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Noise Gating (aka the Phonetic Typewriter)


Michael Lee

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Noise gates are an integral part of most ITC systems. They are a subset of something called expanders, whose job is to expand the dynamic range of a certain ranges of the signal. Below a gate, noise is attenuated. Above the gate, the signal is amplified to achieve more clarity. 

If you listen to the raw sound of a typical entropy / noise source, it's sounds pretty boring, as if there's nothing interesting or "paranormal" going on. However, when you expand or noise gate the signal, you emphasize the slight variations from random and presumably the weak signals from spirit. 

Most of the time, gating can be performed in software, as the first in a chain of effects. It can also be performed in hardware using a noise gate guitar pedal. 

Typical the noise gate works as follows. It first waits for voltage or samples above a user-defined threshold. When these "spikes" are detected, a "gate" is opened which allows sound through for a pre-determined period of time, before closing the gate. The gate can either allow sound above a second threshold or all sound during the open phase.

In my scripts, I wrote my own gate for a 1.024-second clip, which first detects all of the "spikes" above a threshold, then convolves those spikes to a window function (~100 ms). The resultant window is then sample-by-sample multiplied with the original clip.

In addition, if the noise-gated signal sounds too choppy because the gate is short (<50 ms), we can apply time-stretching techniques to make the spikes sound more realistic. The simplest idea is to convolve the signal with an all-pass filter, which is similar to adding reverberation (room echo).

Attached is a Python script that allows you to select input and output devices, and then interactively control a noise gate on whatever noise / phoneme source you like. I also provide a Windows executable of Noise Gater for people who don't want to install Python and the dependent modules.

From recent tests, I have determined that this gate is more sensitive than the Neewer noise gate guitar pedal that is often recommended. However, the software gate is not 100% real-time, it is always time lagged by 2 seconds.

Finally, to get you started. I also have a Scrambled phoneme file (150 ms) that you can play with your cellphone into your PC audio input. Keep the volume low (25%) for best results. You need a mix of noise (affected by spirit) and recorded audio for gating to work, otherwise you'll just be gating the same recorded audio over and over again. 

noise_gater.py

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