Create Hafler circuit (ambient rear channels) using Equalizer APO?

[Bryan Chow]

I was watching this video https://www.youtube.com/watch?v=SNsmG7HHczU&ab_channel=PSAudio and am intrigued by it.

It's called a Hafler circuit https://en.wikipedia.org/wiki/Hafler_circuit

Hafler decoding matrix  Left Front  Right Front Left Back   Right Back
Left Total              1.0         0.0         1.0         -1.0
Right Total             0.0         1.0         -1.0        1.0

Right now I'm simply copying the front channels to the rears for a fuller sound:

# Upmix stereo to 4-channel
If: inputChannelCount == 2
Copy: SL=L
Copy: SR=R
Channel: SL
Preamp: -3 dB
Channel: SR
Preamp: -3 dB
EndIf:

Is it possible to simulate a Hafler circuit using Equalizer APO?

[Bryan Chow]

Ok I figured it out:

Copy: LSHIFT=-1.0*L
Copy: RSHIFT=-1.0*R
Copy: SL=L+RSHIFT
Copy: SR=R+LSHIFT

It seems to work, when L and R channels are the same (stereo mono) the surrounds are silent.

[Peter Verbeek]

Nice. One tip for the upmixing. Use the pre-stage command for detection if the input is stereo. Without it the inputChannelCount could be not detecting stereo (see example Stage command wiki).

Stage: pre-mix
If: inputChannelCount == 2
Copy: LSHIFT=-1.0*L
Copy: RSHIFT=-1.0*R
Copy: SL=L+RSHIFT
Copy: SR=R+LSHIFT
EndIf:
Stage: post-mix

[Bryan Chow]

Yes I do have the pre-mix command.

Thanks Peter!

The Hafler circuit ambient surround sound is quite interesting. Seems to be more pleasant than just copying fronts to rears (with reduced volume).

[Peter Verbeek]

So the phase inversion does do good for the sound? And how's the volume on the rear speakers? Needs there be any reduction or amplification? Anyway, perhaps I should incorporate it on the Peace effects panel.

[Bryan Chow]

Yes! Any sound common in both speakers is removed, you're left with ambient cues. So compared with what I did before (copy front to rear), I no longer get vocals behind me which makes sense.

Technical rationale, from wiki:

"The rear ambient sounds, applause, and coughs from the audience are sometimes received out of phase by the stereo microphones, while sounds from the musicians mostly are in synchronous phase. Thus, if rear speakers are fed with the difference between the stereo channels, audience noises and reverberation from the auditorium may be heard from behind the listener."

[Peter Verbeek]

Well, that's a very interesting and useful sound effect. Is it mostly useful for music recorded in theaters do you think?

[Bryan Chow]

I've tried quite a few types of music and recordings they all work quite well. On some recordings (musicals etc with centralized vocals), the vocals pretty much disappear from the rear. On others like pop music the effect is less pronounced, but I also hear different instruments emphasized in the rear channels depending on how they are mixed in the left and right. It feels much more natural than copying front to rears.

I tried to post a recording to YouTube for you to hear but the video was muted due to copyrighted music. Give it a try!

[Peter Verbeek]

Unfortunately I don't have a 5.1/7.1 setup to try it out. But I take your word, well experience, for it :)

Are you familiar with the Peace equalizer interface? Its effects panel has several interesting effects. This Hafler circuit is an interesting effect too although I'm not sure which term to use as people could be searching it by other terms.

[Bryan Chow]

I do use Peace sometimes but it overwrites my complicated config.txt so I have to back it up, run Peace, find the commands then incorporate them into my config.txt which is a bit of a hassle. What I find really useful is the Editor, especially for visualizing my filters from REW together with my Equalizer APO filters and preamp commands.

You can consider using the term "ambiphonics", which the Hafler circuit is a type of. It will be cool to see it in Peace :)

[Peter Verbeek]

I'll certainly try it to be able to listen to the effect. Thanks. I didn't think out that. Also thanks for the term "Ambiphonics". It seems a bit nicer then a name but I'll mention both.

Yes, Peace does need a command to work. Working with Peace and the Configuration Editor can be hassle. Therefore I have a command window in Peace.
"Visualizing" like the Peace filters graph? Basically the same as the analysis panel, only with more features.

[Bryan Chow]

You can also apply the Hafler circuit commands to your front channels to hear what the rears would sound like. You'll know it's working correctly if mono music becomes silent, while stereo music generally have vocals reduced (or even disappears) and you hear more audio that sounds like ambient cues.

[Fabo W]

better use somenting like Halo Upmix VST