Crest factor and PAPR

The crest factor is a measurement of a waveform’s peak values compared to its effective value. It’s often used to determine the extremeness of peaks in a waveform. A waveform with a crest factor of 1 indicates no peaks, such as a square wave or direct current. On the other hand, sound waves tend to have high crest factors and so do DAB+ modulated signals. The crest factor is calculated by dividing the peak amplitude of a waveform by its RMS value. DAB transmitter signals will have a high crest factor.

The Peak-to-Average Power Ratio (PAPR) is another measurement that is the square of the crest factor. It is calculated by squaring the peak amplitude and dividing it by the squared RMS value. When expressed in decibels, crest factor, and PAPR are equivalent (not equal!) because of the way decibels are calculated for power ratios versus amplitude ratios. Both of these measurements are dimensionless quantities, with the crest factor being defined as a positive real number and commonly stated as the ratio of two whole numbers (e.g. 2:1).

PAPR is expressed in decibels because it is a power ratio. The crest factor is an important factor in loudspeaker testing standards and is usually expressed in decibels. The minimum possible crest factor is 1, which is equivalent to 1:1 or 0 dB.

As an example, a PAPR of 13dB means a difference in peak power of 20:1

10*LOG 20 = 13 dB and 1013/10 = 20