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 Forum index » DIY Hardware and Software » Microcontrollers and Programmable Logic
All pass filter with variable frequency
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VA1



Joined: Aug 20, 2018
Posts: 53
Location: Nederland

PostPosted: Fri Aug 31, 2018 6:35 am    Post subject: All pass filter with variable frequency
Subject description: all pass filter variable frequency
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Hi, i like to have a allpass filter with variable frequency so i can make a phaser.
I dont understand what is the difference between allpass and delay line ?

The examples online dont give anything about variable frequency.
I already have coefficients for VCF, i hope i can re use them for allpass filter.

Any ideas ?
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JovianPyx



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PostPosted: Fri Aug 31, 2018 7:22 am    Post subject: Re: All pass filter with variable frequency
Subject description: all pass filter variable frequency
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VA1 wrote:
Hi, i like to have a allpass filter with variable frequency so i can make a phaser.
I dont understand what is the difference between allpass and delay line ?


A delay line is a collection of delay elements or memories each of which will delay the signal by exactly one sample time. A typical delay line in program code starts with an array of elements each capable of holding one digital sample. A read/write index or pointer into the array is advanced each sample time.

An all pass filter is a filter which has little effect directly on the spectral content of the input signal, but can delay it by as much as one sample time. The actual amount of delay is controlled by a number from 0.0 to 1.0.

Quote:
The examples online dont give anything about variable frequency.
I already have coefficients for VCF, i hope i can re use them for allpass filter.

Any ideas ?


The all pass filter is not a filter that attenuates certain frequencies such as how a SVF works, it delays the signal by a fractional variable amount up to one sample time. Another way to state that is to say it shifts the phase. All pass filters can be used with modulation driving their delay amount in a phaser.

EDIT ADD: More digital all pass filter information can be found here: https://authors.library.caltech.edu/12029/1/REGprocieee88.pdf

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VA1



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PostPosted: Fri Aug 31, 2018 9:31 am    Post subject: Reply with quote  Mark this post and the followings unread

I have seen that one, hard to program things from professor info.
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g.gabba



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PostPosted: Sat Sep 01, 2018 10:16 am    Post subject: Reply with quote  Mark this post and the followings unread

dont know if that matters in the digital realm, but

as for example is mentioned here:
http://www.analog.com/media/en/training-seminars/tutorials/MT-223.pdf
u can archive an allpass by using an state variable filter thru subtracting the BP Signal from the input signal

something similar is done in the steiner filter from sir yves
http://yusynth.net/Modular/Commun/STEINERVCF/Steiner-improved-sch.gif
by feeding the input simultaniously in the HP and LP input
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JovianPyx



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PostPosted: Sat Sep 01, 2018 11:04 am    Post subject: Reply with quote  Mark this post and the followings unread

While using filters like SVF for all pass is intriguing and interesting, I've seen much simpler ways to get all pass in analog, one circuit uses only one opamp, but I don't know if there are other ramifications.

In the digital domain, the process is also very simple and "tunable" with respect to the delay.

The MXR Phase-100 analog phaser schematic

There you can see it has 6 stages of modulated all pass filters (each using only one opamp).

That could be used as a template to model a digital phaser.

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VA1



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PostPosted: Sun Sep 02, 2018 6:28 am    Post subject: Reply with quote  Mark this post and the followings unread

A allpass should be really simple without many code, or in analog 1 opamp indeed.

take this 1 pole lowpass with a & b coefficients :
Code:

#define ProcessLP( p , in , a , b ) \
lTemp = in; \
lTemp += p.lastin; \
lTemp *= a; \
lTemp >>= 15; \
\
lTemp2 = p.lastout; \
lTemp2 *= b; \
lTemp2 >>= 15; \
\
p.lastout = lTemp + lTemp2; \
p.lastin = in; \


What do need change to get lowpass, it should be that simple.
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Grumble



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PostPosted: Sun Sep 02, 2018 12:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have used several MAX261 switched capacitor filters in my synth and they have an all pass filter possibility (which I don't use).
They work pretty well, but in the lower frequencies there is some bleeding of the clock frequency.
What I usually do is to have the clock frequency for the filter made by an AD9833 Programmable Waveform Generator, which on its turn is programmed with an arduino.
So in the end you have quite a circuit, but it will work!
HERE is an Youtube film where I use my filter.

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VA1



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PostPosted: Mon Sep 03, 2018 6:47 am    Post subject: Reply with quote  Mark this post and the followings unread

Digital only please, maybe for a next synth analog.
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Grumble



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PostPosted: Mon Sep 03, 2018 7:23 am    Post subject: Reply with quote  Mark this post and the followings unread

That wasn't clear in your first post Rolling Eyes
The forum index doesn't gave it away either : Microcontrollers and Programmable Logic

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