Quality Lies in the Details SIDEBAR 2: The Audio Precision System One
Most of the measurements Stereophile makes on review products are performed with the Audio Precision System One, a computer-based audio test system. This instrument, introduced in 1985, has so revolutionized audio testing that it's hard to imagine life without one.
The System One is a large box that interfaces to an IBM-compatible personal computer through a custom board in the computer. The box contains analog signal generators and analyzers controlled by commands from the personal computer. Instead of adjusting front-panel knobs on an array of specialized test equipment and reading dials, the operator controls the System One's functions from the computer keyboard. If you need a 500mV, 1kHz, ultra-low-distortion sinewave, for example, you just enter "500m" and "1k" in the appropriate fields of the computer display.
A set of test parameters is stored as a test file, retrieved, then used to make the measurement on the device under test. The graph created with the measured data can then be output to a printer for a hard copy. Because the system is software-based, the measurement data can be manipulated later. For example, graph parameters, such as horizontal and vertical scale, can be adjusted, data from two measurements can be appended to the same graph (as we do in amplifier distortion vs output level into different impedances), and the difference between two sets of data can be calculated and displayed. This is just a tiny sampling of the power of the System One's software-based audio-analysis capability.
The System One allows tremendous control over the test signals generated. For example, the generator can be instructed to sweep between specified frequencies at a specified level and invoke an equalization curve, such as the RIAA curve, for making phono-stage measurements. The generator can also be made to regulate itself to produce a specified condition at the device-under-test's output—0.5% THD from an amplifier, for example.
Stereophile purchased the Dual Domain upgrade for the System One, which allows us to create and analyze both digital and analog signals. Rather than being dependent on whatever signals happen to be on test CDs, we can create digitally any signal we want—including complex signals—and output them in TosLink optical, S/PDIF, or AES/EBU formats. (Most of the test signals on the second and third Stereophile Test CDs, for example, were created in the digital domain with the System One.) The Dual Domain option also provides for analysis of digital signals—such as when measuring an analog/digital converter.
The Audio Precision System One isn't inexpensive, but it's become an indispensable tool in measuring and evaluating the technical performance of the products we review, as well as for presenting that information in graphic form for publication.—Robert Harley