audacity/lib-src/libsoxr

SoX Resampler Library       Copyright (c) 2007-12 robs@users.sourceforge.net

The SoX Resampler library `libsoxr' performs one-dimensional sample-rate
conversion - it may be used, for example, to resample PCM-encoded audio.
For higher-dimensional resampling, such as for visual-image processing, you
should look elsewhere.

It aims to be fast (e.g. multi-channel resampling can utilise multiple
CPU-cores), high-quality (i.e. bit-perfect up to 32-bits, for occupied-
bandwidths up to 98%¹), and to allow any rational or irrational resampling
ratio.  Phase-response, preserved bandwidth, aliasing, and rejection level
parameters are all configurable; alternatively, simple `preset'
configurations may be selected.

The resampler is currently available either as part of `libsox' (the audio
file-format and effect library), or stand-alone as `libsoxr' (this package).
The interfaces to libsox and libsoxr are slightly different, with that of
libsoxr designed specifically for resampling.  An application requiring
support for other effects, or for reading-from or writing-to audio files or
devices, should use libsox (or other libraries such as libsndfile or
libavformat).

Libsoxr provides a simple API that allows interfacing using any commonly-
used sample format or buffering scheme: clients may use either floating-
point or integer sample formats, and, in the case of multiple channels,
either channel-interleaved, or split channels.  The API is currently
documented in the header file `soxr.h' and by example code in the 'examples'
directory.

For compatibility with the popular `libsamplerate' library, the header file
`soxr-lsr.h' is provided and may be used as an alternative API.  For details
of that API, see http://www.mega-nerd.com/SRC/api.html.  Note however, that
libsoxr does not provide a full emulation of libsamplerate; in particular,
`vari-speeding' is not supported.

For build and installation instructions, see the file `INSTALL'; for
copyright and licensing information, see the file `LICENCE'.

The resampler was inspired by Laurent De Soras' paper `The Quest For The
Perfect Resampler', http://ldesoras.free.fr/doc/articles/resampler-en.pdf;
it combines Julius O. Smith's `Bandlimited Interpolation' technique
(https://ccrma.stanford.edu/~jos/resample/resample.pdf) with FFT-based
over-sampling.

Its use of FFTs means that, for real-time resampling, libsoxr may have a
higher latency than non-FFT based resamplers.  E.g. when using the `High
Quality' configuration to resample between 44100Hz and 48kHz, the latency is
around 1000 output samples, i.e. roughly 20 ms.

¹ Note that no practical resampler can be bit-perfect to 100% bandwidth.