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Tomba

Brass and DC offset

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So Ive been recording for a while now and i remember i discovered this weird thing a few years back and I've never got a good answer about it.

Whenever I end up looking at a waveform of brass instruments there is almost always some kind of DC off-set ranging from minimal to ....well lots. lets say the waveform looks like its at least 2/3rds on the compression side of the 0 line and 1/3rd on the rarefaction side.

This is is no way a problem for me I'm just curious to see if there is an interesting boffiny type answer as to why the wavefront of a horn produces so much DC off-set at a microphone compared to other high SPL sounds.

Usually I'm using ribbons but Ive found the same thing with moving coil mics. Also I find its the same if i go straight to the computer or if the sound lives on tape for a while before i dump it into the computer for whatever reason and end up actually looking at the recorded waveform.

Any Clues?

Cheers.

T.

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hey Captain Terrific

Well I suppose I'm just talking about my own. I suppose in theory you'd be able to make out a dc offset in a stab on a master unless the mastering engineer has done some dc off-set magic.

Ive noticed it recording different players in completely different studios and also at my home studio. So I guess I'm kinda convinced it just happens and its not as a result of something that was off on a particular day although id love to have this confirmed by other people.

I'm more just plain curious as to why it happens.

Cheers.

T.

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hmmm, it's interesting... Brass instruments proper (as in trumpet, trombone... you know, Brass Instruments) or 'Jazz'Brass' where a sax is a 'horn'?

And all on different mic/pre etc combos? What sort of mics?

And in what sort of rooms recorded? Dead/Live? Are the players playing loud?

Sorry for all the questions, but i think this interesting too

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Well how about that, huh? Weird. And I was reading and rereading the original post for different instruments, mics, pres, interface etc, desperate to find a common link that could be the culprit, but it turns out that brass is simply weird.

But how can this be? On a sustained note, is the offset constant, or has a slight oscillation to it? I'm intrigued now.

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Hey Captain. Like i said. Its usually a ribbon, (dx-77s, R-84s, Coles 4038s) but some times dynamics (57's, AKG D12s, 421s)

I reckon I've seen it on most microphones.

In most instances I end up with the microphone somewhere between 2.5 and 3.5 feet from the bell of the horn and tilted slightly of axis from looking straight down the barrel of the gun so to speak.

Preamps usualy Neve 81 series at one studio or 1064s.at another. Or I might use the AEA ribbon pre (TRP) for super clean tracks.

It doesn't seem to make a difference. I'm almost convinced its got something to do with the acoustic property's of the wave front of brass instruments.

I must stress that I'm not convinced its really a big problem. The horns sound good and i don't think its putting enough off-set in my mixes to hurt peoples speakers or anything. I still seem to be getting enough dynamic range to give the horns a good squeeze with a compressor.

I just find it odd. No other sounds do this. Ive thought about the fact that maybe at the beginning of a horn note there is a single loud burst of pressure that forces the ribbon of diaphragm into a wild swing before it starts to vibrate and produce a note. but then i thought you'd only every really see that for a moment. The DC off set i see is sustained all the way to the end of the note.

So i feel forced to to feel that there is some really weird spooky property about horns that can consistently hold a suspended vibrating element in compression and establish its own 0 line on top of this.

?????

What else does that?

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...I'm almost convinced its got something to do with the acoustic property's of the wave front of brass instruments...

This is what I'm thinking too... I wonder if it does it on bagpipes?

I have a tuba laying round here somewhere, perhaps i should do some experiments myself this weekend

Like you, I find this pretty intriguing.

Cheers for the new project!

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When I think about it, it makes sense, on a very basic level. Brass instruments create sound by the movement of air in one direction. It flows through and out of the instrument. So the sound being produced has an inherent push in one direction, which translates to a DC offset in the electronics world.

The sound created by most other instruments that come to mind are more of an even vibration, a push and a pull that evens out closer to 0. Like the pluck of a string.

It's so basic that it might not be correct...but it just makes sense, haha

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