Electronic Design

Audio Measurements

After following all the advice in this article, you will want to know if your efforts are successful. Fortunately, standard measurement techniques have developed over the years to help measure audio quality.

THD+N is an abbreviation for Total Harmonic Distortion plus Noise. THD+N is a measure of audio fidelity. If you feed a sine wave into an ideal audio system, you get the same sine wave out. In the real world, though, you get the original sine wave plus added harmonic frequencies and noise. Harmonic frequencies are integer multiples of the original sine wave (Fig. A).

Typically, a THD+N analyzer uses a notch filter to remove the test signal. What remains are the harmonics and any noise induced or generated by the device under test. The ratio between the output signal level and the input signal level is the THD+N measurement. In PC audio, this is usually expressed as a dB ratio.

Frequency response is a measure of the flatness of the output response over the audio band. The frequency response is measured by applying a series of sine waves, one at a time, over the audio band and looking at the output levels. The signal level at 1 kHz is generally used as the reference level. A frequency response of 20 Hz to 20 kHz ±0.5 dB means that the maximum variation in signal level from the level (at 1 kHz) is ±0.5 dB.

Dynamic range is the ratio of the full-scale signal level to the rms level of the noise floor. For digital devices, this is generally measured as THD+N with a small test signal (usually -40 or -60 dBV). This small signal is applied in order to exercise the digital registers. Dynamic range is typically measured over a 20-Hz to 20-kHz bandwidth. Note that 0 dBV = 1 V rms.

Crosstalk is a measure of how much a particular signal bleeds into another signal path (Fig. B). It's measured by applying a signal to one path and measuring that signal's level on another signal path. Crosstalk is expressed as the dB ratio of the two signals. Let's say that in Figure B, a 1-kHz 0-dBV tone is applied to signal path A and no signal is applied to path B. If we measure a 1-kHz signal of —80 dBV at signal path B, then the crosstalk from A to B is —80 dB at 1 kHz.

The PC Audio Quality Measurement Standard is the industry standard for the measurement techniques of PC Audio Quality. You can get the standard at the Cirrus Logic web site: www.cirrus.com/ftp/pubs/meas100.pdf.