Surround Sound Switch #2: Commutators

Previously:

Mother Of all Relay Boxes

Colouring In

We're on the trail of a switch to rotate the sound stage in a room with eight satellite speakers. I've modified the previous diagram to include this component, labelled "7P8T Switch" - it has 7 Poles or circuits to switch, and 8 Throws or settings to accommodate our target of eight compass point orientations. Notice that one of the eight outputs of this switch has now been attached to the previously orphaned speaker number 8.

I've also coloured the wires connecting the amplifier output stages to the switch inputs, and those connecting switch outputs to loudspeakers. When working with multi speaker systems, both ends of every wire should readily be identifiable at all times. I use the following colour scheme, which agrees with the applicable Consumer Electronics Association (CEA®) Standard where possible:

(-)	Common Negative	Black
FL	Front Left	White
FR	Front Right	Red
C	Centre	Green
SL	Surround Left	Blue
SR	Surround Right	Grey
SBL	Surround Back Left	Brown
SBR	Surround Back Right	Orange*
SB	Surround Back*	Yellow
SW	Sub-Woofer	Violet*

* CEA® uses "Tan" for SBR, "Purple" for SW, and has no SB channel distinct from SBL/SBR. ANSI/CEA-863-A uses "Khaki" for SBR. My wire stocks are limited to the palette of the electronic colour code.

Spot the error?

Looking back to the diagram, the wire colours indicate that when the switch is in its initial (default) position, the SBL output is connected to speaker 1 (brown wires), SL to speaker 2 (blue wires), and so on. There is still no connection to speaker 8 in this switch position, since there is no yellow wire coming out of the receiver. As the switch is operated, these default connections are broken and various others made.

If you use wire markers of the type shown here, you'll be painfully aware their numbers don't lend themselves to a logical speaker numbering. The electronic colour code doesn't correspond to the CEA® audio source colour scheme. Best to ignore these numbers and just concentrate on using the correct colour marker for each wire. And maybe that's just as well, given the manufacturing error evident in these French made markers. Noticed it yet? Hint: it's in the right hand set.

Rolling Commutators

Okay, so what form does this switch take? Well, it need not be a literal 7P8T switch. Such a thing would be able to connect any input to any output, for each given setting or position of the switch; whereas we have certain considerations of symmetry to satisfy. For example, wherever the Front Left & Right (FL/FR) signals go, we'll surely want the Centre signal (C) to end up between those two. And we want to assign every signal exclusively to its own, unique destination speaker: no two distinct signals should ever become connected together. Assuming that we're uninterested in "mirroring" left and right (maybe to watch an upside down TV while practising handstands?) the symmetry group we're aiming for is just the set of rotations - and not the reflections - of a regular octagon.

Electro-mechanically speaking, the simplest solution is a sliding contact arrangement known as a commutator. Picture a row (let's call it a comb) of seven or eight electrical fingers or brushes labelled abcdefgh, in sliding contact with a copper circuit board. Yes, a comb of brushes, a comb whose every tooth is a brush; stay with me here. We may take these to be the amplifier output signals, and may choose h to be either "no signal", or else the amplifier's central SB output, if such exists. On this board too is a pattern of contact pads, linked in chains to eight output pins, numbered 12345678 along the edge.

As recently as the first moon landing, an engineer in need of such a commutator might take a stroll down Glasgow's Stockwell Street on a Saturday afternoon, to a shop called Radio, Mechanical and Electrical (R.M.E.) Surplus Supplies Limited. Approaching one of the gentlemen serving behind the horseshoe shaped counter, he'd ask for a seven or eight pole switch. Momentarily sinking from view, that gentleman would presently resurface clutching a massive iron, copper and bakelite contraption which, he would explain, had just been ripped from a decommissioned submarine a few days earlier. It would weigh about two kilograms in new money. "One pound and five shillings", he'd venture, causing the hot ash to fall from his cigarette into the surrounding component trays. But it was always possible to make a deal.

This diagram illustrates such a linear switch, with its comb in the starting position (first row).

In the initial configuration shown here, it's easy to trace that input a is connected to output 1, b to 2, c to 3, and so on. Now, operate the switch once. That's to say, comb the inputs, by moving the row of brushes down by one step. Assuming your eyesight is much better than mine, you should see that input a now connects to output 2, b to 3, c to 4, and so on, up to input g, which now connects to (previously disconnected) output 8; and input h, which has now looped around to connect with output 1.

Having satisfied ourselves that the switching arrangement works as advertised, we can easily see how this design might be transformed into a rotary configuration, such as a flat disc or cylinder. Actually, there's one little niggle worth mentioning about that. Did you notice this switch was implemented using just a single layer of substrate? This is intimately connected to the fact that the underlying symmetry we're modelling (rotations of an octagon, remember?) contains no reflections or other crossovers. But if we want a commutator without an end stop, allowing the control dial to spin freely in all directions, then we'll have to introduce another topological layer to provide the multiple helical connections required between channels. Otherwise these will interfere with the comb wiper connection tracks.

The commutator design has one great advantage over most others: there is just a single point of sliding contact per channel, where the brush meets the copper. Most of the alternative designs we'll be examining later have multiple contact points along each constituent circuit, leading to increased contact resistance per channel, concomitant power loss, and more seriously, greatly increased non-linearity.

Nitpicker's Corner

"But isn't a commutator designed to keep electric flow direction constant under rotation?" - Okay, here's what to do. Dig up your house, cement it to the top of the switch rotor, and spin. Now whatever direction your house points, the music will always come from geographic North. You're welcome!

Next time: We travel to beautiful Bulgaria in the hunt for a 7P8T switch.

Surround Sound Switch #1: The MORB

Mono - Stereo - Quad - Octo!

An octagonal pattern is a good general purpose arrangement for the surround sound satellite loudspeakers in a room. Not because of the wealth of media available in 8.1 or 8.2 formats (there are none), nor because it follows faithfully the researchers' and manufacturers' recommendations about ideal speaker placement (it doesn't). It's a good pattern because it so readily accommodates various others. Everything from 1.0 up to 7.2 in fact, the only exceptions being formats incorporating so-called front presence speakers.

Note: the ".1" and occasional ".2" or ".x" always refer to the sub woofer(s). Low frequency effects (LFE) being non-directional, these components can be placed almost anywhere in the room that's convenient, although if you're trying to fill a larger auditorium space, it's quite handy being able to place two diagonally opposed LFE sources rather than just one. In this series, as in real life, LFE sources won't get many more mentions.

More interestingly, the availability of that eighth idler satellite (numbered 8 in the diagram) allows any of these patterns to be rotated into any of eight orientations - think of the major compass points. Here I've numbered the speakers clockwise as seen from above, and arbitrarily selected the normally idle one to be eight.

From an AV viewpoint, most rooms have one default purpose or usage, which might be watching TV or movies, and so there is a natural "front" wall to the sound stage. But there may also be a frequently used projector, mounted to use another wall or corner mounted screen, in which case it would be useful to be able to rotate the sound stage to become centred in that direction. Again, surround sound music might best be enjoyed by sitting at yet another wall, perhaps to take advantage of the better stereo separation afforded by the longer side of the room. And so on.

Corner Cases

Incidentally, if you think eight orientations separated by 45° constitute a bit of overkill, and that four or even two separated by 90° increments should be enough for anyone, remember that many rooms acquiring a new projector along a wall will already be equipped with a television mounted on a corner unit.

Given the sheer quantity of sophisticated audio signal processing performed every millisecond by a typical modern AV receiver, it comes as a surprise to find that many of even the most recent models do not support this simple mapping of amplifier outputs to particular speaker terminals. What about crafting your own custom firmware to do the job? Well, these embedded systems ain't Linux. When it comes to sound codecs and audio processing, the template was set by the original Dolby pioneers in this area, killing off a thriving hobbyist community as bycatch; and it reads, Thou shalt not.

Seriously, I've tried every variation of approaching the manufacturers directly, seeking any option whatsoever of letting me map the preamp outputs to loudspeaker amplifier inputs. I've tried shaking my European consumer legislation threateningly at them, demanding transparency and disclosure. And I do have to admit, their objections do make perfect sense.

The octoroom is a compromise. But then, so is almost every surround sound system installed today. People will always push their surround speakers back into room corners, no matter what generations of psychoacoustic researchers say about their ideal placement. We might as well try to get the most out of that.

MORB-1
© Audio Design Associates, Inc

Did I mention that we're dealing exclusively with wired speakers here? Well, we are. No Bluetooth, no WiFi, no Cat 6. Just big fat high-current, multi-stranded copper wires. So that leaves us with one remaining option: a speaker switch.

Meet the MORB

Maybe what we need then is the Mother Of all Relay Boxes! This little beauty used a 12VDC trigger to switch up to eight amplified channel signals between two alternative sets of speakers. Originally designed to channel cinema sound into either one of two rooms, with a little bit of cross-wiring, the literature states:

MORB-1 can also be used to spin a room. Here we are talking about a room that has a stationary surround sound speaker array with a video display that spins to face two different directions. As the TV turns from one position to the other, the speaker array can be altered so that the proper speaker channels face the user at all times.

Were there only two out of the possible eight orientations of interest to us, which will usually be the case, then this might have been just the job. Unfortunately, the link to the MORB-1 cut sheet, while taking us to many alternative and very impressive modern systems, no longer leads to any specifications about the MORB itself. I get the impression from the Audio Design Associates website that the landing page for this old school piece of low tech is kept around for purely sentimental reasons. Last time I saw an original MORB-1 on ebay, it was going for about £400.

While we can only now guess what was contained within its 93 cubic inches (approx. 6½" x 5" x 3"), it seems safe to say it probably used substantial electromechanical relays, because: three inches tall! Also, any electronic solid state alternative would have carried the twin penalties of crossover distortion and non-linearity, disturbing both frequency response and the dynamic balance between soft and loud sounds.

Room Preparation

Already this promises to be a long journey, with multiple disruptive disturbances of the home cinema setup in prospect; better start defining some boundaries, and some interfaces.

The eight satellite speakers of the octoroom are permanently wired in place, using the thickest multi-strand copper wire available for the purpose. This turns the whole room into a single rigid component, which I'd like to be able to disconnect and reconnect with ease. Step up Euronetwork Ltd., suppliers of this 7.1 Speaker Wall Faceplate with 14 gold plated Binding Post Connectors, suitable for 4mm banana plugs. Of course the octoroom needs 16 sockets not 14, but luckily there's some space at the bottom - and the same company offers a single speaker version of the faceplate. I'll just drill a couple holes and transplant these... damn, they're different. Aw well, close enough.

While we're in the banana market, let's also make the AV receiver itself a little easier to slide out and into position. Following a YouTube breadcrumb trail from Big Clive, via Julian Ilett, and ending up at Techmoan's door, I discover this handy tip for making the unit's terminal posts compatible with banana plugs: pull the bungs out with a bent paperclip (I used a screw nail but the principle's the same, just debung'em). Then sling a load of gold plated banana cables between the seven AV receiver output terminal post pairs and the original seven post pairs on the wall plate, ignoring the extra eighth pair for the moment, and enjoy the new freedom of movement afforded by your Euronetwork hardware (any political irony detected in this sentence is entirely intentional).



Next time: Rolling your own Commutator.

Haken: Aquarius (2010)

Above: Celestial Elixir recorded at the twelfth ProgPower USA (PPUSA XII, September 17 2011, Center Stage Atlanta GA). This YouTube performance is taken from the official DVD, available from Lucid Lounge Studios.

After their self-produced Enter the 5th Dimension demo in 2008, the English progressive metal band Haken released their debut album Aquarius in 2010. Richard Henshall composed all of the music, and Ross Jennings wrote the words, for this concept album about a child born a mermaid.

Such a professional level of composition and arrangement has achieved recognition at the Guitar Pro format Tab site songsterr.com, where an extraordinarily detailed tab has been uploaded by prolific contributor (158 tabs at the last count) Chris. This instrumental version of the song is so well transcribed, it's a neat treat to listen to its realistic playback:

http://www.songsterr.com/a/wsa/haken-celestial-elixir-tab-s91733t3