Doepfer

HI LIST,
some info on the subject from dieter:

bakis:  "do you think that it would make sense to make a new poll 
about a comb. filter module??or it is similar to the phase shifter 
module??"

 
dieter doepfer:  "There exists not only one "comb filter" module 
type. A comb filter may be realized with phase shifters (i.e. phase 
shift) or delay units (i.e. time shift) or even with several notch 
filters in series. The main difference is the spacing between 
the "holes" of the combs on the frequency axis. The effect resulting 
from phase shifting is called phasing, the effect resulting from time 
shifting is called flanging (earlier made with two tape recoders by 
alternately slowing down the reels or flanges with the fingers). The 
most versatile is the last solution with separate notch filter that 
may be controlled by different CV sources (e.g. independent LFOs or 
Shepard CVs or random or or or ...). In addition feedback can be used 
(like filter feedback). Somewhere I have details of the difference 
between phasing and flanging (pictures of the frequency distribution 
of the "holes", as far as I remember the holes are equal spaced for 
the flanger and logarithmic for the phaser). But as I'm very busy for 
the fair I don't have the time to search the documents. Maybe someone 
of the newsgroup knows detailes too (Florian ?). As far as I remember 
it was in an issue of the Polyphony magazine (about 1983/84). By the 
way a very interesting magazine for technical details of modular 
synthesizers but they stopped it middle of the eighties. Maybe 
someone has scanned the magazine."

...so...anyone??florian??

synthfreak
athens-greece
[doepfer_a100] group moderator

Hi Bakis

> dieter doepfer:  "[...]Maybe someone
> of the newsgroup knows detailes too (Florian ?).[...]

The "comb filter" thing I already mentioned in the mail from 4th of
February. I wrote 
"[...]by an allpass filter (which is used also for phaser) or by a delay.
Both do similar but technically not identical effects."
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I thought this was very clever, since I had not to explain the difference
in detail ;-) ok, now you've got me...

The difference between the (analogue) phaser and the flanger in reality is
the technical way, how a signal delay is created.

The delay in the flanger is realized with some kind of signal storage,
which does not change the signal itself generally. The stored signal is
kept for a while and then played back. This is realized either with digital
storage or in older flangers and guitar efx with so called bucket bridge
delays.

The phase works also with a delay, but this time the delay is realized with
an so called allpass filter. An allpass filter is a circuit which does
not(!) filter. This sounds senseless as long as you think filtering is the
only thing that a filter does; but a filter does a lot more: 
- it does a very(!) small delay (this is, what the phaser wants to use)
- and it shifts the phase of partial notes above the cutoff-frequency
  (yes an allpass filter has still something like a cutoff-frequency,
  though it does not cutoff anything). This phaseshift is originally 
  not intended, but it is typical for all analogue filter designs. 


As we already learned: A comb effect is generated by elimination of partial
tones, which happen when a delayed signal is mixed together with the non
delayed signal. Now you can imagine, that the phase shift of the partial
tones in  the allpass will cause, that other partials will be eliminated.

So far the technical background.


__Which__ special frequencies are eliminated and how this looks (linear or
logarithmic...) I do not remember. Sorry. I know there is a paper somewhere
deep in the big paper sink in my room ;-) but I cannot promise, that I will
find it. So I fear the answer won't be better than the one, you already
received by Dieter.

Florian



-- 
Florian Anwander                  |ConSol*  HP-Support
Tel.   +49.89.45841-133           |Consulting&Solutions Software GmbH
Fax    +49.89.45841-139           |Franziskanerstr. 38, D-81669 München
email: florian.anwander@consol.de |http://www.consol.de

thanks florian!!
synthfreak






--- In Doepfer_a100@y..., Florian Anwander <Florian.Anwander@c...> 
wrote:
> Hi Bakis
> 
> > dieter doepfer:  "[...]Maybe someone
> > of the newsgroup knows detailes too (Florian ?).[...]
> 
> The "comb filter" thing I already mentioned in the mail from 4th of
> February. I wrote 
> "[...]by an allpass filter (which is used also for phaser) or by a 
delay.
> Both do similar but technically not identical effects."
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> I thought this was very clever, since I had not to explain the 
difference
> in detail ;-) ok, now you've got me...
> 
> The difference between the (analogue) phaser and the flanger in 
reality is
> the technical way, how a signal delay is created.
> 
> The delay in the flanger is realized with some kind of signal 
storage,
> which does not change the signal itself generally. The stored 
signal is
> kept for a while and then played back. This is realized either with 
digital
> storage or in older flangers and guitar efx with so called bucket 
bridge
> delays.
> 
> The phase works also with a delay, but this time the delay is 
realized with
> an so called allpass filter. An allpass filter is a circuit which 
does
> not(!) filter. This sounds senseless as long as you think filtering 
is the
> only thing that a filter does; but a filter does a lot more: 
> - it does a very(!) small delay (this is, what the phaser wants to 
use)
> - and it shifts the phase of partial notes above the cutoff-
frequency
>   (yes an allpass filter has still something like a cutoff-
frequency,
>   though it does not cutoff anything). This phaseshift is 
originally 
>   not intended, but it is typical for all analogue filter designs. 
> 
> 
> As we already learned: A comb effect is generated by elimination of 
partial
> tones, which happen when a delayed signal is mixed together with 
the non
> delayed signal. Now you can imagine, that the phase shift of the 
partial
> tones in  the allpass will cause, that other partials will be 
eliminated.
> 
> So far the technical background.
> 
> 
> __Which__ special frequencies are eliminated and how this looks 
(linear or
> logarithmic...) I do not remember. Sorry. I know there is a paper 
somewhere
> deep in the big paper sink in my room ;-) but I cannot promise, 
that I will
> find it. So I fear the answer won't be better than the one, you 
already
> received by Dieter.
> 
> Florian
> 
> 
> 
> -- 
> Florian Anwander                  |ConSol*  HP-Support
> Tel.   +49.89.45841-133           |Consulting&Solutions Software 
GmbH
> Fax    +49.89.45841-139           |Franziskanerstr. 38, D-81669 
München
> email: florian.anwander@c... |http://www.consol.de