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Distortion Trimming
If you have access to an audio signal generator and an oscilloscope, the VCA's can be trimmed for minimum distortion. Connect the signal generator to both inputs and the oscilloscope to one output. Set the signal generator to give a 10Hz sine wave (or the lowest frequency available if it does not go this low) with an amplitude of about 3V peak-to-peak (1V RMS). Set the unit to expansion mode and adjust the range control to about mid-way.

There will probably be some noticeable distortion on the waveform displayed on the 'scope, at the zero crossing points. Adjust the relevant preset to minimise this distortion. This will occur when the zero-crossing distortion is equal on the rising and falling slopes of the waveform. Repeat this for the other channel. The unit will not normally be operating at such a low frequency so the problem will not be significant in practice even though it may look quite bad on the 'scope.

If you do not have access to suitable test equipment, the presets should be left in the centre positions.

In Use
Use of this unit obviously depends on the original source material and what you are trying to achieve. I will give a few examples here.

My main use for the unit has been to attempt to improve poor quality recordings of music. Obviously there are limits to what can be achieved - to quote an old cliche "you can't make a silk purse out of a sows ear". If you use the range control in moderation you should be able to achieve useful improvements without adding too many undesirable effects.

Expansion
Compilation albums suffering from groove jamming, and lower quality pre-recorded cassettes, can benefit from some expansion. This will tend to bring out the bass and vocals, and reduce the effects of noise. Normally turning the control up to about one third is sufficient, you are only trying to compensate for the compression that was used when the record or cassette was recorded.

Setting the expansion too high tends to cause a couple of noticeable effects. The most apparent will be a pumping sound on the bass - difficult to describe but easy to hear. A more subtle effect will be a fluttering variation in level, similar to that obtained from a poor quality tape suffering from drop-outs. If either of these effects are audible the dynamic range should be turned down so that they are not objectionable.

Expansion can also improve the sound from tapes recorded on systems with automatic level controls, although sometimes these level controls are so drastic that little can be done to eliminate their effects.

It is also worth trying some expansion on MW and LW radio stations, providing you have good reception without too much interference and noise. A vast amount of compression is used on AM broadcasting to ensure there is sufficient signal to mask the background noise. Most independent local radio stations broadcasting on FM also use compression in varying degrees, as well as limiting to cope with presenters who are not too proficient with the level controls. This explains why the BBC national stations often sound so much better on decent equipment.

Compression
If you have a recording that is distorted due to excessive recording level, a little compression can reduce the audible effects significantly. Compression also helps reduce the effects of drop-outs on cheaper cassettes, and can make bad scratches on records less pronounced.

Some live recordings - particularly those which are genuinely live and have not been processed in the studio - can sound echoy and hollow. A little compression will calm the reverberations and place the vocalist back with the instrumentalists where he belongs.

When CD was first launched there was a rush by record companies to release old material in this new format. Most of these were superb, but in some cases they really shouldn't have bothered. I have heard a couple of examples where the master tapes were clearly in poor condition with noticeable tape hiss and drop-outs. In some cases the recording company used an excessive amount of analogue expansion when transferring them to CD, giving audible pumping effects. Such poor CD's are not limited to the budget labels - indeed some of the worse examples come from major record companies who realy should know better. Poor quality CD's like this benefit from some compression to mask the problems, and reduce the dynamic range to the original intended level.

Compression is also useful when recording CD's for playing on a car cassette player. The advantage of some compression is that you won't have to turn up the volume to hear a quieter section over the engine and road noise, only to be deafened by a louder section. Obviously this is more of a problem with the lower quality equipment fitted by car manufacturers.

In most cases the separate/combined switch would be left in the "separate" position so that each channel operates independently. The "combined" position can be used when processing mono recordings or recordings where there is very little difference between the two channels. The "combined" setting is also useful when dealing with tape problems such as drop-outs and distortion which often affect one channel more than the other.

As I stated previously, compression and expansion should be used in moderation. Turning the control up too high will often cause unwanted effects to occur, which may be worse than the problem you are trying to reduce! The centre "off" position on the compression/expansion switch is useful for quick comparisons - you will often find that the unit is having more effect than you thought when you were twiddling with the control.