Lexicon Aussteuerungsanzeige
Level meters serve as an aid for the correct Einpegeln of a device or system. It is a measuring device whose characteristics depend on the application. The characteristics can be the subject of standards. Common characteristics are VU and PPM.
With the VU meter, the scale goes from -20 to +3. It is a dB scale whose zero point is fixed at +4dBu, where the "red range" also begins. The measuring range is therefore from -16dBu to +7dBu. A second scale goes from 0% to 100%, where 100% coincides with the zero point at 4dBu. The instrument is relatively sluggish and therefore cannot follow fast signal peaks. It is therefore more suitable for estimating approximately the loudness of a sound signal. When faced with the task of monitoring Overshoot at short signal peaks, a different characteristic is needed:
The PPM (Peak Programme Meter) is 30 times faster than the VU meter, making it suitable for monitoring signal peaks. There are several different scales for the PPM, since national standardization committees have made different specifications here. In Germany, a scale standardized by DIN is used which ranges from -50dB to +5dB, with the "red range" starting at 0dB. This zero point corresponds to a level of +6dBu. At -9dB there is a marker for a measuring level. Again, there may be another scale from 0% to 100%, with 100% falling at the zero point.
The meaning of the % scale on both instruments lies in its connection to broadcasting, and is intended to denote the degree of modulation. Here, 100% modulation denotes the maximum permissible modulation of the carrier signal, above which signal distortion and/or disciplinary action by the monitoring authorities is to be feared.
The zero (or 100%) point of instruments does not normally denote the point at which overdrive begins in a normal instrument. Almost all instruments are more or less resistant to overdriving beyond this point. The zero point corresponds to full scale, which is the maximum level that should be reached in normal operation. It depends on the device class as well as on national conventions or even individual working habits how much reserve is available or desired beyond this point.
The level achieved in normal operation is always below the level at full scale. The measuring level set by DIN at -9dB corresponds to the average level of "ordinary dance music", which is distorted in such a way that its peaks reach 0dB. This is a rather arbitrary determination, which obviously depends on the type of signal. Significant here is the peak factor (Crest Factor) of the signal, as the measure of how high the peaks of the signal turn out in relation to the average signal level. This ratio dictates how far one must stay below full scale at the normal level to avoid clipping at signal peaks. The lower the factor, the closer you can get to full scale when modulating the normal level. You can limit the peak factor with a limiter or dynamics processor, which allows you to increase the average level without getting into overdrive. However, this can have sonic disadvantages.
See also the Wikipedia articles: VU meter, peak meter.