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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Re: Audible differences posted by MAD MAN HARJU on January 6, 2005 at 04:07:17:
It is difficult for me to understand why there is so much debate about THD and whether we can ever measure all that we can hear.
First, THD is a simple test that normally only nominally challenges the component under test. This is especially true with 1KHz THD. However, it can be detected by instrumentation at very low levels (120-140dB below the test tone), because it is so easy to null the fundamental frequency and only look at the residual. Even noise and specific harmonics can be seen by FFT processing the residual of the THD analyzer
The problems with THD are:
1. There is no single number that is meaningful, except for a particular piece of equipment, such as a tube preamp. This is because the higher order odd harmonics are FAR more significant than the lower order harmonics, such as 2nd and 3rd harmonic. Without a very steep weighting applied to the harmonics, the measured result is usually misleading.
Usually, people just give a single number for a given component such as an IC and this is mostly useless.
2. Noise will dominate the test at normal working levels for a component. This is because of the wide bandwidth (80KHz in my analyzer) and the distortion can hide in the noise. Only spectrum analysis or FFT averaging can remove this noise. Then crossover distortion artifacts can be seen in IC's, etc.
3. The test is too easy to really stress the component under test. It doesn't give the wide range of change in input that music will do, including risetime, IM, change in music level, and problems at frequency extremes. It also isn't sensitive to FM distortion that we have already shown can be generated by application of negative feedback.
Knowing the limitations of harmonic distortion, it can still be a very useful tool, IF you compensate for its limitations with subsequent spectrum analysis and spectral weighting.
It is true that the results may appear to be too low in level to be audible, but if higher order distortion is detected, this implies that multitone IM, and real music inputs will produce significantly higher distortion levels than measured by the harmonic method.
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Follow Ups
- Re: Audible differences - john curl 10:14:18 01/10/05 (1)
- Re: Audible differences - john curl 15:27:49 01/10/05 (0)
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