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Macintosh Audio Subsystem Distortion

CPU and disk benchmarks provide information about raw computer performance capability; this benchmark focuses on audio performance, an important part of the Macintosh environment. This procedure provides a simple way to compare different sound i/o systems, by estimating the harmonic distortion at 1 kHz (and noise floor).

Preparation: Connect the computer's sound output jack to the sound input jack; use a stereo mini-plug (3.5 mm) to mini-plug cable. This should be done before proceeding, because the following steps will generate a relatively loud (if played over the computer's speakers) 1 kHz tone. Make the measurement: Launch Mac the Scope. Select 1 kHz Distortion Measurement from the QuickAnalyzer menu (this menu is present only if you have the scripts that were installed with the complete download package). Select Sound In from the Input menu. Adjust the test signal level (click the loudspeaker icon in the Waavebox console for the Waavebox OUTPUT LEVEL panel). If the panel level is greater than about 230, but the signal is too small, adjust the Computer System Volume setting in Apple's Monitors and Sound Control Panel (or the volume buttons on your keyboard, if present). If the trace "jumps" around, or the signal level indicator in the Mac the Scope Console flashes red, the signal is clipping, and the level should be reduced. The signal level also may be monitored by clicking on the icon in the upper left corner of the scope Console display bezel. The Estimate... window shown above may be activated by clicking on the S/N and Distortion button in the Toolbar (or from the Measure menu). This completes the procedure. The readings obtained will depend on the settings of both output level controls. Built-in sound usually gives the best results at levels between - 6 and - 20 dB FS (FS = full scale digital level), probably because of interaction between the differential nonlinearity characteristics of the computer's D/A and A/D converters. As a guide, lower Computer System Volume Settings will give lower output noise, for a given Waavebox level setting. Problems? Be sure that the Waavebox DIGITAL LOCK latency is set to 2048 (the standard default); Hann, Cosine or Square Top weighting function (Hann required for S/N and Distortion estimation); TIME BASE EXPANSION x1; the X2 resolution enhancement button is on, and Sampling is 44100 Hz. For more information on using Mac the Scope and Waavebox, please see the Quick Reference Guide document enclosed with the software.

We would be happy to learn about (send to softnoise) your measurement results (use MACSOUND as the subject line).

Here are some links to similar measurements made on Macintosh and other computer platforms.

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