(23 May 1995)
Norman McLeod
A little expansion on the previous piece on difference signals might be in order.
To recap, you can listen to the difference signal of a stereo programme by unhooking your normal loudspeaker connections, and wiring one speaker between the red 'hot' terminals of the left and right channels.
This technique, sometimes called the 'Hafler' connection among the hi-fi fraternity, can be used, alongside the normal stereo speakers, to wire a third, rear speaker giving an enhanced effect of space and depth. You might like to try this connection if you have a third speaker available to you. It does not have to be identical to the other two - in fact any small speaker will do for a quick trial.
Try playing Marvin Gaye's original Motown recording of "I heard it through the Grapevine" through this system, or something from a similar era of stereo production. You might find the effect of the third speaker quite intriguing.
This connection relies on giving added prominence to the difference components to enhance the stereo effect. If the source is in mono, the rear speaker should fall completely silent. So it is with our solo connection of one speaker.
If you switch to mono on the amplifier or tuner, you should hear virtually nothing coming out of the difference speaker. If you have a balance control, moving it from side to side will cause the volume to rise at each extreme to normal level, and cancel out at or very near the centre.
Go easy on the volume control - don't advance it any higher than you would for normal listening. The difference signal is almost always much quieter than the sum signal, but it is still being produced from full-amplitude excursions from each of your power amplifier stages. You ought not to advance the volume control so much, in your enthusiasm to hear small difference signals, that you drive the output stages of your amplifier into clipping.
So what are we listening to? With FM radio, it's a demodulated version of a signal which, while it was winging its way to you, was riding aboard a supersonic sub- carrier centred on 38 kHz on top of the normal mono modulation. This signal is not detected by mono receivers, or stereo sets switched to mono. The amount of difference signal is a guide to the amount of stereo effect in the programme material.
If there is no difference signal apart from noise and distortion, then the programme material is in mono whatever the lights or indicators may say on your receiver.
A few years ago, the BBC started putting out the pilot tone on Radio 4 FM continuously, regardless of whether the programme was in stereo or not. Similarly, all TV NICAM decoders light a 'stereo' beacon whenever they get a NICAM signal, regardless of whether or not the contents of left and right channels are identical. With the 'difference' connection, you can hear exactly how much stereo you are really getting for your money.
Beware, however, of some FM tuners which have a 'hi-blend' system to reduce the stereo separation (and noise) at high frequencies on weak stations, and can't always be turned off... This can mislead you into thinking there is less 'difference' than is actually broadcast, and may make the difference signal possess a curiously muffled tonal balance.
Any readers wishing to comment on what I have said are welcome to call me on 01 273 684 172 or send e-mail to normac@fastnet.co.uk
Copyright NJ McLeod 1995