The HP Model 8903A Audio Analyzer is a complete audio measurement system covering the frequency range of 20 Hz to 100 kHz. It combines a low-distortion signal source with a signal analyzer. The analyzer can perform distortion analysis, frequency count, and ac level, dc level, SINAD, and signal-to-noise measurements. The Audio Analyzer reduces the number of instruments required in many applications involving audio signal characterization. The Audio Analyzer is easy to use. All measurements are selected by one or two keystrokes. For distortion measurements, the Audio Analyzer automatically tunes to and levels the input signal. Measurement and output ranges are automatically selected for maximum resolution and accuracy. Furthermore, tuning is independent of the source. Thus, the source can be set to one frequency while the analyzer is measuring the distortion on a signal at another frequency (i.e., there is no need to tune the analyzer to the source). The combined capabilities of the instrument are enhanced by microprocessor control, resulting in more capability than would be available from separate instruments. For example, when making signal-to-noise measurements, the Audio Analyzer monitors the ac input level while turning the source on and off. The microprocessor then computes and displays the ratio of the on and off levels. The ratio can be displayed in either % or dB. In addition, the source can be swept. This makes measurements such as frequency response or complete distortion characterization simple to perform. Microprocessor control allows flexible entry of source parameters and versatile display formats. For example, ac level can be displayed in V, mV, dBV, watts, or as a ratio (in % or dB) referenced to an entered or measured value. Virtually all functions are remotely programmable through the Hewlett-Packard Interface Bus (HP-IB1). Programming is easy and straightforward, and all measurements are made through a single input connector. This eliminates the need to switch between multiple inputs under remote control and reduces software development time and hardware costs. The Audio Analyzer measures the true rms level on all measurements. True rms measurements assure greater accuracy when measuring complex waveforms and noise. Accurate distortion measurements typically can be made to less than 0.003% (-90 dB) between 20 Hz and 20 kHz at a IV level.