Secret Rabbit Code (aka libsamplerate) is a Sample Rate Converter for
audio. One example of where such a thing would be useful is
converting audio from the CD sample rate of 44.1kHz to the 48kHz
sample rate used by DAT players.

SRC is capable of arbitrary and time varying conversions ; from
downsampling by a factor of 12 to upsampling by the same
factor. Arbitrary in this case means that the ratio of input and
output sample rates can be an irrational number. The conversion ratio
can also vary with time for speeding up and slowing down effects.

SRC provides a small set of converters to allow quality to be traded
off against computation cost. The current best converter provides a
signal-to-noise ratio of 97dB with -3dB passband extending from DC to
96% of the theoretical best bandwidth for a given pair of input and
output sample rates.