The answer is simple in that there is very little you can do apart from set the recording levels carefully. If the record level is too low, then the signal-to-noise ratio will be poor - and your recording will be drowned out by noises coming from the electronics and the tape. If the record level is too high, then the peaks of your recording will be clipped and the signal will sound harsh. Most recorders have a record level meter that is often labelled in 'VU' or volume units. On this scale, a value of 0dB represents a safe recording level. However clipping does not normally occur until about 3-6dB above this level. Thus you should aim to make your recording peak close to but below 0dB VU, and use the extra headroom for the occasional loud noise that might occur.

Normally, by distortion, we mean the introduction into our signal of frequency components not present in the original. This also gives us a method for esatblishing the quantity of distortion: input a signal with known frequency components and find size of the largest frequency component in the recorded signal not present in the original. You can then express the size of this component as a percentage size of the largest true component, or as an amplitude ratio in dB.

Timbre is the third dimension of our perceptual sensations of sound. Once you have dealt with differences in loudness and differences in pitch, any other differences between two signals must be differences in timbre. Thus two musical instruments can play the same note (pitch) at roughly equal loudnesses, and so any quality difference must be one of timbre.

When a signal is recorded onto analogue magnetic tape, the amplitude of the signal is reflected in the 'magnetisation' of the tape: roughly meaning the amount of magnetism stored on the tape. Recording tape consists of a plastic backing coated with small metal-oxide crystals. Each crystal can be turned into a small permanent magnet by applying an external electro-magnetic field (from the record tape head), and the magnetised crystals can themselves induce a small electrical current in a coil position next to the tape (in the playback head). By this means we can use a signal to change the magnetic properties of the crystals on the tape and conversely use the magnetic properties of the crystals to recreate an electrical signal. Hey presto: a tape recorder!

A way of measuring how much extraneous signal is added to our recording by the electronics and the tape. For analogue tape recorders most noise is generated by random fluctuations in the magnetic properties of the crystals on the tape. Ideally we would want such noise to be much smaller than the signal we want to record. We can measure how good or bad a tape recorder is by measuring the size of the noise generated by the recorder and comparing it to the size of the signal we are recording. The ratio of the amplitude of the signal to the amplitude of the noise is called the signal-to-noise ratio. It is often expressed in dB.

This is a little tricky: ideally we would want to create a set of standard sounds: sinusoidal pressure waves of different frequencies but constant size. We could then simply measure the voltage output from the microphone as a function of frequency. Unfortunately, to produce sounds we need a loudspeaker and how do we know what the frequency response of the loudspeaker is? To get the response of a loudspeaker we need to use a microphone to measure the size of the pressure variations it generates. But then how do we know the frequency reponse of the microphone? And thus we are back to where we started! Basically we have to rely on a reference loudspeaker for which we know its response, or a reference microphone for which we know its reponse. We normally use the latter, since it is much easier to build a microphone with an essentially flat frequency response than it is to build a speaker. So ... we use a speaker to generate some sinusoidal pressure waves and monitor the voltage produced by the test microphone and by the reference microphone. Any differences will be due to deficiencies in the test microphone.