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High speed sound recording and slowing it down.

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High speed sound recording and slowing it down.

Postby GilCarera » Mon Jul 18, 2011 12:09 pm

Hi Everyone.

I'm not an expert in the field of sound recording, and I need Your help in one of the projects I'm currently working at.

I need to be able to record sound sequences - each one 40 to 60 seconds long, at very high speed. The sound is beyond the human hearing range on both ends - from 3 Hz to 55 kHz, and everything in between. But this is not an issue.

By "speed" I mean sampling rate, probably (correct me if I'm wrong). What I have now is a very simple equipment (well, comparing to pro tech), that allows me to record at 96 kHz sampling rate and 16-bit depth.

Every sequence need to be slowed down and played at 44.1 kHz and no less. I'm looking for a way to record my input and then slow it down without any loss of quality and without any significant software "intrusion" into the signal. The problem is, right now I can only slow it down to about half of the original speed before there is no more data in the signal. And I need to be able to slow it down much more - by much more I mean "as much as humanely possible without having to be a billionaire". The more the better. Anything better than half, meaning twice the playing time of the original signal is good. 50x slower or more would be ideal.

So I want to ask You:

- is this possible?
- if yes, how expensive would it be?
- what kind of equipment would I need?
- what is the current technological limit for samplig rate of an audio waveform, and therefore for lossless slowing down of the signal?

Thank You.
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Re: High speed sound recording and slowing it down.

Postby Jumpeyspyder » Mon Jul 18, 2011 12:18 pm

Hi GilCarera

You can get interfaces capable of recording at 192 khz that should give you the resolution.

The cheapest I'm aware of is M-Audio Audiophile 192 but Im sure there are plenty of other options.

What I don't know, is if there is a mic that will go that high.
(A couple of days ago I could have reccomended posting in the SOS mic forum but there isn't one anymore )

Hope this is of some help
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Re: High speed sound recording and slowing it down.

Postby Wonks » Mon Jul 18, 2011 12:35 pm

If yu have a PC then a cheap DAW program such as Reaper will gve you most of the tools you need. As Jumpy says, you really want a 192kHz recording rate, then you can play that back at 44.1kHz to give you a 4.3:1 ratio.This should bring the 55kHz sounds down to 12.6kHz - within the audible range.

If you want to get the frequency even lower you can play back the 192kHz file at slower rates than 44.1 if the DAW sofware and your audio interface allow it (e.g. 22.05kHz). Then you can take the resulting output file and then convert the file into a 44.1kHz format (this is not the same as simply playing it back at 44.1kHz).

Beyond this, you are looking at pitch shifting algorithms which may give you another octave downward shift of reasonable sound quality.

But all this down shifting will put an awful lot of sound energy in the low and very low frequency range. You will easily blow speakers if you are not very careful and any resulting files should go through a high pass filter that cuts out sound below the frequency range of your playback speakers.

Likewise if you record very low frequencies and speed up the playback to get them into an audiable frequency range, you can shift a lot of sound energy into very high frequencies which won't do the HF components of yur playback system much good either, so a low pass filter is required here.

And as Jumpy again said, getting a single suitable mic to cover that range could be a very tall order indeed.
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