Trinaural decoding equations for 3 speaker stereo matrix ?

[WithTarragon]

Originally Posted by KSTR

The basic TriField equations are :
L' = 0.5[(sin(phi) + w)L + (sin(phi) - w)R],
R' correspondingly, and
C = sqrt(2)cos(phi)(L+R)

A certain selection of phi and w should yield the trinaural amplitude relationships but I haven't tried to solve the eq's
EDIT: Once I progress to try a 3-way full TriField matrix with the coeffs that Gerzon recommends, and with DSP linear phase, I'll know....

The variables, phi and w, what are they?

I am guessing they are weights (perhaps vectors and not constants) that are a function of something ?

[KSTR]

Those are just scalar weight constants.

Phi is sort of a "projection blend control" angle and is used from 15° to 75°.
90° gives no center signal and pure L/R, while 0° gives an M/S-decoding.

W is the stereo width parameter and is nominally 1, allowing for fine tuning.

See UsPat#5594800, fig4 and description.

[KSTR]

@Markus : Tannoy 15"

[Elias]

Originally Posted by bzfcocon

Looking at some of the articles quoted in the thread, I am not sure I understand: where do 1st order low/high filters come into play? I thought matrixing was done at all frequencies, or is Trifield different ?

Are there actually 3-speakers solutions that do NOT assume that the center speaker is identical to L and R ? The approach sounds promising and I would like to test it, but not if it means another bulky loudspeaker in the middle. How about if the center would only reproduce >100Hz, would the concept still work and would it change the matrixing ?

Interesting thread, btw.

In Trifield the audio band is divided in two parts and both parts have a matrix but the coefficients are a bit different for each band. The transition freq is somewhere in few kHz.

I think there is no benefit to use a matrix at bass freqs (since bass is mixed mostly mono anyway), so it would be feasible to matrix satellite speakers and keep bass separate. Or as you say put bass only in side speakers and play them ordinary stereo at low freqs.

At the moment in my experimental system the bass is not matrixed below about 100 Hz. I don't see any problems with that.


- Elias

[Elias]

Originally Posted by KSTR

Personally I was thinking about linear phase filters because 1st order analog, while summing linphase, has 90° phase between bands, and acoustic centers of same bands are far apart (speaker-2-speaker distance) so the there could be some off-axis ill effects that might be avoidable with round kneed, gentle slope linphase filters with compact, non-ringing FIR kernels.

I'm FIR based anyway and integrating the Trifield matrixing would not cause any computational resource penalties... still waiting for my new coax drivers (4pcs) but then I'll be eager to try TriField and find out if it significantly betters the simpler Trinaural.

On the other hand, by using linear phase filters do you think you could arrive in same perception as with original Trifield matrix ?

I believe Gerzon did not just put some numbers in his articles and patent but they were based on extensive listening tests. I also assume his filters were not linear phase.

So if you change the filters you'll get something else perhaps but not Trifield ?


- Elias

[bzfcocon]

Originally Posted by Elias

In Trifield the audio band is divided in two parts and both parts have a matrix but the coefficients are a bit different for each band. The transition freq is somewhere in few kHz.

Thanks ! I've checked the Gerzon article again and I see it's all there, missed it the first time.
So basically the split is done to "strech" the higher frequencies further apart (less power goes to center, more goes to L and R)

I wonder if one cannot get a similar effect "built-in" the loudspeakers themselves by boosting the highs on the L+R and rolling them off for the center ? The whole should still sum flat and it's not exactly tri-field, but it could still work.

Originally Posted by Elias

I think there is no benefit to use a matrix at bass freqs (since bass is mixed mostly mono anyway), so it would be feasible to matrix satellite speakers and keep bass separate. Or as you say put bass only in side speakers and play them ordinary stereo at low freqs.

At the moment in my experimental system the bass is not matrixed below about 100 Hz. I don't see any problems with that.

I see. Checked the article and he also explicitly says that below ~150 Hz you can suit yourself depending on the bass capability of your speakers, as the phi angle is uncritical.
So I'll give it a try sometime.

[bzfcocon]

Originally Posted by bzfcocon

T
I wonder if one cannot get a similar effect "built-in" the loudspeakers themselves by boosting the highs on the L+R and rolling them off for the center ? The whole should still sum flat and it's not exactly tri-field, but it could still work.

Did some quick maths based on the paper.

For low freq (phi = 35 deg) you get:
C3 ~ 0.58(L+R) L3 ~ 0.78L - 0.21R (similar for R3)

For highs ((phi = 55 deg)) you get:
C3 ~ 0.40(L+R) L3 ~ 0.90L - 0.09R (similar for R3)

So you can't achieve exactly the same effect by rolling off the high freq on the center, because the phi also controls the "mix" in the L and R, but one might get away with rolling off the center ~3dB down above 5kHz and still getting a good effect, while avoiding additional high/low pass filtering in the matrix circuit.

[Soundtrackmixer]

Originally Posted by markus76

If the conclusion is 3 channels/speakers is better than 2 speaker stereo, then yes. This was 80 years ago

Their conclusion was five speakers up front is optimal for mapping a frontal soundstage, three was the minium, and two was unacceptable.

This conclusion was the basis for movie theaters installing 5 speakers behind the screen - though it was shrunk to three in the early eighties.

[Soundtrackmixer]

Originally Posted by markus76

If the conclusion is 3 channels/speakers is better than 2 speaker stereo, then yes. This was 80 years ago

Their conclusion was five speakers up front is optimal for mapping a frontal soundstage, three was the minium, and two was unacceptable.

This conclusion was the basis for movie theaters installing 5 speakers behind the screen - though it was shrunk to three in the early eighties.

[Elias]

Originally Posted by Rudolf

Must be weird kinds of music you are listening to.
I prefer music were - by and large - the harmonics roll off with rising frequency.

Rudolf

Originally Posted by markus76

Sounds like a constructed case. Any specific recording that would contain such signals?

A metal plate hit by a wooden stick In some music there is two metal plates hitting each others in a consecutive on-off manner