Monday, 16 May 2016

Surround Sound Switch #7: Wrapping Up

Spinning The Room

This is a quick summary of the contents and conclusions reached in my recent series of six articles on the subject of Surround Sound Stage Rotation switch designs and prototypes. The series is about various ways of rotating the sound stage of a surround sound / home cinema audio system, so as to make any chosen wall or corner the focus of the action.

Said action takes place in an arena I've dubbed the octoroom. This is a bit like a normal rectangular room, but with a satellite speaker in every corner, and another in the centre of each wall.

Part 1: The Mother Of all Relay Boxes

I start out by examining the ready-made solutions available in the market. This doesn't take too long, as there are none. The hopelessness of seeking help from the audio kit manufacturers is bemoaned.

I spend most of our session together longing for an earlier time, when things like the MORB-1 were available in shops.

Part 2: Rolling your own Commutator

I detail my personal colour scheme for octoroom wiring, explaining its minor deviations from the relevant standards. Then it's on to another pipe dream, this time involving acres of pristine copper plated (or more likely brass, or other alloy) substrate. An imaginary comb made of brushes is used to illustrate the ideal to which our prototypes can hopefully converge.

I spend most of our session together longing for an earlier time, when such commutators were available in shops.

Part 3: Bulgaria (rotary switches)

The ideal 8-pole commutator can be simulated by helically wiring a suitable stack of wafer switches. I discover 7P8T palladium contact rotaries for sale in Bulgaria, and buy them for research. They turn out to be ex-telecomms system components, too fragile, difficult to wire, and otherwise unsuitable for audio use. But they inspire a passive rotary switch design, which eventually becomes my first successful prototype.

A new feature dubbed Mode 5 is introduced, for the specialist who needs to analyse custom curated surround sound music recordings. It allows a 5.x remix to be "stretched out" over the whole 7.x room, without adding in any sound processing by the receiver.

Now that concrete prototypes are beginning to emerge, I describe a scheme for quickly and conveniently swapping them in and out of the home cinema system. The scheme is based on Bulgin 8-pin, cable- and panel-mounting, plugs and sockets.

Part 4: Group Theory (toggle switches)

The mathematical area of permutations teaches that a single 8PDT switch, suitably wired, can rotate our sound stage through any single angle that's a multiple of 45°. Such rotations can also be composed, or applied one after the other, simply by stringing two or more such switches in series, in any order. So, we can choose a suitable chain of three "basis rotators", say 45°, 90° and 180°, and by selectively turning certain ones on and off, achieve any multiple of the atomic 45° rotation.

Eight pole toggle switches exist, albeit outside the unspoken, hobbyist budgetary scope of this series. But usefully, permutation theory also shows that a safe implementation of the 90° rotation can equally be achieved by splitting our 8PDT switch into two more readily and cheaply available 4PDT units ganged together, and that the 180° can similarly be reached by this means, or even by ganging together four DPDT units.

What is meant by safe in this context, is that under failure conditions, when one or more of the component switches fails to operate, no damage other than a seriously mixed up surround sound image will be caused. Amplifier outputs will not become cross-connected, nor asked to drive two or more loudspeakers in parallel. Sadly, the same can't be said about the 45° rotation stage.

There's a brief, unintelligible diversion, something about binary clocks, I dunno...

I make two more successful passive prototypes based entirely on 4PDT toggle switches - first some big Hong Kong ones with screw terminals, then smaller switches with solder lugs. Each prototype contains a 90° and a 180° rotator, as well as the new Mode 5 feature, which takes up one further 4PDT switch for a total of five. The 45° rotator has been dropped temporarily, as there's no easy way to guarantee that its two associated 4PDT switches will always be operated simultaneously and kept forever out of the potentially destructive one on, one off state.

Part 5: Relayer (electromagnetic relays)

Essentially the same audio circuit can be transcribed from the toggle switches in part 4 to the 4PDT electromagnetic relays in this part. With the addition of a 12V PSU and a 4-bit hexadecimal thumbwheel switch, prototype number 4 - the first active device in the series - is born.

The 45° rotator is reintroduced, since the two 4PDT relays that constitute it can now be guaranteed driven together and kept synchronised. Even under rare fault conditions, e.g. a relay coil burning out, the risk of damage can at least be mitigated by assessing which failure mode - amplifier outputs shorted together, or loudspeakers becoming paralleled up - is the less serious, and wiring the switch contacts accordingly. A free online circuit simulator is used to pre-verify the audio wiring schematic.

I finally have a full 8-position, manually operated, prototype sound stage rotator.

Part 6: Arduino (remote control)

No sooner has it arrived, than the hex thumbwheel switch is replaced by an Arduino Uno, driving the relays and relay pairs through bipolar npn transistors. A wiring self-test program is written, seen operating in a YouTube video. This verifies again that all audio pathways are switched correctly, as the compass orientation rotates, and as Mode 5 is switched on and off.

Home is an MB4 project box
The test wiring is removed, and an IR receiver module is interfaced to the Arduino. Suitable IR codes are obtained by "sniffing" an old Sony BD player remote; these are then embedded into the code, and verified to operate as expected.

Some speculation about future development occurs, but prototype number 5 feels like the logical end of this road. There's life after prototyping, of course. I still have to design a suitable custom PCB, using just the bare ATmega328P chip and a 16MHz crystal, so I can keep my Arduino Uno board for future projects. Still have to stick it all in a box. And so on and on...


Thanks to my wife for putting up with (a) so many odd deliveries of random munitions from Amazon, Ebay, Maplin (hi Scott!) and RS Components, not to mention international arms shipments from USA, Hong Kong, Germany and Bulgaria; and (b) the too many hours I spent locked away in the man-cave, playing with screwdrivers, soldering irons, and ticking devices bristling with hundreds of multicoloured wires.

Special mention to Georgi, my Bulgarian rocket scientist colleague, for pushing me to the Arduino limit, and convincing me there would be merit in these investigations. Without his input, I'd have contented myself with a twisting plug and socket manual solution.

The End

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