If you are serious about making quality recordings, then you need to accurately monitor the audio signal level being fed into the recording device. This is to ensure that the signal level is within a range that the recorder can accept.

In particular, correct audio signal levels are quite important for modern digital recorders. These do not tolerate any amount of excess signal level and will severely distort such signals.

Dynamic range

Any audio signal, be it speech or music, varies constantly in level and the difference between the highest and lowest levels is called the "dynamic range".

When recording, it’s important that the lowest signal levels must be sufficiently above the "noise floor" of the recording equipment, to prevent them from being buried in noise. On the other hand, the highest signal levels must be kept low enough to prevent signal overload and the inevitable distortion that accompanies this.

Ensuring that an audio signal stays within these bounds can be quite difficult unless its level is accurately monitored using a meter. This meter must respond not just to the average signal level but to peak levels as well.

Fig.1 illustrates why it is so import-ant to get the signal levels correct. Note that each waveform shown is not the audio signal itself but the instantaneous signal level plotted against time. These signal level variations occur constantly in music and speech. In music, for example, the level may range from soft passages to quite loud passages.

Fig.1: this diagram shows why it is important to set the correct signal level for recording. In "A", the average signal level has been set too low, resulting in lots of background noise. In "B", the level is too high and the recording system will overload and distort. Diagram "C" shows the correct level - ie, well above the noise floor but with the peaks below the maximum recording level.

Fig.1(a) shows an example of a recording that’s been made with the signal level set too low. What happens here is that lowest signal levels are lost within the noise and so only noise signals will be heard at these levels. The higher signal levels are above the noise floor but the overall sound quality will be rather poor, with lots of background noise.

Conversely, Fig.1(b) shows what happens if the average signal level is too high. Here, the upper levels go above the maximum level that the recording device can handle without distortion.

For magnetic tape recording, some degree of signal peaking above the maximum level can be tolerated. That’s because magnetic tape compresses the signal rather than severely clipping it. However, as previously indicated, this is not true for digital recordings where any signal that goes above the maximum is simply clipped.

The ideal recording level is shown in Fig.1(c). This is where the signal levels are well above the noise floor but do not exceed the maximum level. By doing this, we ensure both low distortion and the best possible signal-to-noise ratio.