Odd Waveform

Has anybody ever seen a signal like the top one here? It's a 'click' or 'screech' sound. the middle is a cymbal crash and bottom is a voice. what's the top one?
(btw, we're looking at .002 seconds here-)

Whatever it is it has a frequency of 3.75KHz.

Whatever it is it has a frequency of 3.75KHz.

Which is not a million miles from a beat frequency between 44.1 and 48kHz? 

Whatever it is it has a frequency of 3.75KHz.

how did you figure that out?
and- why is it so jagged and angular?

Whatever it is it has a frequency of 3.75KHz.

Which is not a million miles from a beat frequency between 44.1 and 48kHz? 

well...3.75k is an audible frequency- 44.1 and 48khz are not- I'm not clear on how that works, 44.1k, etc.-
but it is a long way from 3.75khz...
what gets me is its jaggedness;angular; not like a normal wave-

That's only because it's at a frequency that's not so far from the sampling rate, and the display isn't showing a smoothed curve between samples.  There's nothing abnormal in that.

Paul

Which is not a million miles from a beat frequency between 44.1 and 48kHz? 

well...3.75k is an audible frequency- 44.1 and 48khz are not- I'm not clear on how that works, 44.1k, etc.-
but it is a long way from 3.75khz...

What ryclark means is that if you subtract 44.1 from 48 (which is how the beat might appear) you get 3.9k. If you allow for a measuring error in the 3.75k and it's actually 3.9k, this might explain it.

Note that the track concerned is sampled at 22.05kHz.

Paul

22.05k with harmonics?

Although if that waveform is as big as it looks, I suspect that it has nothing to do with sampling at all. And that's why I didn't comment on it in the first place; I don't think that we have anything like enough information about it, or an actual sample. Looks more like the sort of interference you'd get from a dodgy switch being thrown than anything else.

how did you figure that out?

Sample rate 22.5K, 6 samples per wave. 22.5/6=3.75. Voila!

Hmm... when I checked it along the whole wave, it wasn't exactly consistent - and I would have expected a sample interference pattern to be rather more regular than this one - which is why I think it's interference.

WOOOOW!!!
i'm not so good at using this forum, and i want to reply to all your thoughts here...

Sample rate 22.5K, 6 samples per wave. 22.5/6=3.75. Voila!
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so it seems you're agreeing the wave is what... a harmonic of the recording frequency; 22.05khz...? is that what you mean by a 'beat'?
i have no knowledge of how sampling works- the recording software takes samples 6 times per wave...what wave?

Note that the track concerned is sampled at 22.05kHz.

Paul

Paul, how did you know that?

22.05k with harmonics?

Although if that waveform is as big as it looks, I suspect that it has nothing to do with sampling at all. And that's why I didn't comment on it in the first place; I don't think that we have anything like enough information about it, or an actual sample. Looks more like the sort of interference you'd get from a dodgy switch being thrown than anything else.

SteveG, what do you mean by 'big'? amplitude?

That's only because it's at a frequency that's not so far from the sampling rate, and the display isn't showing a smoothed curve between samples.  There's nothing abnormal in that.

Paul

Paul, so, you mean that since the 'recorded frequency' is close to a beat frequency (or harmonic) of the 22.05khz sampling rate, ther resulting recorded data would then be jagged and angular? like distorted?

because it's not a 'perfect' consistent sine wave or whatever you want to call it- (looks sortof like a jagged sine wave, right?)

Hmm... when I checked it along the whole wave, it wasn't exactly consistent - and I would have expected a sample interference pattern to be rather more regular than this one - which is why I think it's interference.

so, SteveG, as interference- what kind of interference could it be? like an electromechanical unplugging/plugging of a mic? other physical event? or a digital one?

If you're interested enough- i made a 6 minute video of the making of these images; scaling them down to the .002 seconds from the original file in audacity; and, i have the .wav file and the original .ftr file the .wav was made from too... i'd love to figure out what this thing is...

That's only because it's at a frequency that's not so far from the sampling rate, and the display isn't showing a smoothed curve between samples.  There's nothing abnormal in that.

Paul

Paul, so it's just the display not showing curves- that are actually there?

It's the nature of digital audio.  The curves are not really there, but neither are the straight lines - only the samples.  To make the sequence of samples more natural to work with, the program displays lines between the points; some programs fit a curve to the points, which is closer to the effect of playing back.  For lower frequencies this looks perfectly natural, but at frequencies closer to the sampling frequency you can get this strange-looking effect - but once you understand it, you realise it can be ignored.

One way to look at this is to remember that no frequency that is higher than half the sampling frequency can be represented by sampling (for the sampling to work, you must actually filter them out as a part of the process); if you play back the jagged wave formed by joining the dots there will be lots of higher frequencies, but then you filter them out (because they can't have been there originally) and you are left with the smooth waveform you expect.

I knew the sample rate because it's listed in the details to the left of the waveform!  Note how the dots (indicating the actual sample values) are much closer in the central wave, which is sampled at 44.1kHz.

Paul

SteveG, what do you mean by 'big'? amplitude?

Judging from the way the sample you posted is scaled, it looks to be normalised to a +/- value of 1, and the amplitude uses all of it. So that would be pretty much a full amplitude signal - ie, pretty loud.

It's the nature of digital audio.  The curves are not really there, but neither are the straight lines - only the samples.  To make the sequence of samples more natural to work with, the program displays lines between the points; some programs fit a curve to the points, which is closer to the effect of playing back. 

Generally very close! Thing about real-world waveforms is that they can't change their levels instantaneously, and all the samples represent is 'steering points' towards a final analogue waveform that can be realised in a physical sense. So if you looked at the output from your sound device on an analogue oscilloscope you would see the curves, and you wouldn't (or shouldn't!) see any samples.