[sdiy] vocoder filters

[David G Dixon]

So, today I have done some simulations, and have discovered that I can
replicate the frequency response and delay characteristics of Jurgen
Haible’s 8-pole (4-stage) BP filters with my own 4-pole (2-stage) BP filters
with a Q of about 3 (for each stage).  The output frequency responses look
virtually identical, and given an input sine wave (or square wave) at the
filter’s corner frequency, the output signal is up to full amplitude within
5 or 6 periods, for both filters.  When the input signal is turned off, the
ringing characteristics of both filters are also essentially identical, with
the signal dying away within 5 or 6 periods.

 

>From this, I conclude that I can get the same results that Jurgen Haible got
with about half of the filter hardware, as I suspected.  This is nice, since
I have already laid out and built a PCB with 12 of these 4-pole BP filters
on it for a fixed filter bank, and I can use the exact same layout for the
vocoder.

 

  _____  

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Magnus
Danielson
Sent: Sunday, September 01, 2019 11:25 PM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] vocoder filters

 

Dear David,

Many other vocoders used higher Q setups, and Jürgen and I agreed that you
need to avoid that. The reason is that higher Q gets you into higher group
delay, making the filter more sluggish to respond and rings longer after the
end of a sound. It also causes different time-shifts between bands to become
bigger than if you use low Q.

Jürgen was inspired by the EMS3000 vocoder until I sent him the EMS5000
manual and schematic, which derailed the project a little. :)

So, it is not recommended to go for higher Q, and I think you get why.

Cheers,
Magnus

On 2019-08-31 06:05, David G Dixon wrote:

Hey SDIY Team!

 

I’m thinking about building a vocoder, and I have a general question about
the bandpass filters.

 

I’ve looked at Jurgen Haible’s Living Vocoder, and he used 8-pole filters
with low Q.  These give a reasonably broad band with fairly steep slopes.
He makes the filters from two pairs of LP and HP.

 

I was thinking about using BP filter sections, but just 4-pole, and with
higher Q (around 10).  This gives a somewhat narrower band, and the slope is
steep near the corner, but fairly shallow around the skirt.  This idea uses
a lot fewer components (about half as many).

 

What I’m asking is, does anybody here have any insight into what the
“proper” approach to vocoder filters would be?  What is the design goal?  Do
you want significant overlap from one band to the next, or should they be
fairly distinct?  I guess I’m just looking for some general guidelines and
conventional wisdom.

 

Cheers,

Dave Dixon






_______________________________________________
Synth-diy mailing list
Synth-diy at synth-diy.org
http://synth-diy.org/mailman/listinfo/synth-diy

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20190903/6cd2f24f/attachment.htm>