MyPCTV sound and the Hafler Principle - getting good quality surround sound on a budget.

What is not widely known today is that the modern Dolby Surround Sounds et al are based on an elegantly simple way of connecting analog sound systems together for surround sound called the Hafler Principle, developed in the early 70's by David Hafler. There is more information on a stereo track than a stereo picks up, but the Hafler system does - at the very least it does a very good job of giving a reasonable approximation of a "3D" ambient sound.

It works by approximating the empirical way sound typically is heard. Ambient sounds in a room, the music, applause, talking from the audience etc are usually received out of phase by the recording microphones in a "live" environment, while sound from the musicians is generally in phase. So, give rear speakers the difference between the stereo channels out of phase and all the audience noises and echoes etc can be heard from behind the listener.

In essence you add 2 more ( rear ) speakers and connect them up in such a way that they process only the difference between the two stereo signals. This involves nothing more than wiring the 4 speakers up in a slightly different way. So, for the cost of 2 more speakers, a few bits of wire and some insulation tape you can get very passable surround sound.

Most modern systems like Dolby are just improvements (complications?) on this principle, with the ability to add a bit of delay into the rear speakers and manipulate (eg amplify) their signals. More than this, it would seem that, in theory, if you pass a modern DVD's digital 5.1 type signal through a stereo analog amp and through a Hafler circuit, it does a pretty fine job of decoding the signal. I haven't gone into this in enough detail to know how true it is, but after a few days of listening it certainly fools your ears enough to be believable.

Here is a website that describes the circuit - the first diagram is what I have rigged up, as you can see it is very simple - and here is an original paper from 1971 describing various permutations if anyone is interested, and here is a more recent paper describing the tricks of this "Acoustic Illusion Engine"

An aside - my father had this setup rigged up way back then, with four big bass reflex speakers and a valve amplifier, but all this was considered dreadfully old fashioned by the 80's when transistor amps held sway. Today of course people will pay huge sums of money for valve amplifiers, 5.1 surround sound systems and dedicated subwoofers to make up the bass lost by all the tiddly desktop and shelf loudspeakers.

No doubt all the manufacturers of 5.1, 6.1, 7.1 surround systems etc etc will swear blind that the quality of this elegant little bodge is way, way inferior to the shiny glory of their products, but it's pretty good and the price performance is awesome - so give it a go first!

So why bring all this up now, and whats it got to do with MyPCTV?

This Xmas we received a 2 x 64 AMD chip, 15.1" screen laptop with Ferrari badge on it - but it was a Dell, we bought it ourselves, the Ferrari is from a car sticker set - and the only Vista we have is the view of the back garden .

Anyway, the 15.1" screen also means you can watch Web TV, DVDS etc at a bit of a distance. This was a boon for us as the PS2 had given up the ghost on New Year's Eve (perfect timing), and we wanted to keep the kids out the adults' hair as we let it all down at New Year!

The laptop could step in and play the DVDs, CDs, games and Web TV etc etc that kids see to need these days (between beating seven bells out of each other and covering the floor in Lego that is)...but it quickly becomes clear that setting up a laptop to watch at even a moderate distance means the turned up sound is totally cr*p out of those tinny PC speakers, and the sort of desktop speakers you get for PC's are not that much better.

However, we have an old spare stereo amp, and 2 older but still fairly useful bookshelf speakers that could be pressed into service. I was also curious about whether it was possible to rig up a simple Hafler surround sound system, to give a high quality yet low cost, simple entry level Web TV audio system using the stuff that might already be in a house's living room / lounge / parlour / den etc....thus smoothing the path of the PCTV invasion.

At any rate, a quick trip to Tottenham Court Road (the London Geek Market) and £25 for 2 more (on sale) bookshelf speakers later, and we had rigged up a simple Hafler circuit as descrbed.

Another aside - on Subwoofers - these are mainly required on PC desktop and consumer mainstream 5.1 sound systems because the speakers used are usually too small to develop much bass. However, by putting even these 4 small speakers close to wall corners a fair bit of bass was generated. For higher end sound on our main MyPCTV rig I have found, as my father's generation knew, that decent floorstanding speakers will easily give enough bass to make a subwoofer unnecessary - and the bass is good quality, hi fi bass to boot.

Result No 1: The sound is good - awesomely good considering the minimal expenditure. (In fact, I would recommend getting a low cost stereo amp and 4 reasonable speakers and connecting a Hafler circuit rather than one of those lower end 5.1 sound setups - they are just so awfully tinny in comparison). Now, if one wanted a bigger screen the simplest thing to do would be to get a flatscreen with a VGA input for the laptop, but in the interests of experimenting with going Web TV with minimal new investment, the Next Step is to see what I can bodge to to connect the laptop up to the old analog TV we already have....

Result No 2: Found all the kids in a bed, heads together with laptop balanced precariously across knees, watching a DVD......ah well!