Understanding Digital Signal Processing, Second Edition

Appendix E. Decibels (dB and dBm)

This appendix introduces the logarithmic function used to improve the magnitude resolution of frequency-domain plots when we evaluate signal spectra, digital filter magnitude responses, and window function magnitude responses. When we use a logarithmic function to plot signal levels in the frequency domain, the vertical axis unit of measure is decibels.

USING LOGARITHMS TO DETERMINE RELATIVE SIGNAL POWER

In discussing decibels, it's interesting to see how this unit of measure evolved. When comparing continuous (analog) signal levels, early specialists in electronic communications found it useful to define a measure of the difference in powers of two signals. If that difference was treated as the logarithm of a ratio ...