The physics involved sets the


The physics involved sets the success. The physics is very simple. You take the left channel, and then add the waveform of the right channel upside down.


If the recording is of a person speaking recorded with one microphone, and they are of the very left hand side so all their voice comes from the left channel, and there is nothing in the right – then when you add them together you get a mono recording with full voice. Mover them right and repeat, and the same thing happens, dispite that channel being flipped upside down. However, if they are exactly equal – as in when the subject is right in the middle – flipping one upside down exactly cancels out the original. Think of a sine wave shape – if in the left channel it starts by going up, but on the right it starts by going down, then adding them together produces perfect cancellation.

In a music track – the bass might be a bit to the left, the piano a bit to the right, and the voice dead centre. Do the flip and combine trick and the piano and bass will still be there but the voice has magicaly vanished.

Now the problem. Voices will usually be treated in the recording – reverb and other effects. If the reverb added to the voice is in mono – equally split left and right, then it too gets cancelled. The problem comes that now people use stereo reverb, often modelling real buildings or types of location, and the reverb is different so it sounds more real. This means the cancellation will not be complete – and a common result is that the voic e gets cut, but leaves a ghostly reverb sound. You will of course also cancel any other sounds that appear central. This is different on every recording. They will be mixed to sound good, not to cancel properly. Some systems to cancel vocals really are as simple as flip and add, while others add in filtering so that the cancellation is more in the speach range – 250Hz to maybe 4KHz or so. Others use variable phasing to allow the cancellation to be optimised. Duets of course rarely work as the two voices will be spaced apart and if you cancel one, the other gets through. 

The conclusion of all this is that NO voice canceller is guaranteed to work. It really depends on how the recording was done. Early Beatles rcordings nearly always fail, and early Elton John were well known for leaving a ghostly Elton very present in the mix.

A trailer song is just a song really – and some may be available as ready made tracks. If they are, I'd simply buy it – too much effort and too variable quality to try to remove the bit you don't want. Even the best ones will remove bits you would rather have kept – making the end result weak in mamny cases.



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