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Objective testing - audio equipment should be assessed by scientific measurement, not by misconceived 'listening tests'

A post by David Mellor
Wednesday November 30, 2005
How do you know whether your equipment sounds good? Does it sound good to you? Maybe you just like the distortions it creates.
Objective testing - audio equipment should be assessed by scientific measurement, not by misconceived 'listening tests'

Sound equipment of any kind has precisely three functions...

1. To handle sound transparently without adding anything or taking anything away.

2. To allow the user a wide range of freedom in manipulating sound to their requirements.

3. To offer operational convenience.

Anything else is incidental, and is probably linked to marketing - selling the equipment to you, or you selling studio time to a client.

Let's consider here point number one - handling sound transparently...

The first basic requirement is that all frequencies within the range of human hearing should be treated equally. In other words, the equipment has a flat frequency response.

Linked to that is the requirement that no band of frequencies should be advanced or delayed in time with respect to the others. We would describe that as a linear phase system.

In practice, the ear is not that sensitive to phase, therefore a system that displays minimum phase shift is often deemed acceptable. This means that the phase changes smoothly through the frequency range without sudden changes.

The second requirement is that the shape of the waveform should not change, other than being made higher or lower in level. To change the shape of the waveform creates distortion. Distortion creates additional frequencies that were not present in the original, therefore such a system would not be transparent.

The third requirement is for low noise, and therefore a good signal-to-noise ratio.

And that's it. To satisfy the criterion of transparency, that is all that is required.

All of these parameters are very easy to measure, so if a system is tested objectively and found to show sufficiently good figures in all respects, then it is a perfect system.

But...

Although the tests are easy, it is difficult to predict every situation under which the system will work in real life.

For instance, a real-world input signal might contain frequencies above the range of human hearing. Perhaps the system might not be able to cope – slew rate limiting in an analog system, or aliasing in a digital system. Simple tests might not reveal these problems.

Digital systems in particular can suffer from problems with radio frequency interference. If tests are carried out when there is no interference present, then the equipment might seem to be good. But put it into a real-life scenario and things might change.

Objective testing is undoubtedly the ultimate form of assessment. But often the ear has to hear a problem first, and then means must be sought to devise a suitable objective test.

One last point - just because a piece of equipment sounds 'good' doesn't mean it is transparent. Perhaps it has introduced an anomaly or distortion of some kind that you just happen to like.

The trouble is, someone else might not like it, or perhaps it might not suit every kind of program material.

As I said, sound equipment has precisely three functions...

1. To handle sound transparently without adding anything or taking anything away.

2. To allow the user a wide range of freedom in manipulating sound to their requirements.

3. To offer operational convenience.

A post by David Mellor
Wednesday November 30, 2005 ARCHIVE
David Mellor has been creating music and recording in professional and home studios for more than 30 years. This website is all about learning how to improve and have more fun with music and recording. If you enjoy creating music and recording it, then you're definitely in the right place :-)
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