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About the visibility of distortion
The entire history of sound reproduction has evolved from attempts to bring the illusion closer to the original. And although the path has been traversed, it is still very, very far from fully approaching live sound. Differences in numerous parameters can be measured, but many of them remain out of the sight of hardware developers. One of the main characteristics that a consumer with any background always pays attention to is the coefficient of non-linear distortion (THD). And what is the value of this coefficient fairly objectively indicates the quality of the device? The impatient can immediately find an attempt at an answer to this question at the end. For the rest, let's continue. This coefficient, which is also called the coefficient of total harmonic distortion, is the percentage ratio of the effective amplitude of the harmonic components at the output of the device (amplifier, tape recorder, etc.) to the effective amplitude of the fundamental frequency signal when a sinusoidal signal of this frequency is applied to the input of the device. Thus, it allows one to quantify the nonlinearity of the transfer characteristic, which manifests itself in the appearance in the output signal of spectral components (harmonics) that are absent in the input signal. In other words, there is a qualitative change in the spectrum of the musical signal. In addition to the objective harmonic distortions present in the audible sound signal, there is the problem of distortions that are absent in real sound, but are felt due to subjective harmonics that occur in the cochlea at high sound pressure values. The human hearing aid is a non-linear system. The non-linearity of hearing is manifested in the fact that when a sinusoidal sound with a frequency f is exposed to the eardrum, harmonics of this sound with frequencies 2f, 3f, etc. are generated in the hearing aid. Since these harmonics do not exist in the primary affecting tone, they are called subjective harmonics. Naturally, this further complicates the idea of the maximum permissible level of harmonics in the audio path. With an increase in the intensity of the primary tone, the magnitude of subjective harmonics increases sharply and may even exceed the intensity of the fundamental tone. This circumstance gives grounds for the assumption that sounds with a frequency of less than 100 Hz are felt not by themselves, but because of the subjective harmonics they create, falling into the frequency range above 100 Hz, i.e. due to non-linear hearing. The physical causes of the resulting hardware distortions in different devices are of a different nature, and the contribution of each to the overall distortion of the entire path is not the same. Distortions of modern CD-players have very low values and are almost imperceptible against the background of distortions of other blocks. For acoustic systems, the most significant are low-frequency distortions caused by the bass head, and the standard specifies requirements only for the second and third harmonics in the frequency range up to 250 Hz. And for a very good sounding speaker system, they can be within 1% or even a little more. In analog tape recorders, the main problem associated with the physical basis of recording on magnetic tape is the third harmonic, the values \u3b\u333bof which are usually given in the instructions for information. But the maximum value at which, for example, noise level measurements are always made is XNUMX% for a frequency of XNUMX Hz. The distortions of the electronic part of tape recorders are much lower. Both in the case of acoustics and for analog tape recorders, due to the fact that distortions are mainly low-frequency, their subjective visibility drops significantly due to the masking effect (which consists in the fact that the higher frequency is better heard from two simultaneously sounding signals). So the main source of distortion in your path will be the power amplifier, in which, in turn, the main one is the non-linearity of the transfer characteristics of active elements: transistors and vacuum tubes, and in transformer amplifiers, the non-linear distortion of the transformer is also added, associated with the non-linearity of the magnetization curve. Obviously, on the one hand, the distortion depends on the shape of the nonlinearity of the transfer characteristic, but also on the nature of the input signal. For example, the transfer response of an amplifier with soft clipping at large amplitudes will not cause any distortion for sinusoidal signals below the clipping level, and as the signal increases above this level, distortions appear and will increase. This nature of the limitation is mainly inherent in tube amplifiers, which to some extent may serve as one of the reasons for the preference of such amplifiers by listeners. And this feature was used by NAD in a series of their sensational “soft-limiting” amplifiers produced since the early 80s: the ability to turn on the mode with imitation of tube clipping created a large army of fans of NAD transistor amplifiers. In contrast, the center-cut (notch) characteristic of an amplifier, which is common with transistor models, will distort musical and small sine wave signals, and will decrease as the signal level increases. Thus, the distortion depends not only on the shape of the transfer characteristic, but also on the statistical distribution of the input signal levels, which for musical programs is close to the noise signal. Therefore, in addition to measuring SOI using a sinusoidal signal, it is possible to measure the nonlinear distortions of amplifying devices using the sum of three sinusoidal or noise signals, which, in the light of the foregoing, give a more objective picture of distortion. Unfortunately, the latter have not received international recognition and wide distribution. The so-called "transistor paradox" convincingly demonstrates the insufficiently developed technique for measuring SOI. Indeed, how to explain that, according to the results of numerous subjective examinations, tube amplifiers with SOI, hundreds and even thousands of times larger than those of transistor ones, receive a clear preference? An analysis of the spectral composition of the distortions of tube and transistor amplifiers shows their significant difference: in tube amplifiers, the main contribution to distortion is made by low-order harmonics, and their intensity proportionally decreases with increasing harmonic number, in a transistor the spectrum is much wider, and the intensity of the components does not lend itself to any regularity. Obviously, taking into account the masking effect, the influence of low-order distortions on the subjective perception of the harmonic components is weakened, and thus the role of higher harmonics is emphasized. Thus, for a more correct assessment of distortions, it would be necessary to introduce weight coefficients in the summation of harmonics when determining the effective amplitude of distortions, and the influence of higher harmonics should increase. However, there are no generally accepted methods for such measurements. For a typical form of non-linearity of the "step" type, the level of perceptibility of distortion to the ear for a sinusoidal signal is 0,1%, and for musical signals 1%. THD is measured over a frequency range of 40 Hz to 16 kHz and over a range of levels from nominal output level down to minus 23 dB. The THD of modern amplifiers is usually in the range from 0,001 to 296. For Hi-Fi class amplifiers, international standards (IEC 581-6 and others) set a distortion standard of 0,7%. To check the visibility of distortions in your home system, you can use special recordings with an introduced, strictly established level of distortion. For example, the test CD "MY DISC" (Sheffild Lab) has a dozen tracks of separate sine and musical recordings with distortion levels of 0,03%, 0,1%, and so on with gradually increasing distortion up to 10%. I am sure that the results of listening to such recordings will be amazing for many. Author: Alexey Grudinin
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