# Gerzon's Trifield

#### gnarly

##### Senior Member
Early in the Speaker Testing: Why mono is better thread in the ASR Video Chanel forum, Gerzon's Trifield technique came up, by KSTR and a few others.

I would like to try to implement the technique, and have started this new thread towards that goal. Seems too off-topic to the mono thread, to pursue it there.
@KSTR, i hope you see this and will chime in....and i apologize for quoting you below, jumping threads (and even forums). Not sure what the correct protolcol is here....

Anyway, I have Gerzon's AES paper http://decoy.iki.fi/dsound/ambisoni...for Multispeaker Stereo (TRIFIELD)_Gerzon.pdf

And Meridians paper

So now I'm trying to implement the technique with an open architecture processor, which I believe should be able to handle it pretty easily.

Here's snip of the schematic from the Meridian paper:

My question at this point is how are the MS MATRIX components made, specifically the Summed (M) and the Difference(S).
In Gerson's AES paper, he show's them as:

I'm not sure how "matrix form" translates to 'loudpeaker feed signals form'.........

"It is actually very simple to implement, You do not need three source channels for that. Just set L and R according to the simple formulas L' = L - R/2 and R' = R - L/2. The center channel is simply the average and can be obtained passively at the power amp input.
Maybe I should open a thread for this...."

Sorry if you did open a thread, and i missed it. Sure will be grateful to you and anyone else who can help get me rolling.

#### dwkdnvr

##### Senior Member
Timely post, as I've been thinking about Trifield recently as well.

Be careful in interpretation - what KSTR describes in your quoted section is one potential 'derived center' approach, but it is not Trifield. The M/S calcs should be straightforward. M = 0.4 (L + R) and S=0.4 (L-R). (Note that in matrix form it ends up not actually being required to compute these depending on the implementation details)

I implemented the Trifield algorithm in Reaper JS a decade or so ago, and found that it worked pretty well. The setup I used it in was specific to my office setup at the time though, and when we moved I never re-created it. I"m now curious about potentially setting it up in the garage/shop and potentially the living room, although placing the C speaker might be a challenge.

I believe this is very close to what I came up with: https://www.diyaudio.com/community/...els-out-of-stereo-signal.193973/#post-2661028

I'm now thinking about trying to replicate this in CamillaDSP. You can use biquads for the various high/low pass filters, but it might be better to use linear phase FIR filters (e.g designed in Rephase) to avoid unpredictable delay. Those filters plus simple mixer config to re-merge the channels is all that should be needed, I think. It lacks the interactive flexibility of the JS implementation, though.

OP
G

#### gnarly

##### Senior Member
Timely post, as I've been thinking about Trifield recently as well.

Aah good. And thanks for the reply.
Be careful in interpretation - what KSTR describes in your quoted section is one potential 'derived center' approach, but it is not Trifield. The M/S calcs should be straightforward. M = 0.4 (L + R) and S=0.4 (L-R). (Note that in matrix form it ends up not actually being required to compute these depending on the implementation details)
That's what I thought after reading more....a 'derived center' approach compared to true Trifield.
Question: where does the 0.4 coefficient come from?
My math skills have long fled....Excel gives 2^(-1/2) (as used in the above snip from Gerzon's AES paper) as 0.707. ????
I implemented the Trifield algorithm in Reaper JS a decade or so ago, and found that it worked pretty well. The setup I used it in was specific to my office setup at the time though, and when we moved I never re-created it. I"m now curious about potentially setting it up in the garage/shop and potentially the living room, although placing the C speaker might be a challenge.

Cool.
I can already see i will have to have keep my center speaker in line with L & R (no equidistant arc) , so it will need a delay and possibly/probably some mild attenuation.

Thx for that link...will study, but I have zero experience with that software.
I'm now thinking about trying to replicate this in CamillaDSP. You can use biquads for the various high/low pass filters, but it might be better to use linear phase FIR filters (e.g designed in Rephase) to avoid unpredictable delay. Those filters plus simple mixer config to re-merge the channels is all that should be needed, I think. It lacks the interactive flexibility of the JS implementation, though.

I'm using open architecture q-sys which has drag and drop filters.... in any flavor, included various order FIR high and low pass.
If I use FIR for the hpf and lpf, I'm thinking i will need to delay the MATRIX MS "S" output by the amount of FIR delay. Correct?

Seems like the "S" line needs either an all-pass, as talked about in paper, or delay to compensate for use of FIR. ????

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