Allpass filters that oscillate !
Posted: Fri Jan 05, 2018 12:19 pmHi Adam, Spogg and the gang,
I'm glad you're enjoying my classic spring reverb, and as Adam discovered, it can become unruly in a hurry, just like the physical springers.
The phenomenon occurs in the allpass section of the delay line, where several allpass filters are cascaded in the classic hookup proposed by Manfred Schroeder in 1962.
An allpass filter by design has unity gain across the design bandwidth in this case, audio.
So what's the point of a filter with a flat passband ? They are used for phase correction in telephone systems, broadcast, etc.
Their phase can shift from either -180 to 0 or 0 to +180 degrees for a 1 pole filter, depending on the hookup. Phase shift creates delay that is frequency sensitive. Cascading several of them with differing delays creates a very complex phase profile and delay profile, ideal for 'smearing' the 'rattle' decay that emanates from the primary delay network. That's why you see them placed after the primary delay elements in a typical reverb.
The feedback gain has to be kept well below .98 to prevent oscillation at certain frequencies that ADD due to the various phase shifts of the multiple allpass filters. Anything above .5 and you have to be careful.
As you may imagine. by the same token, other frequencies CANCEL for the same reason.
My Allpass demonstrator shows this, comparing 1 allpass element with 4 elements, cascaded.
Have fun with another Dead Simple demonstrator !
ROXY
I'm glad you're enjoying my classic spring reverb, and as Adam discovered, it can become unruly in a hurry, just like the physical springers.
The phenomenon occurs in the allpass section of the delay line, where several allpass filters are cascaded in the classic hookup proposed by Manfred Schroeder in 1962.
An allpass filter by design has unity gain across the design bandwidth in this case, audio.
So what's the point of a filter with a flat passband ? They are used for phase correction in telephone systems, broadcast, etc.
Their phase can shift from either -180 to 0 or 0 to +180 degrees for a 1 pole filter, depending on the hookup. Phase shift creates delay that is frequency sensitive. Cascading several of them with differing delays creates a very complex phase profile and delay profile, ideal for 'smearing' the 'rattle' decay that emanates from the primary delay network. That's why you see them placed after the primary delay elements in a typical reverb.
The feedback gain has to be kept well below .98 to prevent oscillation at certain frequencies that ADD due to the various phase shifts of the multiple allpass filters. Anything above .5 and you have to be careful.
As you may imagine. by the same token, other frequencies CANCEL for the same reason.
My Allpass demonstrator shows this, comparing 1 allpass element with 4 elements, cascaded.
Have fun with another Dead Simple demonstrator !
ROXY
