A PC as a tachometer

[sperho]

I took what was said earlier and did some playing around...I used a 1 second snippet from that company's video about chaps, with the saw taching out at 12200.

Running the frequency analysis in Adobe Audition, I got a familiar graph...note the peaks, indicated by the arrows:

The peaks are at the following Hz, which calculate into RPMs as mentioned earlier, correcting for the various harmonic factors:

• 208 hZ = 1x > 12480 RPM
• 417 hZ = 2x > 12510 RPM (417*60/2)
• 624 hz = 3x > 12480 RPM
• 832 hZ = 4x > 12480 RPM
• 1246 hZ = 6x > 12460 RPM
• 3321 hZ = 16x > 12453 RPM
• 4978

= 24x > 12445 RPM

Using the harmonics helps verify the readings, and they all average out to 12471 RPM, which is pretty close to the tach reading. The slight discrepancy MIGHT be from the video compression causing a slight increase in frequency...either way, it's pretty close.

Thoughts?

Well done. We agree to within about 4% and you're 2% away from BobL. You might play around with the FFT settings and windowing functions to give you better looking power spectrums to find the fundamental more easily. You can see what my setting were in my graphic attached about half way down the thread.

 

[sperho]

You can find the rpm as you have done by analyzing the spectrum like you have but you have also found several of the problems involved with doing it.

Are you trying to offer dial up tach service?

In order to have enough data to find the fundamental frequency you need to have verification by the harmonics which you have plotted.

You have also found one of my main reasons for my leaving the recording industry years ago, an almost complete lack of reliable reference standards in the various digital compression and manipulation algorithms.

You have guessed quite correctly that the video codex treatment of the audio has skewed the results......the weird part which you can verify is pull the sample several more times from the site and the spectrum can and will shift in frequency. For the largest variation download the sample from a different computer using a different ISP and you will see.

The other thing you have found is the reflections involved in the recoding environment can make it difficult to near impossible to get an accurate read of the fundamental due to swamping of the fundamental and harmonics by standing waves and doppler pitch shifted reflections

A stick on transducer on the saw itself can eliminate those variables and would be quite accurate

As far as lakesides suggestions it is quite possible and can be done for rather low cost today.

Is there really a market for such a thing? Could be

Yes, there are errors introduced in this method, but most of the error is in these examples are associated with the precision of the power spectrum, which is in part affected by the number of samples analyzed. With my number of samples, the error is a few Hz, which corresponds to 60 RPM/Hz. Given craneage's, BobL, and my results are all +/-2% of each other with us all using different software, FFT sizes, and windowing functions and then reading the power spikes manually, I'd say that the method is quite sufficient for chainsaw tuning. Not picking on your post because you have valid points, just explaining for those who might be interested...

To answer your subject line question: Save money. I had the software and laptop to do this and it worked; saws tuned...

 

[Nitroman]

Good idea, If you could rig some way to have a constant pressure on the saw in the cut to reduce that variable I can see it being a great tuning tool. A programmer could come up with a GUI for a freq counter that would display in real time and hold at peak.

I'd thought about just putting a long wire on my tach and having my wife monitor it from a couple steps away. I don't know how far the signal will carry down the wire tho.

Ian

I proposed this idea in the Hotsaw forum and was b!tchslapped by the respondents.

 

[BlackCatBone]

I proposed this idea in the Hotsaw forum and was b!tchslapped by the respondents.

That's usually what happens to the first guy with a possibly good idea!

 

[rxe]

Why don't you just hook up an oscilloscope to an inductive coupler on the HT lead? You'd get the frequency and current flow through the plug at the same time - very good for fixing high speed ignition problems.

I've got a scope, I'll post some pics when I have time to set up a test. Comparing electronic ignition and points is quite interesting.

 

[blsnelling]

Try this one. There are a couple shots of the tach.
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/9W1ds6kULmE&hl=en"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/9W1ds6kULmE&hl=en" type="application/x-shockwave-flash" wmode="transparent" width="425" height="344"></embed></object>

 

[Erick]

So all you need is a good audio file????

 

[sperho]

Why don't you just hook up an oscilloscope to an inductive coupler on the HT lead?

If I still had one, I probably would have.

I've got a scope, I'll post some pics when I have time to set up a test. Comparing electronic ignition and points is quite interesting.

Let us know how it goes...

 

[sperho]

So all you need is a good audio file????

Yes...

 

[sperho]

Try this one. There are a couple shots of the tach.
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/9W1ds6kULmE&hl=en"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/9W1ds6kULmE&hl=en" type="application/x-shockwave-flash" wmode="transparent" width="425" height="344"></embed></object>

I'll take a look at your video tonight after the young ones are sawing their own logs...

 

[sperho]

Re: blsnelling's video above

Keep in mind that the saw is varying in RPM as the cut is made, some cuts more than others. Thus, I'm mainly putting a middle of the road value here. The tune values are plus/minus 84 RPM because of various reasons including length of time recording (thus analyzed) and sampling rate. So for ultra-fine tuning as you might want in racing, this method is not ideal for tuning. For the rest of us, it's perfectly fine. Set the saw a few hundred under max RPM and be happy. Thanks for posting the video bls, it helps us understand the accuracy and precision limitations of the audio recording/frequency analysis method. What appears to me to be the larges variable in how well the saw performs in the cut is the skill of the operator being able to maintain the optimal amount of cut force for a given tuning.

1st snippet (cut): ~9,500
2nd snippet (tune): 15,018 +/-84
3rd snippet (cut): ~10,020
4th snippet (tune): 15,018 +/-84
5th snippet (cut): ~9,500

 

[nrguenth]

Why?

So I'm just a newbie to AS but, why? Why go to all of this trouble? I've got 2 'scopes, a spectrum analyzer, and a LOT of pc's with various sound cards and audio processing software. (I'm a network engineer and a closet musician) When I want to tune my saw I use my $25 inductive tachometer...:monkey:

It seems like killing yourself trying to find a way to hammer nails with a fish....

 

[sperho]

So I'm just a newbie to AS but, why? Why go to all of this trouble? I've got 2 'scopes, a spectrum analyzer, and a LOT of pc's with various sound cards and audio processing software. (I'm a network engineer and a closet musician) When I want to tune my saw I use my $25 inductive tachometer...:monkey:

It seems like killing yourself trying to find a way to hammer nails with a fish....

Killing myself? In 2 minutes flat I can be checking my tune, which I might do once/year. Of course, I've loads of experience in collecting this kind of data and analyzing, so other's mileage may vary.

It was also an experiment, which worked, and was free, and is quick for me, so..............to each their own, eh?

What's the refresh rate on that $25 tach and where did you buy it?

 

[brncreeper]

Tiny Tachs are relatively cheap and easy to get (e-bay). Finding horsepower/torque would be a much better waste of time rather than finding than several methods of measuring rpm. Although finding the rpm is a part of finding horsepower.

 

[sperho]

Tiny Tachs are relatively cheap and easy to get (e-bay). Finding horsepower/torque would be a much better waste of time rather than finding than several methods of measuring rpm. Although finding the rpm is a part of finding horsepower.

I cut wood to burn for myself. All I care about is that my saw runs reasonably well and it doesn't blow up because I've set it too lean. If I want to find more horsepower by a means that is more involved than drilling a few holes in the muffler, I'll buy a bigger saw. Seems like a lot more people here "waste" time trying to eek out HP out of their saw for no practical reason; at least I've saved some cash with my waste of time.

 

[rxe]

So I'm just a newbie to AS but, why? Why go to all of this trouble?

Because we can! And it's fun.

 

[nrguenth]

Killing myself? In 2 minutes flat I can be checking my tune, which I might do once/year. Of course, I've loads of experience in collecting this kind of data and analyzing, so other's mileage may vary.

It was also an experiment, which worked, and was free, and is quick for me, so..............to each their own, eh?

What's the refresh rate on that $25 tach and where did you buy it?

Tach can be found here
Spec sheet lists sample rate of ~1 second, with resolution of 10rpm. Don't get me wrong, I have no problem with doing something for the sake of seeing if it can be done. I frequently take the road less traveled for no other reason than that it beats sitting around watching American Idol... First time through I missed your response to PES+:

To answer your subject line question: Save money. I had the software and laptop to do this and it worked; saws tuned...

Had I seen it I wouldn't have posted at all...

 

[tatra805]

following with interest....

2 cents maybe they help

How i get rid of backgroundsounds and isolate engine sounds to the place where they occur:

After the rebuild of my Tatra engine, to check each cylinder by sound, we hold a screwdriver on the valve cover of each cylinder and hold our ear to it.

Looks funny but to eliminate all the sounds around it (aircooled V8 revving makes a hell of lot of noise) you can realy spot-listen that way and scan the engine to find out where something is making a different sound. (eg valves)

Now, i figure if you would stick a cheapo microphone (as used to skype) to a screwdriver and hold it against the engine you would be nicely able by adjusting the mic level to come to a more dedicated signal for analysis.


Just be carefull not to cut your neck...




edit: stick the mic to some clamp, or vise-grip and clamp it to any metal part of the saw.

 

[sperho]

How i get rid of backgroundsounds and isolate engine sounds to the place where they occur

Actually, you don't have to worry about getting rid of background noise. The sound pressure levels coming out of a chainsaw at full throttle dominate all other sounds being recorded by the microphone. Also, automatic gain control on video recorders and the like seems to work just fine to prevent clipping. I've analyzed files from my iRiver H340 (over which I have control of the gain), a laptop microphone running Cool Edit, video cameras with AGC, and digital cameras with a video feature. All seem to do an adequate job for how I use a tach with a saw.

 

[blsnelling]

Re: blsnelling's video above

Keep in mind that the saw is varying in RPM as the cut is made, some cuts more than others. Thus, I'm mainly putting a middle of the road value here. The tune values are plus/minus 84 RPM because of various reasons including length of time recording (thus analyzed) and sampling rate. So for ultra-fine tuning as you might want in racing, this method is not ideal for tuning. For the rest of us, it's perfectly fine. Set the saw a few hundred under max RPM and be happy. Thanks for posting the video bls, it helps us understand the accuracy and precision limitations of the audio recording/frequency analysis method. What appears to me to be the larges variable in how well the saw performs in the cut is the skill of the operator being able to maintain the optimal amount of cut force for a given tuning.

1st snippet (cut): ~9,500
2nd snippet (tune): 15,018 +/-84
3rd snippet (cut): ~10,020
4th snippet (tune): 15,018 +/-84
5th snippet (cut): ~9,500

Thanks for the analysis. I find it very interesting. The difference in RPM in the cut is very close to the difference in cut times between the two saws.