An audio analyzer is a test and measurement instrument used to objectively quantify the audio performance of electronic and electro-acoustical devices. Audio quality metrics cover a wide variety of parameters, including level, gain, noise, harmonic and intermodulation distortion, frequency response, relative phase of signals, interchannel crosstalk, and more. In addition, many manufacturers have requirements for behavior and connectivity of audio devices that require specific tests and confirmations. Audio analysis requires that the device under test receive a stimulus signal of known characteristics, with which the output signal (response) may be compared by the analyzer in order to determine differences expressed in the specific measurements. This signal may be generated or controlled by the analyzer itself or may come from another source (e.g., a recording) as long as characteristics relative to the desired measurement are defined. As test and measurement equipment, audio analyzers are required to provide performance well beyond that of the typical devices under test (DUTs). High quality audio analyzers must demonstrate vanishingly low levels of noise, distortion and interference in order to be deemed worthwhile, and must do so consistently and reliably to be trusted by engineers and designers. For example, while a commercial CD player can achieve a total harmonic distortion plus noise (THD+N) ratio of approximately −98 dB at 1 kHz, a high quality audio analyzer may exhibit THD+N as low as −121 dB (this is the specified typical performance of the Audio Precision APx555). Audio analyzers are used in both development and production of products. A design engineer will find it very useful when understanding and refining product performance, while a production engineer will wish to perform tests to rapidly confirm that units meet specifications. Very often audio analyzers are optimized for one of these two cases. Current popular audio analyzer models include: APx585 and APx555 (from Audio Precision), dScope M1 and Series III (from Spectral Measurement, formerly Prism Sound), U8903A (from Agilent) and the UPP and UPV analyzers (from Rohde & Schwarz). History One of the earliest reliable sources used for audio test was the first product made by Hewlett-Packard in 1939, the HP200A audio oscillator. The clever and inexpensive design of the HP200A allowed testers to generate very high quality, low distortion sine waves that could be used for testing. This was followed by the company's introduction of the HP320A and HP320B Distortion Analyzers in 1941. These early analyzers could only determine total