Sound quality standard
The so-called sound quality refers to the fidelity of the audio signal after transmission and processing. At present, the industry recognized sound quality standards are divided into 4 levels, namely the CD-DA quality of digital compact disc, its signal bandwidth is 10Hz~20kHz; FM broadcast FM quality, its signal bandwidth is 20Hz~15kHz; AM broadcast AM quality, its signal The bandwidth is 50Hz~7kHz; the voice quality of the phone has a signal bandwidth of 200Hz~3400Hz. It can be seen that the digital compact disc has the highest sound quality and the telephone has the lowest voice quality. In addition to the frequency range, other methods and indicators are often used to further describe the sound quality standards for different uses.
For analog audio, the more frequency components of the reproduced sound, the smaller the distortion and interference, the higher the sound fidelity, and the better the sound quality. For example, in communication science, the level of sound quality is measured by indicators such as distortion and signal-to-noise ratio in addition to the frequency range of the audio signal. For digital audio, the more components that reproduce the sound frequency, the smaller the bit error rate and the better the sound quality. Usually measured by the digital rate (or storage capacity), the higher the sampling frequency, the larger the number of quantization bits, the more the number of channels, the larger the storage capacity, of course, the fidelity is high and the sound quality is good.
The sound categories are different and the sound quality requirements are different. For example, the sound quality fidelity is mainly reflected in clear, undistorted, and reproduced flat sound images; the fidelity of the music is required to be high, and the spatial sound image is mainly embodied in multi-channel analog stereo surround sound, or virtual two-channel. 3D surround sound and other methods to reproduce all the sound images of the original sound source.
The use of audio signals is different, and the quality standards for compression are different. For example, the telephone quality audio signal adopts the ITU-TG?711 standard, 8 kHz sampling, 8 bit quantization, and a code rate of 64 Kbps. The AM broadcast uses the ITU-TG® 722 standard, 16 kHz sampling, 14-bit quantization, and a code rate of 224 Kbps. The high-fidelity stereo audio compression standard is jointly developed by ISO and ITU-T. The CD11172-3MPEG audio standard is 48kHz, 44.1kHz, 32kHz sampling, and the digital rate of each channel is 32Kbps~448Kbps, which is suitable for CD-DA discs.
If the sound quality is too high, the equipment is complicated; otherwise, it cannot satisfy the application. Generally speaking, the principle is "enough, not wasteful".
Sound quality evaluation method
There are two methods for evaluating the quality of reproduced sounds, subjective evaluation and objective evaluation. E.g:
Voice quality
Methods for assessing the quality of speech coding are subjective and objective. At present, the subjective assessment is the subjective assessment, which is measured by subjective scoring (MOS). It is divided into the following five levels: 5 (excellent), no perceptual distortion; 4 (good), just perceived distortion, but not annoying; 3 (middle) , perceive distortion, slightly annoying; 2 (poor), hateful, but not offensive; 1 (bad), extremely annoying, offensive. Generally, if the reproduced speech frequency is above 7 kHz, MOS can evaluate 5 points. This evaluation standard is widely used in multimedia technology and communication, such as video telephony, video conferencing, voice e-mail, voice mail, and so on.
2. Music quality
The quality of music quality depends on many factors, such as sound source characteristics (sound pressure, frequency, spectrum, etc.), and signal characteristics of audio equipment (such as distortion, frequency response, dynamic range, signal-to-noise ratio, transient characteristics, stereo Separation degree, etc., sound field characteristics (such as direct sound, pre-reflected sound, reverberant sound, cross-correlation between the ears, reference vibration, sound absorption rate, etc.), auditory characteristics (such as loudness curve, audible range, various senses of hearing) )Wait. Therefore, it is difficult to evaluate the sound quality of the audio equipment.
The following two methods are usually used: one is to use the instrument to test technical indicators; the other is to listen to various sound effects subjectively. Due to the complex nature of the sound quality of the music, the personal color of the subjective evaluation is relatively strong, and the existing sound testing technology can only reflect its fidelity from some aspects. Therefore, to date, there is no internationally recognized evaluation standard that can truly quantify the fidelity of music. However, it has also been reported that the International Telecommunication Union (ITU-T) has recently approved a new measurement method called electronic ear that objectively evaluates the sound quality, which can be used for objective listening evaluation of the sound quality of any audio equipment, and can also be used for detecting telephones. Defects in communication speech coding systems.
The music sound quality evaluation methods are summarized as follows:
Subjective listening sound
Generally, according to the three elements of musical sound quality, that is, the change and combination of loudness, pitch and pleasantness, subjectively evaluate various properties of sound quality, such as low frequency loudness for fullness of sound, high frequency loudness for bright sound, and low frequency for weak sound. The high frequency is weak and the sound is clear. In the following, several typical senses of hearing are introduced in combination with sound source, sound field and signal characteristics.
1 three-dimensional
The sound mainly consists of a sense of space (surrounding) of sound, a sense of orientation (direction sense), and a sense of layering (thickness), and a sound having these senses is called stereo. The various sound fields in nature are themselves three-dimensional, and it is the most important feature of the sound source. The German-Bol effect proves that the physiological characteristics of the human ear are: the human ear is on the symmetry axis of the two sound sources. When the sound pressure difference Δp=0dB and the time difference Δt=0ms, the two sound sources are the same. There are two sound sources; when △p>15dB or Δt>3ms, the human ear feels that there are two sound sources, and the sound image moves to the sound pressure or the sound source before the lead, every 5dB sound The pressure difference is equivalent to the time difference of lms. The Haas effect further proves that when △t=5ms~35ms, the human ear feels that there are two sound sources; and when the near reflection sound, the lag direct sound or the time difference of two sound sources △t>50ms, even once The loudness of the reflected sound (also known as near or pre-reflected sound) or the lag sound is many times greater than the loudness of the direct sound or the leading sound, and the sound source orientation is still determined by the direct sound or the leading sound.
According to the physiological characteristics of the human ear, as long as the sound intensity, delay, reverberation, spatial effect, etc. are properly controlled and processed, the man-made manufacturing has a certain time difference Δt, phase difference Δθ, sound pressure. The sound state of the difference ΔP, and the state of the sound wave generated by the original sound source at both ears is exactly the same, and the human can truly and completely feel the three-dimensional sense of the reproduced sound. Compared with monophonic sound, stereo usually has the characteristics of sound image dispersion, proper volume distribution of each part, high definition, and low background noise.
2 sense of positioning
If the sound source is recorded after recording in left, right, up and down, front and back, the sound received and reproduced should be able to reproduce the orientation of the sound source in the original sound field. This is the sense of positioning. According to the physiological characteristics of the human ear, the maximum time difference between the first sound source and the direct sound of the two ears is 0.44ms~0.5ms, and there is also a certain sound pressure difference and phase difference. Physiological psychology proves: 20Hz~200Hz bass mainly *People's two-ear phase difference positioning, 300Hz~4kHz midrange main *sound pressure difference positioning, higher treble main *time difference positioning. It can be seen that the sense of positioning is mainly determined by the direct sound that first reaches both ears, and the one-time reflection sound that arrives at both ears and the reverberation sound that is reflected multiple times in all directions mainly simulate the spatial surround feeling of the sound image.
3 sense of space
The reverberation sound of one reflection and multiple reflections, although lagging directly to the sound, has little effect on the sense of direction of the sound, but the reflected sound always reaches the ears from all directions, which has an important influence on the size of the surrounding space of the auditory sense, so that the human ear is surrounded by the surround. The feeling is that this is the sense of space. The sense of space is more important than the sense of positioning.
4 layers
The sound is high, medium and low frequency, the frequency is balanced, the high-pitched harmonics are rich, clear and slender and not harsh, the midrange is bright and prominent, full and full without hard, the bass is thick and there is no nasal sound.
5 thickness sense
The bass is steady and powerful, heavy and not turbid, the high sound is not lacking, the volume is moderate, there is a certain brightness, the reverberation is suitable, and the distortion is small.
In addition, there are many listening sensations that evaluate sound quality, such as the sense of strength, brightness, presence, softness, tightness, and sense of width.
2. Objective test technical indicators
1 distortion
Harmonic distortion mainly causes the sound to be hard and fragile; while the steady-state or transient intermodulation distortion mainly causes the sound to be rough, sharp and turbid. Both of them deteriorate the sound quality, and if the distortion exceeds 3%, the sound quality deteriorates remarkably. The sound system has the largest distortion of the speaker, and the minimum distortion is generally more than 1%.
Phase distortion mainly causes low-frequency sound blur below 1 kHz, and affects the IF sound level and sound image localization.
The jitter is mainly caused by the unstable motor speed, the unstable pressure of the capstan-pressure pulley, the tape shock and the tape volume caused by the head, and the signal frequency is modulated, and the sound tone is turbid and trembling. Dithering is usually expressed by the rms value of the pitch change. Typically, the recorder's jitter rate is <0.1%, Hi-Fi recorder <0.005%, normal video recorder <0.3%, and video player <0.001%.
2 frequency response and transient response
Frequency response refers to the variation of the gain or sensitivity of the audio equipment with the signal frequency, expressed by the width of the passband and the in-band unevenness (such as the frequency response of the high-quality power amplifier 1Hz~200kHz±ldB). The wider the bandwidth, the better the high and low frequency response: the smaller the unevenness, the better the frequency equalization performance. Generally, the low frequency of 30Hz~150Hz makes the sound have a certain thickness. The low frequency of 150Hz~500Hz makes the sound have a certain intensity. When the low frequency sound pressure is too strong in 300Hz~500Hz, the sound is turbid, the sound is weak when excessive attenuation; the medium frequency is 500Hz~5kHz. Make the sound have a certain brightness, when the sound is excessively strengthened, the sound is hard; when the sound is excessively attenuated, the sound is scattered and floated; the high frequency band of 5 kHz to 10 kHz makes the sound have a certain level and color; when it is excessively strengthened, the sound is sharp; when it is excessively attenuated, the sound is dim. Bored. According to this rule, the frequency response of the sound system can be quantitatively adjusted according to various senses of hearing.
Transient response refers to the ability of the sound system to follow a sudden signal. In essence, it reflects the high-order harmonic distortion of the pulse signal, which seriously affects the transparency and layering of the sound quality. The transient response rate V/μs is usually used. The higher the index, the smaller the harmonic distortion. For example, the conversion rate of a general amplifier is >10V/μs.
3 signal to noise ratio
Signal-to-noise ratio, which is the decibel difference between the signal and the noise level, expressed in S/N or SNR (dB). The level of the noise frequency, the strength of the signal has a different impact on the human ear. Generally, the human ear is most sensitive to noise of 4 to 8 kHz, and the weak signal is more prominent than the strong signal by noise. Different audio equipment, signal-to-noise ratio requirements are not the same, such as Hi-Fi audio requires SNR> 70dB, CD machine requires SNR> 90dB.
4-channel resolution and balance
The channel separation degree refers to the degree of isolation of stereo between different channels, and is represented by the difference between the signal level of one channel and the signal level of the other channel. The bigger the difference, the better. Hi-Fi acoustic separation is generally required to be >50 dB. Channel balance refers to the consistency of the characteristics of the gain and frequency response of the two channels. Otherwise, it will cause a shift in the channel image.
A tablet computer is an electronic device that integrates mobile commerce, mobile communication and mobile entertainment, with a touch recognition LCD screen, handwriting recognition and wireless network communication functions. At present, the tablet computer has become the most popular electronic product.
1.In appearance, the tablet computer looks like a large-screen mobile phone, or more like a separate LCD screen.
2.In terms of hardware configuration, a tablet computer has all the hardware devices of a traditional computer, and has its own unique operating system, compatible with a variety of applications, and has a complete set of computer functions.
3.Tablet PC is a kind of miniaturized computer. Compared with traditional desktop computers, tablet computers are mobile and flexible. Compared with notebook computers, tablet computers are more compact and more portable.
4.Tablet PC is a digital notebook with digital ink function. In daily use, you can use the tablet computer like an ordinary notebook, take notes anytime and anywhere, and leave your own notes in electronic texts and documents.
Tablet Pc Pad,Mobile Tablet,Scratch Pad Tablet Pc,Tablet Pc,Tablets & Presentation Equipment,Educational Tablet
Jingjiang Gisen Technology Co.,Ltd , https://www.gisentech.com