The conventional measurements used in audio tests can also be used to evaluate ADCs and DACs. These measurements include frequency response, signal-to-noise ratio (SNR), interchannel phase, crosstalk, distortion, group delay, polarity, and others. But conversion between the continuous and sampled domains brings several new mechanisms for nonlinearity, particularly for low-level signals. This article looks at problems seen in audio analog-to-digital and digital-to-analog conversion and some of the methods that have evolved to address these issues.
Of course, ADCs and DACs are used in a great number of non-audio applications, often operating at much higher sampling rates than audio converters. Very good oscilloscopes might have bandwidths of 33 GHz and sampling rates up to 100 GS/s, with prices comparable to a Lamborghini. Although audio converters don’t sample at anywhere near that rate, they are required to cover a much larger dynamic range, with high-performance ADCs digitizing at 24 bits and having SNRs over 120 dB. Even a high-end oscilloscope typically uses only an 8-bit digitizer. A 24-bit conversion pushes the measurement of noise and other small-signal performance characteristics to the bleeding edge. Consequently, measurements of such converters require an analyzer of extraordinary analog performance.
This article also includes an interesting text, written by Eric Hodges (Marketing Manager for Audio Precision), detailing how Audio Precision found an ideal test case on Benchmark’s AHB2 amplifier, when they introduced their APx555 analyzer at the 137th International Audio Engineering Society (AES) Convention. John Siau (Benchmark), together with Laurie Fincham and Jayant Datta from THX (a patent holder for amplifier and error-correction technologies used in the AHB2), visited the Audio Precision booth wondering if it was possible to set up a test of the AHB2 with the APx555 — and effectively also test the APx555’s analog performance as indicated by its total harmonic distortion plus noise (THD+N) measurement capability.
Read the full article now available online here.
About the Author
David Mathew is Technical Publications Manager and a senior technical writer at Audio Precision in Beaverton, OR. He has worked as both a mixing engineer and as a technical engineer in the recording and filmmaking industries, and was awarded an Emmy for his sound work in 1988.