Plugin to calculate comparative tuning of recordings
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This plugin estimates the tuning frequency (concert-A) of a recording,
by comparing it with a "reference" recording of the same music that
has a known tuning frequency. The aim is to identify the tuning
frequency of the unknown recording even in difficult cases, such as
where A is tuned more than a semitone below 440Hz.

The reference recording could be another performance at known pitch
or, for example, a MIDI rendering at 440Hz. The two recordings must be
in the same key, although I suppose you could also interpret this
plugin as finding the pitch difference of any transposition between
the two recordings' keys plus any underlying tuning difference.

The plugin expects to receive the two different recordings of the same
piece of music as its two channels of input. (You can provide these
using the --multiplex option in Sonic Annotator, for example.) If you
feed it a single piece of music, you won't get anything worthwhile.

Example usage:

$ export VAMP_PATH=.  # if running from the build directory

$ sonic-annotator -m -d vamp:tuning-difference:tuning-difference:tuningfreq PreludeInCMajorBWV846.mp3 BWV846Egarr.mp3 -w csv --csv-stdout --csv-omit-filename

Here the first file is a MIDI rendering using a piano sample at 440Hz,
and the second is a harpsichord recording tuned with A=397Hz. After
processing, this prints

0.000000000,397.009


Chris Cannam, Feb 2015