added modified MFCC features based on DNN-c and fDNN-c features; it i… #2908
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…s activated using --modified option.
…fied-mel-kaldi
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Any preprint available of the paper mentioned? |
src/feat/mel-computations.h
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| @@ -48,14 +48,16 @@ struct MelBanksOptions { | |||
| BaseFloat vtln_low; // vtln lower cutoff of warping function. | |||
| BaseFloat vtln_high; // vtln upper cutoff of warping function: if negative, added | |||
| // to the Nyquist frequency to get the cutoff. | |||
| bool modified; // If true, use 'modified' MFCC, which uses a breakpoint of | |||
| // 900 instead of 700. | |||
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changes to documentation needed here
| @@ -69,6 +71,13 @@ struct MelBanksOptions { | |||
| opts->Register("vtln-high", &vtln_high, | |||
| "High inflection point in piecewise linear VTLN warping function" | |||
| " (if negative, offset from high-mel-freq"); | |||
| opts->Register("modified", &modified, | |||
| "Modified MFCCs, based on paper 'An alternative to MFCCs for ASR' " | |||
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Please update this documentation to be accurate and fix typos. (the stuff about 1nt and 2nd formant isn't accurate any more, I believe).
src/feat/mel-computations.cc
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| a lot of bins, their diamter is defined by a formula and it's a function of | ||
| the center frequency f of the bin: | ||
| diameter = 30 + 60 f / (f + 500). | ||
| so it increases from 30Hz to 90Hz with a knee around 500Hz. |
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This documentation seems a bit out of date.
src/feat/mel-computations.h
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| // breakpoint_ is 700 for normal mel, or 900 for modified. | ||
| inline BaseFloat InverseMelScale(BaseFloat mel_freq) { | ||
| if (sec_breakpoint_ > 0.0) | ||
| return 3500.0 * (expf((expf(mel_freq) - breakpoint_) / 3500.0) - 1.0); |
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I imagine this should be sec_breakpoint_ instead of 3500.
src/feat/mel-computations.h
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| // and for other purposes. | ||
| BaseFloat breakpoint_; // The breakpoint in the mel scale: 700 normally; | ||
| // 500 if opts.modified is true. | ||
| BaseFloat sec_breakpoint_; // The second breakpoint used in the modified |
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please call this either second_breakpoint_ or breakpoint2_.
src/feat/mel-computations.cc
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| BaseFloat diameter_floor = (next_center - center_freq) * 1.1, | ||
| diameter = 30.0 + 60.0 * (center_freq / (center_freq + breakpoint_)); | ||
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| diameter = pow(diameter * diameter + diameter_floor * diameter_floor, 0.5); |
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I think sqrt would be easier than pow(.., 0.5).
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Tangential thought unrelated to the contents of this MR. It pleases me that someone at last had a look at the feature engineering part of our overall business. MFCC were invented to drop as much "irrelevant" information as possible, when ASR was tiny and puny. With the DNN renaissance, our general approach has changed: just give the network all information you have, and let it figure out what is really correlated. I am not at all sure that the currently "standard" features discard mostly useless information. The field mostly got rid of HMMs (hooray!) which make no sense in modeling speech signals: they decay exponentially, which speech obviously do not, ye-e-e-e-eah. My general feeling is our features are another dinosaur that has outlived its time. |
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…s activated using --modified option.