Overview

This is a Go port of the phase vocoding analysis/resynthesis routines from Tom Erbe's program "SoundHack".

Unlike the original SoundHack, this port does not have a UI and must be used on the command line.

Differences from the original SoundHack

• gopvoc doesn't allow time stretching beyond the maximum or minimum as determined by the given inputs, see below.
• gopvoc uses a slightly different vonn Hann window function than SoundHack
• gopvoc can process AIFF files with an arbitrary number of channels.
• gopvoc can only read and write AIFF files.
• gopvoc pitch shifting can only take a multiplier scale factor for pitch (octave lower is scale factor of 0.5, octave higher is 2.0, etc).
• gopvoc time stretching can only take a multiplier scale factor for time instead of a target output duration.

Commands and Options

Flags and Options

Both time stretching and pitch shifting use the following common set of flags:

Input AIFF file path (required):

-i

Output AIFF file path (required):

-f

Number of requested bands for FFT processing (must be one of: 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096):

-b

Overlap factor (must be one of 0.5, 1, 2, 4):

-o

Scale factor (for time stretching, the amount to mutliply input duration by. For pitch shifting, the pitch shift multiplier):

-s

Windowing function for FFT processing (must be one of: rectangle, hamming, vonhann, kaiser, sinc, triangle, ramp):

-w

Quiet flag (suppress stdout information and progress bar):

-q

Time stretching can do phase locking during resynthesis, to enable it:

-p

Time Stretching

Time stretching is acheived via windowed FFT analysis of the input file, then resynthesis into the output file via overlap add resynthesis.

For a list of flags relevant to time stretching:

./gopvoc time -h

Note that the original SoundHack could crash given certain program states based on extreme stretching multipliers. The maximum or minimum allowed -s scale multiplier is dependent on the FFT window size which in turn is dependent on the number of FFT bands requested by the -b flag. Unlike SoundHack, gopvoc will cap the multiplier within this limit instead of crashing due to a division by zero. Program output will indicate of your requested -s flag has been limited.

Example:

./gopvoc time -i strings.aif -f strings_x10.aif -b 4096 -o 4 -s 10 -w kaiser

The above example takes strings.aif, and stretches it to be 10 times the original length using 4096 FFT bands with an overlap factor of 4, using a kaiser windowing function

Pitch Shifting

Pitch shifting is acheived via windowed FFT analysis of the input file, then resynthesis into the output file via oscillator bank resynthesis.

For a list of flags relevant to pitch shifting:

./gopvoc pitch -h

Example:

./gopvoc pitch -i strings.aif -f strings_octavedown.aif -b 2048 -o 1 -s 0.5

The above example takes strings.aif, and pitch shifts it down one octave (0.5 multipler of any given pitch in Hz is an octave lower) using 2048 FFT bands with an overlap factor of 1.

Build instructions

• Download and Install the Go language for your system. Gopvoc has only been tested and built with Go 1.17.
• Clone this respository
• run go build -o gopvoc
• you should now have an executable ./gopvoc in the current directory or ./gopvoc.exe on Windows.

Binaries have been cross-compiled for OS X amd64 (Intel), arm64 (M1) as well as Linux (amd64) and Windows (amd64).