Implementation Notes » History » Version 4

Chris Cannam, 2014-04-30 05:17 PM

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h1. Method and implementation Notes
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h2. Publications
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 * Main reference. "A Shift-Invariant Latent Variable Model for Automatic Music Transcription":http://www.mitpressjournals.org/doi/abs/10.1162/COMJ_a_00146
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 * MIREX 2012: a simplified method, without the HMM block. "Multiple-F0 Estimation and Note Tracking for MIREX 2012 using a Shift-Invariant Latent Variable Model":http://www.music-ir.org/mirex/abstracts/2012/BD1.pdf
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 * Updated in 2013 as "Multiple-instrument polyphonic music transcription using a temporally constrained shift-invariant model":http://openaccess.city.ac.uk/2155/
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 * For more recent work in MATLAB, see the project "Automatic Music Transcription using efficient Multi-Source SI-PLCA (+GPU support)":/projects/amt_mssiplca_fast.
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We are aiming to produce an "online" implementation (i.e. not requiring all audio before processing starts) in Vamp plugin form, based on the MIREX 2012 method.
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h2. About the method
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The basic flow is audio -> "Constant-Q transform":/projects/constant-q-cpp -> "Probabilistic Latent Component Analysis":http://www.cs.illinois.edu/~paris/pubs/plca-report.pdf using "Expectation-Maximization":http://en.wikipedia.org/wiki/Expectation%E2%80%93maximization_algorithm -> some note-clustering algorithm.
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The CMJ paper uses a hidden Markov model for note clustering, but the MIREX submission used a simple thresholding method. The thresholding method worked quite well, so we should probably use that.
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The published method has a convolution stage to handle fine pitch variations with a 20 cent resolution. This makes the process many times slower, gaining maybe 2% in overall performance, so we should probably omit it to begin with. The diagrams in "Benetos 2013":http://openaccess.city.ac.uk/2155/ illustrate this stage.
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There is no accommodation for percussion; one might preprocess to remove broadband percussive events.
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There is no accommodation for typical qualities of vocal performance (portamento, vibrato etc).
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h2. Implementation notes
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*Constant-Q transform*: the existing code uses it uses Anssi's "MATLAB toolbox":/projects/constant-q-toolbox. This is substantially better than the Constant-Q in the "qm-dsp library":/projects/qm-dsp. A good first step would be to do a good new C++ Constant-Q implementation. See "this project":/projects/constant-q-cpp for that work.
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*Other notes*:
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 * We could potentially parameterise the sparsity level on z ("sz" variable in MATLAB) as a rough correspondence with number of simultaneous notes
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 * The MIREX method also eliminated any polyphony > 4 by dropping weaker notes
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 * No particular temporal constraints -- the template has no "duration" -- meaning input can be broken up as required, could be processed completely frame-by-frame at the cost of having to reinitialise EM at each frame
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h2. How to test
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 * Constant-Q -- compare with Anssi's MATLAB
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 * Random initialisers for EM mean the method doesn't always produce the same output but it generally converges to within 1% say
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 * Can compare pitch-activations (equation 12 in the paper, "z" variable in the MATLAB code)
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 * Test data: Trios dataset (in C4DM datasets) + MIREX development dataset + RWC + MAPS
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See also: Joachim's work and comparison of different CQT methods: http://www.eecs.qmul.ac.uk/~jga/eusipco2012.html