Not that I'm complaining about the job, you understand, but chorus units are definitely the most difficult effects to judge. Unlike fuzz boxes, wah-wahs or compressors where you hear an output distinctly at odds with the input, chorus should just be there — a bigger sound, a richer sound, a moreness. If you can detect impressive wooshings, whirrings and tweetings, the pedal's doing a bad job.

So when coughing up for a chorus and testing it in the store, this is what you don't want to witness:

(a) Lumpiness, an uneven rising and falling in pitch that is the result of driving the variable time delay, which creates the effect from a badly set up LFO. The best units will use a steady sine wave oscillator, but it's often easier and cheaper to build a ramp or triangular LFO which, if poorly designed, will produce that faint lurching in the sound.

(b) Heavy vibrato and by that we mean obvious and unattractive wobblings in the pitch. Vibrato can be handy as an extra special effect when all the controls are on full, but a good chorus should be able to create that 'moreness' without casting the tuning of your guitar adrift on a violent ocean swell.

(c) Hiss, twitterings (leakage from the delay section through the earths), hum and nasty clicks when you hit the footswitch. You might get away with noisy switches on fuzz boxes where the sound is about to become radically different, but not on such a subtle change as chorus.

Alerted to these horrors, we plunge into a mound of pedals only to discover that the manufacturers have as much trouble telling one chorus from another as we do. In this concise sample there were no real howlers. The final sounds varied only slightly and it was the a, b and c tests above that separated the marks.

All the contestants had a depth control (some called it width), but in virtually every case you'd never need it... leave it on full otherwise you'd only be weakening the effect. The exception was the Ibanez CS9 Stereo Chorus (£86) where you could detect a genuine variation beyond the half way mark. The chorus appeared to reach deeper into the harmonics of the guitar and pull on the tone of the instrument. It was only slight, but it helped the Ibanez gain enough points to be voted the fullest sounding pedal of the collection. It also appeared to be the only one with a noise gate which silenced some of the hiss by turning off part of the circuit when nothing was being played. Main disappointment was the limited LFO speed which couldn't drive the pedal fast enough for a Leslie imitation.

Next (and closest) in sound but actually best for facilities, was the Arion SCH-1 Stereo Chorus (£51) which added a tone control alongside the normal rate and depth knobs. True, it only trimmed back the treble boost put in by the chorus circuitry, but at least you had the choice. Stereo works well with chorus pedals, reinforcing the illusion of there being more than one guitar by generating the sound from more than one place. All such pedals have two output sockets converting from mono to stereo operation when you plug in the second lead. The Arion had a small slider switch to change from stereo to mono (one speaker only) so the leads could be left untouched — an added convenience. Wasn't so keen on the pop out cover for the battery compartment which could be easily lost or broken, and the Ibanez tough die-cast metal case won over the SCH-1's less durable plastic variety.

The Ross (£55) and the Tokai (£63.30) paired off well matching each other for hiss (a fraction better than the Arion), hum (hardly any), twitterings (the worst offenders in the bunch, but still acceptable) and strength of sound. They also each had something of an ADT element to their tone which some guitarists like.

There was a touch more 'liveness' to the chorus, a hint of reverb that improved the impression of size even though they were perhaps not as warm as the Ibanez or Arion. Both were also a mite lumpy and the Tokai shared the Ibanez' restriction on the top speed Leslie.

Which leaves the Korg (£65.10) and the Ibanez Bi-Mode (£79) — both oddities.

The Korg is part of the company's chorus/flanger modular PMX system so has no batteries, power supply or sockets of its own but is fitted with an eight pin connector that links it to its PMX brethren. It was the most versatile of all the offerings — plenty of LFO speed, enough depth for a strong chorus and some in reserve for loopy pitch bending effects plus a manual control so you could sweep through the effect by hand. Pity, then, that this versatility is unlikely to escape from the rest of the PMX pedal board.

The Ibanez BC-9 Bi-Mode is a clever scheme in theory but sounds, in practice, as though its contracted the lurgy.

Witness two choruses in one pedal, each with width and rate controls. 'B' is designed for slow and gentle chorusing (a bit like the CS9 but not quite so strong), and A is for faster, vibrato-ish work. Each has a separate output and presumably they're supposed to be mixed in stereo since the pedal gives you no way of swapping between the two settings — they're either both on or both off. It would be nice if they gelled to give an ethereal, unpredictable, meandering chorus. To be honest, they lurched in and out of each other's paths so much I was reaching for the seasick pills. Dramatic, certainly, but unnecessary, I think.

FLANGERS and chorus pedals are very close neighbours, brothers, or to be more biological still, Siamese twins. They share the same theory, design and in many cases components since it's quite possible to get a passable if not perfect chorus sound out of a flanger, though not the other way around.

To make a flanger out of a chorus circuit you have to add regeneration — the facility to send part of your treated signal back through the electronic hoops one more (or many) time(s). The greater the amount of regeneration, the more 'perverted' the sound becomes resulting in that mouthy 'yoy-yoy' type noise that has the twang of an empty bathroom on Friday night.

Of this limited selection, the Tokai TFL-1 (£62.98) turned in the best value for money performance. It could do a reasonable chorus with the regeneration turned to zero, introduced the desired amount of swooping and diving when that control was increased and finally, at maximum, descended into yowling weaves that stopped just short of real, whistling-type feedback. Neither of its rivals went that far so the Tokai gets the prize for guts as well as having a solid if somewhat ugly metal case against the Arion's plastic alternative.

Stereo Flanger SFL-1 it says on top of the Arion (£51) and the arrangement of controls plus the mono/stereo switch is all but identical to the Stereo Chorus from the same company. The separate, panned outputs again provide a spatial ear pleaser, but not as marked as on the chorus units. A strong effect and, like the Tokai, one that has a manual control. This is for applications that undeniably fit under the category 'special' effect. Turn the regeneration up full, the rate to zero then play with the depth and manual controls. Instant conversion of guitar to a sheet of corrugated iron as the frozen delay circuitry within the flanger creates a very hard and short reverb. Carl Marsh of Shriekback describes this sound elsewhere in our supplement as 'steel drum'. Can also be identified as 'Eddie The Shipboard Computer' in The Hitchhiker's Guide To The Galaxy. The Arion almost shared equal first place with the Tokai except for the loseable battery cover and plastic case (see chorus) plus the more worrying element of distortion whenever I bashed a loud chord. Just a dodgy sample, we hope.

The Ibanez FL9 comes further down the scale due to item (a) in our initial checklist — lumpiness. This time, with so much harmonic swooping going on, you can't help hearing a certain wandering in pitch. Uneven LFOs demonstrate themselves in another way on Flangers — they cram all the effect into one end of their travel. So for a while little seems to be happening to your sustaining guitar chord and then, whoop, it goes through a harmonic hair pin bend before returning to a plateau of not-much-to-get-excited-about, thanks. Better to have some degree of flanging activity all the time than a concentration every third second.

All the reviewed pedals ran from 9V PP3 batteries or 9VDC power supplies and most (with the exception of the Korg and the Ross) had separate battery compartments that could be quickly unclipped without disturbing the main circuit board or reaching for the screwdriver. The Ross needed to have its base plate removed and the Korg, as mentioned, fits in its own rack.