[sdiy] vocoder filters
rburnett at richieburnett.co.uk
Sat Aug 31 22:52:41 CEST 2019
>.... I was wondering the other day if there are vocoders with dynamic poles. I'm not sure how to explain this well, but the idea is to use DSP to do sophisticated analysis of the modulator, then dynamically modulate (think VCF) the carrier filters' frequencies accordingly.
Yes, there are definitely vocoders that use filters with dynamic poles...
Take a look at speech CODECs like linear predictive coding, CELP, SPEEX, etc. The coding stage tries to identify the prominent spectral peaks (formants) by an analysis process of "whitening" the spectrum. Then enough information is transmitted to reconstruct the speech from an impulse train excitation passed through a multi-pole filter that imparts the required spectral signature. That's a bit of an oversimplification, but a Google search should fill in the details about the "residual", etc.
A variant of this digital "vocoder" is used in the GSM baseband processing that conveys speech over mobile phone networks with minimal bitrate.
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