Hi: What you all are working on is very interesting. Keep up the great work!
I greatly appreciate the creation of the detailed description of codec2 (https://github.com/drowe67/codec2/blob/main/doc/codec2.pdf <https://github.com/drowe67/codec2/blob/main/doc/codec2.pdf>). With that document, the code becomes easier to follow. But I have a question about the math. I’m not that sharp on DSP stuff, so this may be an obvious question to you all. If this is the wrong place to ask, feel free to redirect me. Page 14, equation 18 makes sense to me - you’re trying to characterize the relationship between Sw() and W() within each band of Sw(). And the W*(k - ROUND(mr)) also makes sense, since you need to remove the “repeating” concept from W() - a spectral copy of W() is present across the bands of Sw() in the voiced case. It’s like a modulo w0. And the fact that the W* becomes W in equation 19 also makes sense - real/even in the time domain gives you real in the frequency domain. But I can’t figure out how (k - ROUND(mr)) becomes (k + ROUND(mr)) in equation 19. Certainly W(k - ROUND(mr)) and W(-k + ROUND(mr)) are the same (even function in frequency domain), but how does W(k - ROUND(mr)) equal W(k + ROUND(mr))? I’m probably missing some property of the FFT that makes this true. Thanks in advance, Bruce KC1FSZ
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