[sdiy] Vocoder dabblings

nvawter at media.mit.edu nvawter at media.mit.edu
Tue Oct 23 17:13:39 CEST 2012


Oh hey just wanted to voice my praise for those vocoder demos.  very  
excellent.

I'm also curious to see just how you implemented the multiplies in the  
SVFs - any fraction-saving, dithering or straight up truncation, etc?




Quoting Tom Wiltshire <tom at electricdruid.net>:

> Hi Richie,
>
> I think that sounds really good! It's indisputably "vocodery", which  
> is what you're after. I suppose having got it working and the  
> structure of it sorted out, the question would then turn to "how  
> does it sound?" and that's where the art takes over from the  
> technical side. Presumably "well-thought of" or "classic" analog  
> vocoders have their own specific characteristics of timing and  
> frequency responses or distortion that give them their particular  
> "sound". Adding modelling to this level of complexity would  
> presumably give a digital vocoder a similar level of character.
>
>> Processor is dsPIC30F6012A 16-bit DSC from Microchip running at  
>> 25MHz. Audio sample rate is 24kHz.  Vocoder is 20 bands (20  
>> analysis + 20 synthesis), each 1/3rd octave, filters are 8th order  
>> Butterworth band-pass response (24dB/oct slope each side)  (160  
>> seperate state-variable filters in total!)
>
> So each filter is two SVFs in series, right? Are these typical  
> 'Chamberlin' SVFs or did you do anything clever to them? The  
> Chamberlin SVF is only supposed to be stable to 1/6th of the sample  
> rate, so I'm guessing you used an alternative algorithm to avoid the  
> need to oversample the filters.
> I'm not up to date on digital filtering, so I'd be interested to  
> hear your thought process when working out what filter design to  
> choose for this application.
>
>> Lowest band is 60Hz, highest band is 6kHz, although the bands in  
>> the lowest octave do little with speech signals.  Sibilance from  
>> modulator above 6kHz is high-pass filtered and mixed in with the  
>> modulated carrier to increase intelligibility.  +3dB/oct spectral  
>> tilt is added for classic sawtooth/square based carriers, and  
>> -3dB/oct spectral tilt is added for white noise carrier.  No  
>> user-interface or spectral display.
>>
>> Modulator audio was either Radio-4 news, or audio sample played  
>> from Akai sampler.  Carriers are various waveforms or white noise  
>> from Roland SH-09, or hoover sound from Juno-2.
>>
>> Never built a vocoder before, either analog or digital so please be kind...
>
> A vocoder is not a simple thing in any incarnation, so I think you  
> should be proud of having pulled it off at all!!
>
> Regards,
> Tom
>
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