Measurement of non-linear distortions on a noise signal. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In the article, the author draws the attention of readers to one practically unused method for measuring the nonlinearity of amplifiers. The results of objective measurements of UMZCH non-linear distortions using this method surprisingly coincide with the results of their subjective assessments during expert listening.

Known methods for measuring nonlinear distortions in sound transmission paths are very diverse [1, 2]. The method of harmonics is widely used as the simplest in experiments and convenient for calculations. Other methods are less common: difference tone, modulated tone, mutual modulation (intermodulation). Transient intermodulation distortion is also measured.

These methods have their own areas of application. In addition, each of them uses special signals that provide the highest efficiency in detecting distortion products. However, this is precisely the reason for their low information content with respect to the integral assessment of distortions introduced into the audio path and significantly affecting the subjective (expert) assessment of the quality of transmission of real audio signals.

The visibility of nonlinear distortions of a real signal is related to how often, if we consider the process in time, or with what probability, if a statistical measure is applied to it, its instantaneous values ​​fall into the region of significant nonlinearity of the sound transmission path. Many, probably, had to observe how, with a decrease in the signal level in an overloaded channel, the hoarseness of the sound disappears. It is the smaller, the less often the signal spikes fall into the overload area.

A typical characteristic of the signal transmission function s in the sound transmission path is shown in fig. 1a. Here: sin, sout - input and output signals normalized by power; W(s) - probability density of instantaneous signal values ​​sin. Section A corresponds to a relatively small non-linearity, and sections B - large. For convenience of analysis, Fig. 1b shows graphs of the probability density distribution W(s) of instantaneous values ​​of two signals of the same power: white (Gaussian) noise (curve 2) and harmonic noise (curve 1). As follows from Fig. 1a, all values ​​of the input signal, limited by the function W(s) for a sinusoid, fall in the section of the transmission characteristic with less nonlinearity, while for the noise signal 16% of the time its values ​​are in the sections of the transmission characteristic with a large nonlinearity. It is clear that the noise signal is subject to much greater distortion than the sinusoidal one.

In [3], the results of studies of the probability density of instantaneous values ​​of signals of natural sounds (speech and music) are presented. In terms of their distribution of levels, they turned out to be much closer to a noise signal than to a harmonic one. Therefore, estimation of non-linear distortions based on the methods listed above gives misconceptions about the actual non-linear distortions of real signals.

Less known measurement methods using noise signals are much more informative [1, 2, 4–9].

One of the methods [4] is used in cinematography and television to measure the nonlinear distortions of a photographic soundtrack [5]. The block diagram of measurement and spectral diagrams for this method are shown in Fig.2.

The measuring signal is generated by the white noise generator of the GBSH, limited with the help of a PF bandpass filter with a frequency band of 3...12 kHz, which is fed to the input of the OI measurement object. The products of non-linear distortions of the PNI (intermodulation) of the noise signal are measured with a voltmeter V after the low-pass filter with weighting in the frequency band of 30 Hz ... 1,2 kHz. The numerical indicator of non-linearity is the ratio, expressed in decibels, of the rms voltage of the distortion products (UС) to the voltage of the reference signal (UВ) generated by the generator built into the device with a frequency of 1 kHz:

KISH \u20d 1 lg (UС / UВ). (one)

The described measurement method is implemented in the 7E-67 device and is successfully used in film studios. On television, a similar device is the INIF meter.

Distortion measurements are also carried out by the harmonic method using a measuring signal in the form of a one-third octave noise band [5-9]. The block diagram and spectral diagrams are shown in fig. 3.

From the pink noise generator generated by the FFT band-pass filter unit for studying the object of measurements of the RI, bands are alternately selected, and the level drop of 3 dB per octave with increasing frequency provides a constant power of the measuring signal in any one-third octave band. Of the voltage distortion products of the signal U1, only its harmonics U2, U3 located in one-third octave bands with average frequencies nf1, where n = 2, 3...,f1 is the average frequency of the measuring signal band, are taken into account. The measurements are carried out by an AC spectrum analyzer connected to the output of the measurement object. The numerical indicator of the harmonic coefficient of the noise signal is determined by the formula:

It should be taken into account that the reliability of measurements with this method depends significantly on the limitation of the bandwidth of the measurement object.

There are other, more sophisticated measurement methods using noise signals. The wide use of such signals in measurements in audio equipment, in the author's opinion, is hindered by a number of factors: the scarcity and high cost of equipment for analyzing random signals, the need to revise standards (for example, output power in amplifiers), and the inertia of thinking of many engineers who are accustomed to sinusoidal signals.

For a practical assessment of the effectiveness of the use of noise signals, the author carried out comparative measurements of nonlinear distortions in several UMZCH according to the standard method (harmonics method) and on a noise signal using a 7E-67 device at the same amplifier overload values. For testing, various UMZCHs were selected in terms of circuitry and element base, designed to sound large rooms (power 100 W or more, all models had overload indicators). In addition, subjective assessments of the quality (SQA) of sound reproduction were carried out on a ten-point scale.

The test results of the nonlinearity of the amplifiers are given in the table. Power amplifiers 1 - 4 - transistor with different feedback depth (A), amplifier 5 - tube. The table shows the values ​​of the coefficient of harmonics of the KG at a frequency of 1 kHz and the coefficient of noise intermodulation according to the device 7E-67.

The high level of distortion in transistor amplifiers with deep overall feedback when measuring nonlinearity with a noise signal is due to the fact that the measuring signal in the form of noise has a high crest factor and contains a fairly wide spectrum of frequencies, creating an even wider range of distortion products, and a significant difference in relation to the KG /KISH for all amplifiers - an increase in intermodulation distortion during short-term overload. It follows from the table that UMZCHs with a greater depth of SOS also have a greater ratio of CG / KIS, receiving, accordingly, low SOC scores.

As a result of the tests, the following conclusions can be drawn:

1. The control of non-linear distortions on a noise signal has a much greater information content, allows you to get closer to the subjective assessment of the quality of sound reproduction.

2. When designing all parts of the sound transmission path, one should strive not only to reduce the harmonic coefficient, but also the noise intermodulation coefficient.

The described method was originally proposed for measuring the nonlinearity of the photographic soundtrack of films (when controlling the quality of the technological process of their replication), therefore, in relation to measurements in high-quality sound transmission paths, including loudspeakers, it is advisable to correct the measurement signal bandwidth.

Professional UMZCH noise intermodulation measurements differ in this case in that this equipment is often used at maximum power, allowing short-term overload. Compared to tube amplifiers, in transistor amplifiers, the maximum current limitation is often more pronounced during overload, which corresponds to a sharp increase in non-linear distortion. In UMZCH used in a home environment, the signal limiting mode is practically not achieved with a correctly selected power, so it is advisable to consider the option of using a technique with limiting the noise signal level. In this case, the difference between amplifiers with different element base is likely to decrease significantly. In addition, it should be borne in mind that there are a number of critical parameters - the frequency band, phase and transient characteristics, the level of intrinsic noise ...

Literature

1. Rakovsky VV Measurements in film sound recording equipment. - M.: Art, 1962, p. 336 - 353.
2. Ishutkin Yu. M., Rakovsky VV Measurements in film sound recording and reproduction equipment. - M.: Art, 1985, p.
3. Shitov AV, Belkin BG Statistical characteristics of signals representing natural sounds and their application in the study of electroacoustic systems. - Proceedings of NIKFI, vol. 56, 1976
4. Rakovsky VV A method for measuring non-linear distortions in a photographic transverse soundtrack. Auth. certificate No. 136573 (1960) - BI, 1961, No. 5.
5. RTM 19-17-72. Movies 35 and 16 mm. Technological regulations for the compensation method of recording negatives, photographic processing, printing positives and quality control of photographic phonograms. - M.: NIKFI, 1972.
6. Penkov G. Verhu is measured for non-linear curvature with a random stationary signal. Measured on a non-linear curvature with a narrow tape from normal noise. - News at NIIKRA, vol. 6. - Sofia, 1966.
7. Zhuravlev VM A method for measuring non-linear distortions using noise bands. Cand. diss. LIKI, 1967.
8. Belkin BG, Bork AA Correlation between coefficients of non-linear distortions measured on noise and sinusoidal signals. - Technique of cinema and television, 1968, No. 7.
9. GOST 16122-78. Loudspeakers. Methods of electroacoustic tests.

Author: A.Syritso, Moscow

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