Now this has gotta interest pickup makers

[frankfalbo]

Thank you, defaced. C'mon, guys, let's help John out. He's having trouble with this one. Someone explain to John that a 1/4000 of a second still shot would show the same shape if that's what's really happening, as would a high speed camera in slow motion. This is not that effect. Someone tell him that you can't make a string do that without lengthening the string to the point of slack. Someone tell him that people have already debunked this visual effect as the artifact of rolling shutters OR rolling CRT refresh rates. Someone tell him that if the human eye can see it on video in real time, without slow motion, then we'd be able to see it, or at least evidence of it with the naked eye under incandescent or sun light. Cmon guys, someone tell him.

[J M Fahey]

Well, you *just* did.

[Steve Conner]

YouTube - Amazing subwoofer ripple camera effect low fps

[Sweetfinger]

When I was in college you could check out a xenon strobe light setup from the engineering lab. The setup had the capability to run several flash units in unison so you could fully illuminate an object and clearly see oscillating motion. One of the cooler things we looked at was a vibrating bass string where you could easily see the fundamental and first couple harmonics. You could also see the three dimensional movement. They did not look like the strings in the first video(which is a scanning artifact) BUT the range of movement captured in the vid is accurate.

[frankfalbo]

The range from the farthest points yes, but not the opposing range of supposed harmonic vibrations within the string that some have suggested is accurately depicted by either rolling light sources in the case of the CRT or rolling image capture in the case of the high speed camera.

[JGundry]

The range from the farthest points yes, but not the opposing range of supposed harmonic vibrations within the string that some have suggested is accurately depicted by either rolling light sources in the case of the CRT or rolling image capture in the case of the high speed camera.

Not quite sure what you are getting at. Do you mean the wave amplitude is correct but the wave shape is warped by the refresh rate? My point is you basically have a strobe light source with a CRT display and that you are seeing a nearly symmetrical wave form of the string harmonic. What's not to like?

[frankfalbo]

Jon you're killing me. Seriously are you just messing with me? Watch all the videos posted then read ALL the comments under the videos. A CRT is a rolling strobe, not a single point, pulsating strobe. Read the comments on the video defaced posted:

I don't think you're really seeing the vibration of the string per se. What you're seeing is aliasing caused by the scan rate of your CCD. The vibration is traveling up and down the string faster than the pixels can be scanned from the CCD. When you have the strings oriented almost horizontally, the vibration looks more true to the real movement.

True. The vibrating string would look nothing like this if you took a still frame at 1/4000. The whole string moves from side to side, rather than creating this s-shaped spaghetti warps, which is an illussion caused by the rolling shutter that records the video.

What I can't believe you're missing is that these vids are NOT slow motion. They're real time. That's how you know the rolling artifact is at play, or else you'd see all of this with the naked eye. I don't discount the fact that there are smaller harmonic subdivisions within the main revolution, but these camera effects are not highlighting that in the least. They are depicting the MAIN revolution, but in a rolling scan where any given "frame" is made up of line images that were taken across a time span. Not all captured at the same time. The string's vibrating frequency (the fundamental, not any harmonics) is fast enough to intersect the rolling rate at different points across the frame. That's why the finger entering the subwoofer video stays "normal" looking. The same effect could be created by oscillating a solid piece of 1/4" steel rod, which you know would not be bending in a harmonic pattern.

...you basically have a strobe light source with a CRT display and that you are seeing a nearly symmetrical wave form of the string harmonic. What's not to like?

No, you're seeing the fundamental mathmatically subdivided by the rolling refresh rate of the CRT. If you were able to adjust the refresh rate on the CRT the visible "subdivisions" would get longer and shorter, and have no correlation to the harmonic subdivisions.

C'mon man, tell us all you're kidding.

[Mark Hammer]

I said this would interest pickup-makers, not divide them!!

Whether the string behaves exactly as the picture seems to depict, I think one can be fairly confident that it draws our attention to the absolute magnitude of string deviation.

And, on David's behalf, I will also add that I think it underscores nicely the considerable differences between the requirements of bass pickups and guitar pickups. Scale length matters.

[frankfalbo]

I said this would interest pickup-makers, not divide them!!

Ha Ha can you do one without the other? I don't think so... your original post did say the string was in slow-mo, and that's part of the hang up I think. If it were a high speed camera in slow-mo the fundamental vibration would be the primary visible motion.

[FunkyKikuchiyo]

This is the same phenomenon of trying to photograph a CRT TV or computer monitor. Ever watch a news report and see a guy's computer in the back making all sorts of flickering/rolling movements? It is because the refresh and scan aren't in sync between the camera and the monitor. With nicer cameras you can sync these up so that you can photograph a computer monitor or TV if necessary. The vibrations of the string and the scan of the camera just aren't in sync. You'd have to have a ridiculously fast camera to accurately show a string vibrating, or maybe "tune" it to the string. What you get is a series of images from practically random points put together to make a moving image, hence the strobing effect. There is useful information in looking, but I think this is an important part of making sense of the data.

[JGundry]

Jon you're killing me. Seriously are you just messing with me? Watch all the videos posted then read ALL the comments under the videos. A CRT is a rolling strobe, not a single point, pulsating strobe. Read the comments on the video defaced posted:


C'mon man, tell us all you're kidding.

Frank, I'm trying to have a serious exchange but it is hard to read past the obnoxious commentary. Maybe it is charming in person but not so much on line. You may learn more without the snark. Given that you will probably be bored to tears by the following.

Okay I had a photo shoot today for Bissell vacs, ( I have been a commercial photographer going on 15 years, I have been getting out of it but I still service a couple of sugar daddy clients, Bissell is one of them). I get a nice shoot every week or two but otherwise it is all pickups and pedals. Sweet set-up really, lots of space for both in my studio. Anyway at the end of the day I thought WTF, why not take a photo, horizontally, with a camera at 1/5000 sec. The horizontal orientation should address the artifact theory.

You can see the standing wave on this tuned down low E string in the photo link below. With the naked eye I could see a hint of a harmonic on the D string under 60 cycle lighting. But for the photo I just winged it so don't ask the frequency. BTW you can also see the super sweet paf clone covers I had tooling done for in this photo.
http://i620.photobucket.com/albums/t...andingWave.jpg

At home I do some modal analysis of free violin plates (I make violins for fun) with a set up there, but no set up to see string standing waves. However I have gone to presentations by famous makers like Joseph Curtin among others. Everything I have seen at these presentations are consistent with what you see with a CRT display. If you look a little bit into it online the harmonic patterns are clearly illustrated even with some photos.

As far as simply seeing a multiple of the fundamental? The harmonics are multiples of the fundamental by definition! We are talking about one string with multiple modes. Not multiple strings for each harmonic. Take the the CRT progressive scan path if you like but it is semantic B.S.. The harmonic is there and that is what you see with the CRT display. Franky I think the first video with the CMOS sensor is probably not too far off from harmonic reality but may have an amplitude that is juiced by the sensor.

BTW the CRT test I did last night was with and "older" color TV set. And if memory serves me right, color CRT tubes are interlaced not progressive scan tubes. Your theory depends upon having a progressive scan tube, yes? Interlaced would produce a strobe effect. I did double major in film/video and photography in college but I graduated in the early 90's so it is a bit fuzzy but I'm pretty sure I'm right.

[defaced]

Might find this of interest. This is more in line with what actually happens. Because we do not pluck the string in the center there can be some longitudinal movement of the transverse wave, but it is not propagating like the first video shows.

If you really want to prove it, take a very long string, tie it between two points, and watch it. If the frequency is low enough, you will clearly see what happens. If you'd like to engineer the test, weight per unit length, tension, and total length determine frequency. I'll let someone who's board dig up the formula. I don't' have my physics book handy to post it.

[JGundry]

Cool but it is just a slice of what the string is doing.

[defaced]

Ok...? So what else is going on?

[FunkyKikuchiyo]

Jon - the difference between interlaced and progressive is that interlace does odd lines and then even lines and alternates, while progressive (which didn't exist until relatively recently) scans all the lines at once, effectively having double the scan rate. You'll still have that rolling pattern with interlace or progressive because they both scan top to bottom. You might see a difference between interlace and progressive if you do them side by side with that sort of a test, but I'm thinking you'd have to have an amazing eye to tell the difference just like that.

Although, I have owned TVs where I could tell what setting it was on without hitting display... I have no idea if that is true for the majority the viewing public. I am rather anal-retentive, after all.