• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). There are daily reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

Gerzon's Trifield

gnarly

Senior Member
Joined
Jun 15, 2021
Messages
320
Likes
348
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:
Trifield schematic.JPG


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:
trifield matrix.JPG


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

Especially after reading KSTR's quote (from earlier linked page)
"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
Joined
Nov 2, 2018
Messages
370
Likes
573
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
Joined
Jun 15, 2021
Messages
320
Likes
348
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. ????
 
Top Bottom