Audio signal and audio analysis Audio is an important medium in multimedia. The frequency range of audio signals that we can hear is about 20Hz-2OkHz, where speech is distributed within 300Hz-4kHz, while music and other natural sounds are distributed in a full range. Sound is recorded or reproduced by analog equipment to become analog audio, and then digitized to digital audio. The audio analysis here refers to the process of taking digital audio signals as the analysis object and digital signal processing as the analysis means to extract a series of characteristics of the signal in the time domain and frequency domain. The principle of audio analysis The principle of audio analysis mainly involves the basic theory of digital signal processing, the basic method of audio analysis, and the measurement and analysis of audio parameters. Digital signal processing is the theoretical basis of audio analysis. Audio parameter measurement and analysis Audio measurement generally includes basic parameters such as signal voltage, frequency, signal-to-noise ratio, and harmonic distortion. Most audio parameters can be composed of these basic parameters. Audio analysis can be divided into time domain analysis, frequency domain analysis, time frequency analysis and other categories. Because the harmonic distortion of the signal is more important for audio measurement, it is separately classified as distortion analysis. The following introduces various audio parameter measurement and audio analysis. In the actual measurement, for convenience, the total signal voltage with noise is usually used instead of the signal voltage to calculate the signal-to-noise ratio. Step signal analysis is relatively simple and is mainly used to detect the response sensitivity of audio equipment to sudden signal changes. There are usually two parameters for step signal analysis, namely the rise time and pulse width of the step response signal. The smaller the rise time, the more sensitive the device's response to sudden signal changes and the better the transient characteristics; the smaller the pulse width, the better the damping characteristics of the device and the more stable the system. Audio analysis instrument The audio analysis instrument mentioned here means that it can not only measure various electroacoustic parameters of various single audio equipment such as microphones, audio amplifiers, speakers and so on, but also test the overall performance of combined audio equipment such as combined audio and mixer Analytical instruments. Various analytical instruments that can be used to measure audio equipment have appeared on the market, such as distortion analyzers, spectrum analyzers, frequency counters, AC voltmeters, DC voltmeters, and audio oscilloscopes. These rack-based hardware instruments based on various functional circuits are easy to use and have high measurement accuracy, and have been widely used at present. Audio equipment manufacturers can use audio analysis equipment to check the performance of the equipment and find defects, so as to improve the design and manufacture of equipment. Consumers can also use audio analysis equipment to evaluate the equipment and choose the appropriate products. Pm Stepper Motor,Square Flange Stepper Motor,High Precision Stepper Motor,Permanent Magnet Stepper Motor Changzhou Sherry International Trading Co., Ltd. , https://www.sherry-motor.com
Audio analysis of various specific frequency ranges has different application fields. For example, the analysis of speech signals between 300-4kHz is mainly used in speech recognition, its purpose is to determine the content of speech or to determine the identity of the speaker; and the analysis of the full range of speech signals between 20-20kHz can be used to Measure the performance of various audio equipment. The so-called audio equipment refers to the various types of electronic equipment used in the entire process of picking up the actual sound to play the sound, such as microphones, power amplifiers, speakers, etc. The main technical indicators of measuring audio equipment are frequency response characteristics, harmonics Distortion, signal-to-noise ratio, dynamic range, etc.
1. Basics of audio analysis technology Fourier transform and signal sampling are the most basic techniques used in audio analysis. The Fourier transform is the basis of spectrum analysis. The spectrum analysis of a signal refers to the frequency structure of the signal, the amplitude and phase of its components, and the frequency distribution law to establish various "spectrums" with the frequency as the horizontal axis, such as Amplitude spectrum, phase spectrum. In the signal, the periodic signal undergoes the Fourier series transform to correspond to the discrete spectrum, and for the non-periodic signal, it can be regarded as the periodic signal with the period T as infinity. = 2Ï€ / T) approaches infinity, so that the discrete spectrum becomes a continuous spectrum. Therefore, the spectrum of aperiodic signals is continuous.
In a computer-centric test system, an analog signal passes through an A / D converter before it enters a digital computer, changing a continuous-time signal into a discrete-time signal, called signal sampling. Then the amplitude is quantized to become a discrete digital signal. In this way, a series of new problems will appear in the frequency domain, and the spectrum will change. After changing from analog signal to digital signal, its Fourier transform also becomes discrete Fourier transform, which involves a series of problems such as sampling theorem, frequency aliasing, truncation and leakage, windowing and window function.
2. Audio analysis methods are usually regarded as a black box system with input and output ports when measuring and analyzing an audio device. Input a certain known signal into the system, and then obtain the output signal from the output terminal for analysis to understand some characteristics of the system. This is the general method of audio analysis. The signal input to the audio device is called the excitation signal. The excitation signal may be a periodic signal such as a sine wave or a square wave, or a random signal such as white noise or pink noise, or a signal such as a double tone, multi-tone, or sine burst. The most commonly used detection and analysis methods include sinusoidal signal detection, pulse signal detection, and maximum length sequence signal detection.
1. Basic parameter measurement The basic parameters to be measured in audio measurement mainly include voltage, frequency and signal-to-noise ratio. Voltage testing can be divided into root mean square voltage (RMS), average voltage and peak voltage.
Frequency is one of the most basic parameters in audio measurement. Usually use high-frequency precision clock as a reference to measure the frequency of the signal. When measuring the frequency, the input signal and the reference clock are counted at the same time within a limited time, and then the count value of the two is compared and multiplied by the frequency of the reference clock to obtain the signal frequency. As the computing speed of the microprocessing chip increases, the frequency of the signal can also be calculated by software using fast Fourier transform.
The signal-to-noise ratio is the basic performance index of audio equipment and the ratio of the effective voltage of the signal to the noise voltage. The calculation formula of the signal-to-noise ratio is: 
2. Time domain analysis Time domain analysis is usually to input a certain test signal into the audio device to be tested, and observe the time domain waveform of the output signal of the device to assess the relevant performance of the device. The most commonly used time-domain analysis test signals are sinusoidal signals, square wave signals, step signals, and single-tone mutation signals. For example, inputting a sinusoidal signal into the device and observing the time domain waveform distortion of the output signal is a time domain analysis method.
Square wave analysis has good abruptness and periodicity. By observing the output signal waveform of the square wave signal of the device, the performance of the device can be well detected. Therefore, the square wave signal has become the most commonly used time domain analysis signal. FIG. 1 is a detailed description of the response signal of the square wave of the audio device within half a period (rising edge). There are several main parameters to describe the square wave response, such as rise time, peak oscillation, overshoot and inclination.
The sinusoidal signal suddenly rises in peak value at a certain moment, forming a sudden change, which is a single-tone sudden change signal. Because the energy of the single-tone mutation signal is concentrated in a narrow frequency range, the single-tone mutation signal is commonly used to detect the response of the audio equipment at a specific frequency. The main purpose of the single-tone mutation signal is to quickly determine the damping characteristics of certain audio equipment, such as speakers. 
3. Frequency domain analysis Frequency domain analysis is an important content of audio analysis. The main basis of frequency domain analysis is the frequency response characteristic curve. The aforementioned sinusoidal detection, pulse detection and maximum length sequence signal detection can all obtain the frequency response of the device. The frequency response curve reflects the distribution of the frequency response of the audio device in the entire audio range. Generally speaking, the frequency component at the peak of the curve has large sound pressure and strong sound pressure during playback; the frequency component at the bottom of the curve has low sound pressure and weak sound. If the peaks and valleys fluctuate too much, it will cause more serious frequency distortion.
4. Time-frequency analysis Time-frequency characteristics describe the changes in the frequency domain characteristics of audio equipment over time on the time axis. The time-frequency characteristic not only describes the response state of the audio device during the frequency change, but also describes the response state of the audio device during the time change, that is, comprehensively describes the response characteristics of the audio device from a three-dimensional perspective. For playback equipment, subjective listening comments, such as whether the bass is clean, the background is clear, the level is clear, the depth of the sound field, etc. are closely related to the time-frequency characteristics of the audio equipment. The time-frequency characteristics of audio equipment is an important aspect of objectively evaluating the performance of audio equipment.
5. Distortion analysis The distortion of audio equipment includes harmonic distortion, intermodulation distortion, phase distortion and transient distortion. The most important thing in audio measurement is harmonic distortion. Harmonic distortion is simply the extra harmonic components that are generated after the sound signal is replayed by the audio equipment. From the perspective of the listener, the sounds emitted by different sounding objects are composed of different fundamental waves and harmonics, and the listener can distinguish the sounding objects according to the characteristics of the sound. If the power amplifier amplifies the tones (the tones are composed of fundamental waves and harmonics) emitted by a certain musical instrument, and after playing through the speaker, the waveform shape, amplitude and phase of the fundamental wave and each harmonic can be distortion-free If it is reproduced, it can be regarded as high-quality playback; otherwise, the sound emitted by the speaker sounds irritable and awkward, and the harmonic distortion has become unbearable, even making it impossible to distinguish the type of sounding instrument. Therefore, harmonic distortion is an important performance index of audio equipment.
There are two methods for measuring harmonic distortion. One is to input the device under test with a sinusoidal signal, and then analyze the frequency components of the response signal of the device to obtain harmonic distortion. Another simpler measurement method is to first use a band stop filter to filter out the fundamental frequency component of the response signal, and then directly measure the voltage of the remaining signal, and compare it with the original response signal to obtain harmonic distortion. Obviously, the harmonic distortion obtained by the second method is THD + N. Since the total voltage value of the signal is used instead of the voltage value of the fundamental frequency component, the resulting harmonic distortion is smaller than the actual value, and the actual harmonic distortion is more The larger, the greater the error.
In actual audio measurement, usually select several frequency points within a certain frequency range, measure the harmonic distortion of each point separately, and then connect each harmonic distortion value to the horizontal axis to form a curve, called harmonic Wave distortion curve. Figure 2 is a graph of the total harmonic distortion and 2nd, 3rd and 4th order harmonic distortion of a power amplifier in the range of 100-1OKHz. 
Take the combination of sound as an example, the term "timbre" is often used when evaluating its performance. The so-called timbre refers to the difference in sound caused by different harmonics. The so-called "balance" of the sound refers to the degree to which the sound of the sound reproduced in the entire frequency band sounds natural. The role of audio analysis instruments is to express various industry terms of evaluation equipment in the form of various quantitative characteristic parameters. The characteristic parameters corresponding to "tone" are the measurement of harmonic distortion, and "balance" refers to the entire equipment. The distribution of frequency response in the audio range.
Generally speaking, a full-featured audio analysis instrument should be able to measure parameters such as signal AC / DC voltage, signal frequency, harmonic distortion, and signal-to-noise ratio. The powerful audio analysis instrument provides spectrum analysis, 1/3 octave analysis, octave analysis, sound pressure level measurement and other functions. If you want to build an audio analysis system, you also need a standard audio signal generator as the excitation signal source.
Audio analysis uses time domain analysis, frequency domain analysis, distortion analysis and other methods as means to evaluate the performance of audio systems by measuring various audio parameters. Audio analysis is a comprehensive analysis involving many test instruments. For ordinary users, it is necessary to select test instruments reasonably according to the parameters they are interested in. It is difficult to establish a complete audio test analysis system.