1 (edited by pinox67 2022-11-02 19:33:00)

Topic: AllPass Filter

Is it possible to act on some configuration parameter of the RME ADI-2 Pro to compensate only the phase for low and/or high frequency?
I mean the equivalent of AllPass filter effects.

2 (edited by KaiS 2022-11-03 09:46:22)

Re: AllPass Filter

The various DA-Filters have different delays.
If you’re lucky one fits.

What exactly are you heading for?

3 (edited by pinox67 2022-11-04 10:39:33)

Re: AllPass Filter

AllPass filters act only on the phase of the signal, anticipating or delaying specific frequency ranges (not a global delay). They can be used in production to attenuate signal peaks without reducing the overall level. Here https://www.uaudio.com/blog/allpass-filters/ some details.
I would like to experience the high frequency phase delay recovery that sometimes occurs in the downstream analog electronic chain. This also indirectly causes an increase in dynamics.

4 (edited by KaiS 2022-11-03 21:52:28)

Re: AllPass Filter

I know about the function of All-Pass filters.
For one of their purposes, phase alignment of multi-way speaker systems, delays can be used too.
Therefore my proposal.


The very special application you are heading for is implemented mainly in mastering processors for radio transmissions, like the famous Optimod in all it’s iterations.

The “Phase Rotation”, as it’s usually called there, reduces audio peaks by a few dB without obviously changing the sound.
This is the point that speaks against your idea to increase audible dynamics by reversing phase shifts:

Peak level will change, but due to the way our hearing works the audible effect will be very limited, and not reflect this change.

This is from someone who has worked with Optimod and it‘s Phase Rotation, me.


Anyway you would need to exactly measure the audio path you want to compensate, for useful results.


What you CAN do with ADI-2:
Select a DA-Filter that suits your taste.
Those indeed have different HF phase / impulse reposeses that do make an audible difference.

Re: AllPass Filter

There is software on PC which does that.

I use acouratepro for the filterdesign (including allpass) and acourateconvolver for playing filtered music.
I use it actually as a crossover (4-way crossover with room correction).
www.audiovero.de
There is also a version where you can filter musicfiles on a NAS/Storage.
acourateNAS.

Description here
https://www.audiovero.de/acourate-wiki/ … d=en:start

Forum also here (german): acourate
https://www.aktives-hoeren.de/viewforum.php?f=40

Tip: you can translate with google translator

6 (edited by pinox67 2022-11-04 10:36:11)

Re: AllPass Filter

Thanks for the tips. As part of some studies that I am conducting, I have created several programs for signal processing. AllPass filters are very simple, first and second order already offer a lot of flexibility.

Analytically, it happens that the application of a filter of this type combined with a linear phase digital DA filter modifies the resulting impulse response, making it resemble a minimum or intermediate phase. Hence, it becomes parameterizable.

By performing simulations on different systems with some degrees of advance in medium-high frequency, I found a slight improvement in the focus of the sound and in dynamics. The filter is used in the opposite way to what is done in production to reduce peaks: it recompacts the energy spread over time, resulting in an increase in maximum signal levels and recompaction of transients.

The extent of the correction naturally depends on the system and must be cut out on each one. Given the simplicity of the AllPass filter and the power of the RME ADI-2 it would be interesting to have it available in its DSP.

7 (edited by KaiS 2022-11-04 10:35:28)

Re: AllPass Filter

Phase rotation is not used in normal music production to reduce peaks.

99% of all music released, during mastering, is run through a peak limiter that more ore less sophisticated cuts off excessive pulses/peaks, without changing the phase relation in the frequency domain.


Depending on the signal path during recording, there are phase shifts in the frequency extremes, cause by the bandpass characters of the incorporated equipment, starting with the microphones.

Due to the multiple stages of recording, processing and multitrack mixing and mastering, any clear definition of these shifts never exist.

So, if you get good results with what you’re doing, go for it, but don’t expect to retrieve an “original” phase relationship.


BTW: The BBE Sonic Maximizer is a commercial device (available as plugin too) that works based on All-Pass filters, and some dynamic processing.
Carefully tuned, not overdone, it can improve dynamic impact in bass and treble.

8 (edited by pinox67 2022-11-04 11:58:55)

Re: AllPass Filter

Thanks again for the useful information. I agree, given the elements involved, it makes little sense to talk about restoring the "original" phase.

However, the possibility of being able to act on these aspects in reproduction offers more degrees of freedom for the sound tuning: one acts casually on the equalization of the levels, why not give some possibility of action on the phases as well? With the digital tools we have today it is not difficult.

Since you have the chance, when you have some time try what I have described on a good system and let me know if you see the effects described: to start using a curve that leads by 1 degree at 1KHz and 10 degrees at 20KHz (a go up for higher frequencies). Then increase or decrease these values. Of course, blind evidence!