> 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
Improving SNR or a better Goertzel?
Started by ●February 25, 2007
Reply by ●March 1, 20072007-03-01
Reply by ●March 2, 20072007-03-02
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
>
Reply by ●March 2, 20072007-03-02
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. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●March 2, 20072007-03-02
"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
Reply by ●March 3, 20072007-03-03
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
