Audio analyzer
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.
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.
These early analyzers could only determine total harmonic distortion and noise combined, and worked by employing a steep notch filter to remove the fundamental frequency of the stimulus signal from the output of the DUT.
The remaining signal was measured as an AC voltage, and thus allowed for the manual calculation of total noise and distortion to approximately 0.1% minimum.
Subsequent products from HP, Wandell & Goltermann, Radford, Marconi, Sound Technology, and Amber continued to refine measurement capabilities from the 1950s through the 1970s, but the model of usage remained relatively constant; signal generators and analyzers were separate pieces of equipment, and testing involved careful tuning of each one by a person with high technical skills.
The combination of PC technology with audio analyzers was adopted by others, including Prism Sound (dScope), Rohde and Schwarz (UPL), and Stanford Research (SR1).
These techniques allow loudspeakers to be characterised in a non-ideal (noisy) environment, without the need for an anechoic chamber, which makes them ideally suited for use in high volume production line manufacturing.
Most quasi-anechoic measurements are based around an impulse response created from a sine wave whose frequency is swept on a logarithmic scale, with a window function applied to remove any acoustic reflections.
The log-swept sine method increases signal-to-noise ratio and also allows measurement of individual distortion harmonics up to the Nyquist frequency, something which previously impossible with older analysis techniques such as MLS (Maximum Length Sequence).
