
>> >>    ...
>>
>> >> > Harmonics are generally: 18, 36, 55, 70, 90, 115, 178, 227Hz.
>>
>> >> Don't the specific numbers depend on engine speed? Is that necessarily
>> >> constant?
>>
>> > I think helicopter rotors are designed to rotate at a fixed speed.
>> > Engine power varies, though, depending on lift and load.
>>
>> > Rune
>>
>> That's true, but the Doppler effect will cause the frequencies heard by a
>> fixed microphone to vary.  I put a couple of  aircraft spectrograms taken 
>> by
>> my company on a web 
>> page.http://home.comcast.net/~pomartel/signatures.html First image is a 
>> small
>> plane passby, second is a helicopter.
>
> Nice images! Haven't seen that sort of stuff for a long time.
>
> What type of plane was it? P51? One or two gazelles?
>
> Rune
>
I couldn't find what type of plane it was, but my boss remembered (or 
deduced from the gram) that the prop had 3 blades.  It was, however a modern 
plane, not a P51.  One helicopter rather than two.

   Best wishes,
   --Phil 

SunLei wrote:
> I think the problem is not the anti-aliasing filter.

Why do you think so? Either it's good enough or it's not. What was 
written that supports your opinion?

> The Goertzel is good for single or multiple sine waves, but it's not good 
> for band-limted signal.

Come again? All sampled signals are band limited to half the sampling 
frequency. It's good if the signal before sampling were band limited 
too. Can you describe a signal that can't be construed as multiple sine 
waves?

   ...

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

I think the problem is not the anti-aliasing filter.
The Goertzel is good for single or multiple sine waves, but  it's not good 
for band-limted signal.
On the condition of rotor-wing aircraft,  some of the hypothetical NOISE 
should be regarded as SIGNALS.
While the Goertzel is not good to clarify signals under such conditions, 
maybe FFT is possible.
But your CPU is a simple processor, Maybe you can try to only compute 
several frequency bins, not all the N points.
For example,
    N = 512,
       ( 50-300)*N/Fs -----Signals
        (600-800)*N/Fs----Noises

    set a Signals/Noises ratio for judgement.


Sun Lei.

<jecottrell65@gmail.com> 
??????:1172414979.284438.69130@q2g2000cwa.googlegroups.com...
> Hello All,
>
> I'll apologize in advance for the poor (none)  DSP knowledge and my
> abuse of terminology and theory....
>
> Over a year ago I started on an acoustic sensor project and received
> some fantastic help from the regulars here. I've run into some
> problems and was hoping for some suggestions.
>
> Background: Detect and classifiy aircraft (piston/prop) from their
> acoustic signatures with a PIC processor (not DSPic.... don't have a
> compiler yet....) I achieved really good success rates using the
> Goertzel algorithm to look for the presence of known frequencies in
> the sample. It works well because the unique frequencies stand out so
> well. Also, there is little or no background noise when the aircraft
> are present.
>
> Now I'm trying to detect and classify rotor-wing aircraft. I'm running
> into problems because the SNR isn't as good. The exhaust noise and tip-
> path noise is getting lost in the general 'helicopter self-
> destruction' noise.
>
> Is there any way to solve this problem?
>
> Interesting freqs: 50-300hz
> Sample rate: 1588hz
> Number of samples: 512 optimum (1024 max, but really slows things
> down)
>
> Thanks,
>
> John
> 

> That's true, but the Doppler effect will cause the frequencies heard by a
> fixed microphone to vary.  I put a couple of  aircraft spectrograms taken by
> my company on a web page.http://home.comcast.net/~pomartel/signatures.html First image is a small
> plane passby, second is a helicopter.

My algorithm takes that into account. The spectrograms illustrate
perfectly what I'm running into. The airplane has a better SNR, the
helicopter not so....

I haven't had a chance to do any more work on this project yet, I'm
tied up with paying work right now. I hope to get back to it next
week.

BTW, yes... most modern helicopters have governed rotor and engine
speeds. Rotors tend to hold fairly fixed RPMs, engines driving those
rotors vary a little more to maintain them in their desired ranges.

Thanks,

John

On 1 Mar, 02:42, "Philip Martel" <pomar...@comcast.net> wrote:
> "Rune Allnor" <all...@tele.ntnu.no> wrote in message
>
> news:1172597519.134222.261390@m58g2000cwm.googlegroups.com...> On 27 Feb, 18:17, Jerry Avins <j...@ieee.org> wrote:
> >> jecottrel...@gmail.com wrote:
>
> >>    ...
>
> >> > Harmonics are generally: 18, 36, 55, 70, 90, 115, 178, 227Hz.
>
> >> Don't the specific numbers depend on engine speed? Is that necessarily
> >> constant?
>
> > I think helicopter rotors are designed to rotate at a fixed speed.
> > Engine power varies, though, depending on lift and load.
>
> > Rune
>
> That's true, but the Doppler effect will cause the frequencies heard by a
> fixed microphone to vary.  I put a couple of  aircraft spectrograms taken by
> my company on a web page.http://home.comcast.net/~pomartel/signatures.html First image is a small
> plane passby, second is a helicopter.

Nice images! Haven't seen that sort of stuff for a long time.

What type of plane was it? P51? One or two gazelles?

Rune

"Rune Allnor" <allnor@tele.ntnu.no> wrote in message 
news:1172597519.134222.261390@m58g2000cwm.googlegroups.com...
> On 27 Feb, 18:17, Jerry Avins <j...@ieee.org> wrote:
>> jecottrel...@gmail.com wrote:
>>
>>    ...
>>
>> > Harmonics are generally: 18, 36, 55, 70, 90, 115, 178, 227Hz.
>>
>> Don't the specific numbers depend on engine speed? Is that necessarily
>> constant?
>
> I think helicopter rotors are designed to rotate at a fixed speed.
> Engine power varies, though, depending on lift and load.
>
> Rune
>
That's true, but the Doppler effect will cause the frequencies heard by a 
fixed microphone to vary.  I put a couple of  aircraft spectrograms taken by 
my company on a web page.
http://home.comcast.net/~pomartel/signatures.html  First image is a small 
plane passby, second is a helicopter.

On 27 Feb, 18:17, Jerry Avins <j...@ieee.org> wrote:
> jecottrel...@gmail.com wrote:
>
>    ...
>
> > Harmonics are generally: 18, 36, 55, 70, 90, 115, 178, 227Hz.
>
> Don't the specific numbers depend on engine speed? Is that necessarily
> constant?

I think helicopter rotors are designed to rotate at a fixed speed.
Engine power varies, though, depending on lift and load.

Rune

On 27 Feb, 17:44, jecottrel...@gmail.com wrote:

> General algorithm is this:
>
> Take sample.
> Look for peak between 50-90Hz.
> Is the peak right around 70Hz?
> Is that peak > 2x the average of the surrounding freqs.

This is where all sorts of things might happen. Where did the
factor 2 come from? What happens if you have a broad peak,
that may be 2 or even 3 bins wide?

> Yes, look for peak at 1.28 x peak of first band (1.28x70 = 90).
> Is that peak > 2x the average of the surrounding....
> etc.
>
> This way, an incremental approach is used and processing is minimized.
> As each parameter is met, confidence grows in the data.
>
> Maybe I should reduce the sample rate to decrease the bin size? I
> really can't increase the number of samples because of processing
> time.

Try to decrease the sample rate. Make sure to use anti-alias
filters before decimation. What happens to the width and SNR
of the peaks?

Rune

jecottrell65@gmail.com wrote:

   ...

> Harmonics are generally: 18, 36, 55, 70, 90, 115, 178, 227Hz.

Don't the specific numbers depend on engine speed? Is that necessarily 
constant?

   ...

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯