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Audio quality measurement seeks to quantify the various forms of corruption present in an audio system or device. The results of such measurement are used to maintain standards in broadcasting, to compile specifications, and to compare pieces of equipment.
Measurement allows limits to be set and maintained for equipment and signal paths, and different pieces of equipment to be compared. While the issue of measurement is controversial, to the extent that Hi-Fi magazines these days tend to shun measurement in favour of listening tests, it is important to realise that audio quality measurement has in the past got a bad name by failing to produce results that correlated well with listening tests. This was because certain basic measurements were used, such as THD measurement, and A-weighted noise measurement, without any proper consideration of whether these related to subjective effects. The proper approach to measurement, which is largely adopted by broadcasters and other audio professionals, is to first devise measurements that can quantify the various forms of corruption in terms of subjective annoyance to a human listener, ideally the most critical listener based on tests using many suitably rested subjects. Once this is done, measurement has the advantage of not being dependent on a particular listener, or his state of hearing on a given day. It also has the advantage of being able to quantify corruption levels that would not be audible to even the most sensitive ear, which is important because a typical audio path from source to listener can involve many items of equipment, and just listening to each is not a guarantee that they will still sound acceptable when cascaded so that all their deficiencies add up.
Sequence testing uses a specific sequence of test signals, for frequency response, noise, distortion etc, generated and measured automatically to carry out a complete quality check on a piece of equipment or signal path. A single 32-second sequence was standardised by the EBU in 1985, incorporating 13 tones (40 Hz–15 kHz at −12 dB) for frequency response measurement, two tones for distortion (1024 Hz/60 Hz at +9 dB) plus crosstalk and compander tests. This sequence, which began with a 110-baud FSK signal for synchronising purposes, also became CCITT standard 0.33 in 1985.
Lindos Electronics expanded the concept, retaining the FSK concept, and inventing segmented sequence testing, which separated each test into a 'segment' starting with an identifying character transmitted as 110-baud FSK so that these could be regarded as 'building blocks' for a complete test suited to a particular situation. Regardless of the mix chosen, the FSK provides both identification and synchronisation for each segment, so that sequence tests sent over networks and even satellite links are automatically responded to by measuring equipment. Thus TUND represents a sequence made up of four segments which test the alignment level, frequency response, noise and distortion in less than a minute, with many other tests, such as Wow and flutter, Headroom, and Crosstalk also available in segments.
The Lindos sequence test system is now a 'de-facto' standard in broadcasting and many other areas of audio testing, with over 25 different segments recognised by Lindos test sets, and the EBU standard is no longer used.
Another approach to automated testing uses a special multitone signal to assess all parameters simultaneously, by analysing the spectrum of the output from the device under test. It relies on the fact that with appropriate choice of frequencies, distortion components and noise can be made to appear between the tones, and measured using digital comb filtering. Even noise and wow and flutter can be extracted from the spectrum in principle.
In practice, though the use of a single brief test is attractive, and might even be used between programmes, this method presents several problems. Digital distortions produce a fine spectrum which can swamp the measurement of true noise in the absence of signal. The composite signal also has a high peak to mean ratio, with peak levels occurring whenever all the tones hit maximum simultaneously. Although the Probability density function can be controlled to some extent, it is not possible to separate out distortion at high level, from low level distortion. Quite high amounts of the former can be considered acceptable, but low level distortion is more critical.
Fast sequence tests are possible, and there have been attempts to make these appear like jingles for incorporation into broadcast programmes!