Audio Signal Difference Extraction

Question

I'm from visual arts background and so, I'm not familiar with technical aspects of sound design. But I'm exploring and learning everyday.

Currently, I'm working on a sound art project, for which I'd need to subtract Audio Signals of Clip B from Clip A to extract the signal difference between the two clips.

Clip A - Field recording of a particular space [ l ] during day time, the crowded hour

Clip B - Field recording of the same location [ l ] during night time, after the crowd has dispersed

So far I've come across some articles on Audio DiffMaker - a Signal Difference Extraction Software, which does exactly what I'm looking for. However, I'm using Mac OS, so I'm unable to use the program. It's not practical for me to run Windows on Bootcamp as all the other programs I use regularly are on Mac platform.

Is there a program that does similar job as Audio DiffMaker? Would Audicity or Ableton Live 9 has the option to extract the Signal Difference from the 2 audio clips?

Looking forward to your constructive advice and suggestions. Many thanks!

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15 Oct 2015

Hi! Thanks a lot for your replies.

And thank you @he_artburns for your detailed advice. You perfectly understood what I was trying to achieve with this experiment. I now understand better why it wouldn’t work with programs like Audio DiffMaker in my situation, and how it might work with the “trick” as you've suggested.

My project is about narrating sonic memories of people and spaces. Just like @he_artburns has mentioned, I’m extracting the “sound of the world”, or rather, sonic memories of the people from a particular space within the world. I hope to measure the weight of these memories, to understand the VOID it would create on the other spectrum, if they were to be removed, from this world.

I've tried to look for similar plug-in's to Waves X-noise, which is either free or will not cost as much, but without much success. What is your opinion on H-EQ Hybrid Equalizer and Z-Noise? I also have Adobe Audition installed. Will that work too?

Pardon me please, if I’m asking the question to something that has an obvious answer. I’m unsure how to go about processing this: “...build a destructive EQ with troughs at every major peak you can see on the spectogram and hear in the night sound. Then copy that EQ shape onto a track with the day sound…”. Could you tell me what this process is called, so that I could read up/ explore more on this?

Thank you :)

Answer 1

In one sense, the answer from "audionuma" is correct, a similar process is possible in both Ableton and Audacity. In Ableton, all the tools you need to do this are contained in the "Utility" plug-in, which you can use to phase-invert one recording and thus "subtract" it from the other....

HOWEVER**

Neither Audio diff-maker nor the more basic process as described by audionuma would work for your situation! It's a cool idea, and I have some other thoughts about how you could achieve it, but the reason these are not the right tools is that phase-inverting audio and recombining it will only remove the component of the audio signal that is IDENTICAL in both recordings. In your case you are looking at two completely different recordings. The sound of the world is always changing and it is extremely unlikely that any component of your two recordings would be exactly alike on a sample by sample level. In other words, you can't directly, mathematically subtract the night sound from the day sound because they're two different sounds. Also, the methods you're considering would not subtract specifically B from A, they would technically subtract each from the other...What you would be left with is a combination of night and day sound in a big mess.

BUT THE GOOD NEWS**

...there is a neat trick I would suggest for you. If you use the night-time recording to "teach" a Noise Reduction plug-in what noise to filter out, then you could apply that plug-in to the Daytime recording and in effect, it would filter out the "shape" of the night noise from the day recording. The result should be an abstracted sound of the crowd, plus if you push the noise reducer hard it will cause warbling distortions at the edges of sounds that are pretty interesting.

Waves X-noise is an easy to use noise-reduction plug-in, but there may be freeware that does the same thing, just look for intelligent noise reduction.

You can also do this to a certain extent manually using an EQ with spectogram analysis (built-in to Ableton, Logic, etc..) Look at an analysis of the night sound and then build a destructive EQ with troughs at every major peak you can see on the spectogram and hear in the night sound. Then copy that EQ shape onto a track with the day sound, listen to your filtered version and adjust to taste.

Answer 2

You might get the difference between signal A and signal B by putting them on two parallel tracks, invert the phase of track B and sum both tracks on a mix bus.