KVR Audio

Girolad · Post by **Girolad** » Fri Jun 16, 2017 5:04 am

Hello all,

I'm after some suggestions for a specific VST...
After scouring the web in an attempt to track down a '1-pole' all-pass filter, it seems all of the EQs so far tested, that do have variable all-pass filters, are of the 2-pole variety (DDMF IIEQ, electri-q, SIR standard EQ).

The closest thing I have found is 'IBP Phase Adjust' plugin from UAD. Here is a screenshot of that, as seen online:

As you can see, its a constant phase shift over 180, and NOT 360 i.e 2-pole, like many of the paragraphics exhibit..
Imageline Fruity Shaper is potentially another one but its not a vst.

It shouldn't be confused with the following either, which I have tried and measured; phase rotator (kilohearts disperser, airwindows nudge) or sample based delay (voxengo PHA, phasebug).
Here is an example of a 'rotator':

Where are the phase police when you need em...

Any ideas?

TIMT · Post by **TIMT** » Fri Jun 16, 2017 6:58 am

Would be a bit of an investment for just an Allpass filter,but EQuilibrium has a 6dB slope one.also,PHA-979's phase rotation is accomplished with an FIR filter not sample delay,hence why there is no group delay(and latency).your other options include VOS's Prefix and Nasty VCS which both do 180 and 90 degree phase rotation with an IIR Allpass filter,alas windows only and 32bit

I just remembered there is also Fractional Delay by Forward Audio,

Girolad · Post by **Girolad** » Fri Jun 16, 2017 2:19 pm

Thanks for the reply; 32bit is ample

I'll try out DMG one.

How ever the voxengo does its thing; its unsuitable here

Girolad · Post by **Girolad** » Fri Jun 16, 2017 5:02 pm

I tested DMG EQuilibrium Demo with VST Analyser and I could not see the all-pass you mention.

EDIT: Found it, you have to have LPF or HPF band selected (not peak/dip) then navigate to Subtype> All pass.

FYI Fractional Delay by Forward Audio is a sample-based delay. I don't know the exact definition but its the same as voxengo PHS in that it shifts LR relative to each other (not to be confused with intrachannel phase shift described here).
Group delay is similar to intrachannel phase shift but is measured in microseconds - On many forums I see folks make the mistake of using 'ms' to denote microseconds. Don't confuse ms (milliseconds) and μs (microseconds)!

The exact relationship between group delay and intra-phase isn't clear to me to this day... no idea how they transform.

PreFIX adjusts the phase but isn't variable in its frequency.
NastyVCS is variable in its frequency but is a 2-pole.

Christian Schüler · Post by **Christian Schüler** » Sun Jun 18, 2017 6:21 pm

Just to shamelessly plug (no pun intended) AriesVerb here: The 'inverter' in the filters section of this plugin can do exactly this. The phase shift at the specified corner frequency is 90°, tending to 180° above it and to 0° below it, just like in the graphs.

Start from an empty init patch and be sure to tick 'filters before output' to have the filter section in the direct path. If you are fine with windows 32 bit, the free alpha version will be sufficient without any cost.

Girolad · Post by **Girolad** » Tue Jun 20, 2017 11:40 am

Thanks for the suggestion. At first glance its not clear how to specify corner frequency? I click Transforms>Inverter in the list, the phase shift shows up in VST analyzer but I've no idea how to adjust frequency.
Unless I RTFM, shouldn't this be more than obvious?

Girolad · Post by **Girolad** » Tue Jun 20, 2017 10:21 pm

I found the freq. adjustment. (see attachment). The thing is why does the curve start at -180 and end at 0 here?

Christian Schüler · Post by **Christian Schüler** » Wed Jun 21, 2017 4:32 pm

Hi Girolad, I just wanted to reply to your post above, but then I read that you already found your way around the controls!

Maybe I was confusing, but do not start off with the preset named "inverter"; start off with the "init" preset.

The reason is that the preset named "inverter" under the transforms category uses input/output rotation to achieve the inversion. This works independently of the all-pass filter, and in fact, as it turns out in the above graph, you have now inverted the action of the all-pass

So, just do the same as you did above but start with the "init" preset. That should do the trick!

Girolad · Post by **Girolad** » Sat Jul 15, 2017 1:03 pm

Hi Christian,

Long reply..
I acannot find all-pass in Filter menu of Ariesverb, where is it?
For time being I've used all pass in Equilibrium. There it seems that there exist two types of all pass it lets you choose...

One all pass (HPF type) starts at 0deg and ends at 180deg.
Another all pass (LPF type) starts at -180deg and ends at 0deg.

Untitled.png

So which one is the most old school / electrical 'all pass'? (Judging from your reply, HPF type might just be inverted LPF type(?)

I don't think the -180 to 0 all pass is possible in the analog domain, only the digital domain, right?

I'm not that experienced at the electrical / RC / trigonometery side of things... I rely on graphs, vst analyser and circuits schematics...

I can give more info about the application. See attachment 2. The phase shift introduced by a low shelf present in S channel of a Mid/Side (Sum/Difference) Network, is compensated with what looks like two more all pass... Essentially I would like to replicate this transfer function. I already know how to Sum/Difference signals, and have a dual mono EQ at hand. I just find it hard to grasp how the all passes work in this circuit as there are two of them. I think the High Shelf introduces 90deg phase shift in any case... It requires quite alot of thinking because Sum and Difference are two halves of the same thing, but with one 180inverted!

attachment2.jpg

The fact that the circuit is old telephoney (600ohm) derived does not help. I would not know where to start to even try to simulate these circuits using a external filter simulator program like Filter Pro etc.

P.s. (may I congratulate you for the low filesize footprint of ariesverb! too many plugins today are bloated I think! 100mb + is ridiculous for just one plugin)

Girolad · Post by **Girolad** » Fri Jul 21, 2017 6:30 pm

Bump, question remains...

mystran · Post by **mystran** » Sat Jul 22, 2017 10:02 am

Girolad wrote: I don't think the -180 to 0 all pass is possible in the analog domain, only the digital domain, right?

Sure it is.. you just invert the polarity to get another 180 degrees phase-shift (in which ever direction makes your plots align, since multiples of 360 are arbitrary as far as phase goes).