Practical Ambisonic "Clock Outputs" Decoder with Special Crosstalk Cancellation

UHJ Decoder

The UHJ decoder uses standard design practices and equations as detailed in the original design papers on the subject. The phase shifters are implemented with standard 2-pole per op-amp circuits rather than the phase sequence networks used in my previous decoder. This has the advantage of flat frequency response and use of fewer capacitors in their reactive region in the signal chain. Recovered B-format is presented externally on the standard 5-pin male XLR output and an external lead feeds this back into the decoder for normal straight through operation. This can be disconnected and used to feed in B-format sources from the outside world if desired.

B-Format Decoder- General

In general the B-format to speaker feed decoder presented here is substantially simplified from the usual concepts. It is assumed that the user has gain controls on each of the external amplifiers so that they can use recorded ambisonic material to set the gain of each speaker to give good results in a listening test. this removes the need for gain controls on this unit. It is further assumed that if the user requires more front gain then the external controls can be adjusted for this too. This decoder has outputs for twelve speaker feeds which have angles fixed internally, spread at 30 degree intervals around the soundstage. Each of the outputs labelled 1 to 12 creates a speaker feed designed for a speaker placed at that clock position, with the 12 O'Clock output being centre forward etc. It is expected that the user can find speaker positions close enough to these 12 possible positions to make errors practically undetectable in a home listening environment. In an experimental environment the speakers can be positioned exactly in any case. The O'Clock nomenclature gives an extremely simple and internationally understood method of expressing approximate angular direction and is seen as a significant advantage of this design. No complex front panel switching or programming is required to assign speaker feeds: The amplifier for a certain speaker is simply plugged into the appropriate output socket. The absence of front panel controls brings down cost and reduces both the perceived and actual complexity of operation. By making some reasonable compromises we have reduced front panel controls to a mains switch and a neon power indicator, which are combined in a rocker switch.

Standard B-Format Decoder Matrix

An internal 12-way jumper can be plugged to give completely standard B-format to speaker feed decoding coefficients for the twelve outputs. In common with the previous design, no shelf filtering options are included. The decoding coefficients produce a cardiod shape around the speaker circle, hence zero output from the speaker directly opposite the maximum direction of a panned sound.

Experimental Phase Shifted Speaker Feed Matrix

The W, X and Y signals are first passed through all-pass phase shift networks. These are the familiar imperfect phase-shift filters as used in UHJ encoding and decoding. This creates 0 and 90 degree versions of W, X and Y signals and 180 and 270 degree versions can then be generated by inversion. Now, using appropriate sine weighted sums of these signals will allow any arbitray phase of W X and Y to be generated. As these phase shifts are wideband and relative, any speaker feed with the correct ambisonic amplitude output can be created, only now it can be created with an arbitrary phase relative to the other speakers. The proposal is to apply a phase angle to a speaker feed which is numerically identical to its angle from centre front. So for speakers at the twelve positions of the clock, the relative phase shift will be as follows.

Clock Phase Shift

1 +330
2 +300
3 +270
4 +240
5 +210
6 +180
7 +150
8 +120
9 +90
10 +60
11 +30
12 0

From this you can see that each pair of opposite speakers are now driven in antiphase.

Mono signals with purely W content will tend to cancel at a point in the exact centre of a perfectly circular and balanced speaker array. Looking at the system slightly differently you can also see that a mono sine wave will spin around the array at its own frequency, with each speaker cone receiving the signal phase-shifted by the angle of it's physical position; A mono sine wave will create a spiralling wavefront. (A dual spiralling wavefront?)

A sine signal presented at position 12 for example, will have equal amplitude cancelling signals coming from positions 3 and 9, and less equal cancelling signals from other opposing speakers. Only the centre front signal from position 12 will have no interfering signal, as its opposite feed position 6 has no signal due to normal ambisonic amplitude based speaker feed generation.

It is hoped that this might provide an enhancement to the usually rather vague directional imaging of first-order ambisonic decoding. The normal amplitudes of a non-shelf filtered speaker feed decoder are maintained with just the phases being altered, so it is difficult to see how it can be made worse. As a cancellation strategy this method, to me, seems more logical than shelf filters as it at least avoids allowing antiphase components to emanate from the speaker opposite the desired maximum sound direction. Of course if it works, you'd do all of this digitally.

Schematics - Unfinished, Unbuilt.

Stereo Input, UHJ Decoder and Recovered B-Format Output

Speaker Feed Phase Shifters

Phase Shifted Speaker Feed Resistor Matrix

Normal Speaker Feed Resistor Matrix

Matrix Sums and Output buffers

Obvious Problems, Thinking Aloud and Wild, Mathematically Ignorant Hand-waving

Why would you want a mono signal to disappear anyway?

The human head is not a "point" receiver and is certainly never at the exact centre, even assuming a perfect array, so any effect would only function in a very small "sweet spot." (I suppose the same applies to normal shelf-filtered decoding anyway, to some extent)

Won't this create a treble emphasis effect as bass signals will cancel quite effectively whereas higher frequencies will not. Maybe you could let the phase shifters be deliberately lower bandwidth to avoid this - best of both worlds then.

Shelf filters - What do they do that might actually be a bit like this, noting the treble thing above.

Isn't this a bit like just sending weighted B-format direct to the speakers with no W signal?

Anyway...

 

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