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[email protected] 3rd July 2007 05:09 PM

What does this wave form mean?
 
1 Attachment(s)
This is the wave form of my inside kick drum mic (re20). Does it indicate that i am having some sort of phase problems? why is the wave form not distributed equally?

thanks
matt

slaves666 3rd July 2007 05:15 PM

looks like you are clipping the input of your AD

matt thomas 3rd July 2007 05:20 PM

its fine, at a guess it'll just be the pressure when the kick head is compressed thats making it lop sided

you can't tell if there is phase problems from just one track btw

if it sounds fine its fine (which I'm guessing it does)

zoom in on the wave and check its not clipped on the top tho

although, even then if it sounds fine it is

narco

[email protected] 3rd July 2007 05:22 PM

thanks

it does sound fine

narcoman 3rd July 2007 05:52 PM

waveforms arent and should not be the same "either side" for the reasons my similarly monickered friend has mentioned....

Drumsound 3rd July 2007 06:09 PM

As recording engineers working for a sonic goal we should rely on out ears not our eyes.
shiee

Dave Peck 3rd July 2007 06:41 PM

It looks like a normal acoustic transient. You see this effect on nearly all acoustic percussive signals, especially close-mic'd.

But a suggestion - while listening to this kick track along with the other drums tracks, try inverting the polarity of this track so the transient is positive instead of negative. It can sometimes make a noticeable (and desireable) difference if percussive transients are going positive, which translates to outward speaker excursion. If it sounds better, it's better.

DP

alfonso 4th July 2007 12:16 AM

Probably it means that the first transient happens when the skin (thus the air) is moving to one direction as hit by the pedal so the overall sound at that stage is influenced heavily by the negative (in this case) portion as detected by the mic. That can happen also with synthesized percussive sounds or samples with very fast attack and decay stages. Some other instruments (many wind ones) show constant DC offsets when recorded, probably because of the constant direction of the cause of vibration, the player's breath.

narcoman 4th July 2007 08:59 AM

Quote:

Originally Posted by Drumsound (Post 1359248)
As recording engineers working for a sonic goal we should rely on out ears not our eyes.
shiee

yeeeeesssssssshiee

e-cue 4th July 2007 09:59 AM

looks like DC offset to me...

unsung 4th July 2007 02:54 PM

Quote:

As recording engineers working for a sonic goal we should rely on out ears not our eyes.
Ya know, I'm sure the guy has figured out by now that he should use his EARS to record and mix stuff. He just was interested in a wierd looking wave, that's all. Really really great advice though.

StudioTinPanAll 4th July 2007 03:53 PM

what is dc offset ??

narcoman 4th July 2007 06:57 PM

Quote:

Originally Posted by StudioTinPanAll (Post 1360890)
what is dc offset ??

its the extra tax they charge for living near GWB.

i'l go........

orange 4th July 2007 09:16 PM

those people saying that you should use your ears and not your eyes are being a bit smart arsed....Although I agree with the "if it sounds good then it is good", there are times when your eyes might pick up problems that your ears would have problems distinguishing. DC offset is a classic example. High frequency (above 20khz) oscillations are another.

Dave Peck 4th July 2007 10:11 PM

Quote:

Originally Posted by StudioTinPanAll (Post 1360890)
what is dc offset ??

DC offset is when the entire waveform is not 'centered' around the zero voltage line. It is offset higher or lower. This can be a result of normal acoustic properties of the audio waveform or it can be caused by various types orf electronic signal processing or types of distortion (intentional or otherwise).

It is generally a good idea to correct DC offset before the signal gets recorded to allow a higher recording level (DC offset will reduce the level at which you can record because either the positive or negative side of the wave will clip prematurely since it is offset in one direction or the other). This DC offset correction can be done with a simple high pass filter that is set to a cutoff frequency well below the fundamental of the audio signal. In this application, you don't want the filter to affect the sound of the track, you just want the track re-centered around zero before it goes to the recording device. A HPF will re-center the signal since DC is, in effect, an infinitely low frequency. Therefore, the HPF 'blocks' the DC component of the signal and moves the entire waveform up or down to re-center it around zero. A high pass filter with a very low cutoff frequency like 15 Hz will work fine.

However, the example of the kick drum posted above is not an example of DC offset. If you study the waveform you will see that there is one big transient at the attack, but after that the bulk of the signal is centered around zero and when it fades out there is no DC component left - the flat line between kick drum hits is at zero, with no DC offset. This particular example is just a normal negative-excursion transient.

DP

[email protected] 4th July 2007 10:18 PM

thanks Dave!

very helpful!


matt

bigbone 4th July 2007 10:25 PM

Quote:

Originally Posted by Dave Peck (Post 1361456)
DC offset is when the entire waveform is not 'centered' around the zero voltage line. It is offset higher or lower. This can be a result of normal acoustic properties of the audio waveform or it can be caused by various types orf electronic signal processing or types of distortion (intentional or otherwise).

It is generally a good idea to correct DC offset before the signal gets recorded to allow a higher recording level (DC offset will reduce the level at which you can record because either the positive or negative side of the wave will clip prematurely since it is offset in one direction or the other). This DC offset correction can be done with a simple high pass filter that is set to a cutoff frequency well below the fundamental of the audio signal. In this application, you don't want the filter to affect the sound of the track, you just want the track re-centered around zero before it goes to the recording device. A HPF will re-center the signal since DC is, in effect, an infinitely low frequency. Therefore, the HPF 'blocks' the DC component of the signal and moves the entire waveform up or down to re-center it around zero. A high pass filter with a very low cutoff frequency like 15 Hz will work fine.

However, the example of the kick drum posted above is not an example of DC offset. If you study the waveform you will see that there is one big transient at the attack, but after that the bulk of the signal is centered around zero and when it fades out there is no DC component left - the flat line between kick drum hits is at zero, with no DC offset. This particular example is just a normal negative-excursion transient.

DP

Thank you Dave , that was go info, not some bull*** for other poster........

thenewyear 4th July 2007 10:27 PM

As Dave says it's not DC offset, it's just an asymmetric waveform.

This is not that unusual a thing to come across with low frequency non-harmonic sources. Although you do also see it quite a bit with brass instruments (as someone else already pointed out) for reasons that I don't quite fully understand. Somebody once used the phrase 'air saturation' when trying to explain it to me. I'm not sure they fully understood it either.

Dave Peck 4th July 2007 11:43 PM

Quote:

Originally Posted by thenewyear (Post 1361474)
As Dave says it's not DC offset, it's just an asymmetric waveform.

This is not that unusual a thing to come across with low frequency non-harmonic sources. Although you do also see it quite a bit with brass instruments (as someone else already pointed out) for reasons that I don't quite fully understand. Somebody once used the phrase 'air saturation' when trying to explain it to me. I'm not sure they fully understood it either.

The reason it happens with things like wind instruments is because the instrument is pushing air out into the room towards the mic. Think of it this way - the trumpet waveform is created by air resonating inside the instument and exiting the bell in a series of 'pulses' of acoustic pressure - the air never goes back INSIDE the bell, so the waveform never goes negative, only positive. Thus, a positive DC offset. This can be corrected before the signal gets recorded by using the high pass filter technique described above, or by other methods like moving the mic further away from the instrument or moving it off-axis.

DP

thenewyear 4th July 2007 11:51 PM

that makes sense. It's interesting, the medium that our acoustic wave is traveling in (the air) is also traveling itself!

Mic the instrument too close and the pitch will go sharp! (OK probably not enough to ever hear it but you get my point. It's like the Doppler effect on a tiny scale).

Dave Peck 5th July 2007 07:40 AM

Quote:

Originally Posted by thenewyear (Post 1361576)
that makes sense. It's interesting, the medium that our acoustic wave is traveling in (the air) is also traveling itself!

Mic the instrument too close and the pitch will go sharp! (OK probably not enough to ever hear it but you get my point. It's like the Doppler effect on a tiny scale).

No, placing the mic closer to the bell may increase the DC offset in the signal that is picked up by the mic but it won't affect the pitch. The only thing that would make the pitch go sharp would be if the 'pulses' in the acoustic waveform happened more rapidly, spaced closer together (meaning at a higher frequency).

DP