The concept of stereo and mono and binaural

                                The concept of stereo and mono and binaural

1. Definition of stereo sound

In fact, the sounds of nature we hear in daily life are stereo sounds. However, the stereo sound mentioned in audio technology is not natural sound, but sound obtained through recording, transmission and playback systems. In order for the listener to obtain the impression of the spatial distribution of sound and create a sense of presence and three-dimensionality, some special processing is required to produce stereo sound.

A stereo system is a system consisting of two or more microphones, transmission channels and speakers (or headphones). With appropriate arrangements, the listener can have a sense of the spatial distribution of the sound source. What is generally called stereo nowadays is actually the abbreviation for stereo broadcast, stereo recording and stereo playback.

In a concert hall, the stereo sound heard by the listener is composed of three parts of sound: direct sound, reflected sound and reverberated sound. Direct sound can help people determine the direction of the sound source; reflected sound gives people a sense of space, and they can feel the size of the concert hall; reverberation gives people a sense of envelopment, and they can feel the sound surrounding it in a three-dimensional space. Reflected sound and reverberation sound work together to comprehensively form the on-site environmental sound atmosphere, which produces the so-called sense of presence. Good stereo sound should be able to reproduce these elements.

2. Mono and two-channel stereo systems

Humans have two ears, so people can judge the location and spatial distribution of sound sources. In other words, the human ears have the ability to perceive the stereo sound field. This is commonly known as the binaural effect.

When we listen to the broadcast of a large orchestra performance, if the sound relay system is only picked up by one microphone (or picked up by several microphones and then mixed together), after passing through an amplification channel, it will be picked up by a loudspeaker or by a When played back by a group of speakers, it is a so-called mono system, as shown in the figure below. Since the sound source reproduced at this time is similar to a point sound source, it cannot reflect the orientation and mouth spatial distribution of various instruments in the orchestra in the actual sound field. It is very different from the effect that people listen to at the performance site, that is, it lacks Three-dimensional feeling. This is the biggest disadvantage of mono playback systems.

In order to obtain a three-dimensional listening effect, people initially tried arranging many microphones in a plane vertically in front of the performance stage, connecting each microphone to its own amplifier, and then connecting the output of each amplifier to another listening room. The same number of speakers arranged in a plane in the sound room are connected one by one. In this way, when you listen to the sound in the listening room, you can get a very similar effect to that of listening to the sound at the performance site, and you can distinguish the orientation and spatial distribution of various instruments, that is, you have a three-dimensional sense. But then it was discovered that the microphones placed above and below the performance site were of little practical use. As long as a row of microphones at the height of the instrument and a row of speakers at the height of the human ear were retained, the effect would be very good. Of course, the greater the number of microphones and corresponding speakers that make up a row, that is, the greater the number of channels, the better the effect will be. But too many channels is impractical. Later, the experiment only used three channels, and the effect was good enough. This was the approach taken in widescreen stereo movies in the 1950s. Subsequently, further experiments found that good results can also be obtained using two channels (dual channel), which is the method used in the past for stereo records, stereo tape recording, and stereo broadcasting.

The two-channel stereo sound transmission system is shown in the figure above. Compared with the mono system, it has made a huge leap forward in terms of improving sound quality, enhancing the sense of presence, and faithfully reproducing the position and spatial distribution of each sound source in the actual sound field. However, the two-channel stereo system only reproduces the direction and spatial distribution of the sound source in front of the listener, and does not establish a stereo field from all directions. Therefore, it has moved from two-channel stereo to four-channel stereo and three-dimensional surround. sound development, and now the panoramic sound system.

3. Characteristics of Stereo Sound

Compared with mono reproduction sound, summarize some of the salient characteristics of stereo sound.

1) Have a clear sense of direction and distribution

In monophonic playback, the sound is emitted from a "point". Even if the sound source is a band's performance, the listener will still clearly feel that the sound is emitted from a point in the speaker. When using multi-channel stereo playback, the listener will clearly feel that the sound source is distributed in a wide range, and can subjectively imagine the location of each instrument in the band, creating an illusion of the location of the sound source. Referred to as audiovisual. The hallucinated sound image reproduces the relative spatial position of the actual sound source, with a clear sense of direction and distribution.

2) Has higher definition

When using monophonic playback, since the position of each sound cannot be distinguished, the sounds from different sound sources are mixed together, which is affected by the masking effect, resulting in lower listening clarity. With a stereo system for playback, the listener can clearly feel that different sound sources come from different directions, and the masking effect between the sound sources is much weakened, so it has higher clarity.

3) Has less background noise

When using monophonic playback, since the background noise and the useful sound are emitted from the same point, the background noise has a great influence. When using stereo playback, the reproduced noise image is dispersed, and the impact of background noise on useful sounds is reduced, making the stereo background noise appear smaller.

4) Have a good sense of space, envelopment and presence

Stereo system playback brings a sense of the original sound field that is unmatched by mono playback. This is because stereo systems can transmit near reflections and reverberant sounds better than mono systems. The reverberation sound in the concert hall is non-directional and surrounds the listener. Although the near-reflection sound is directional, due to the Haas effect, the listener cannot feel the direction of the reflected sound, that is, for the listener, the reverberation sound is non-directional. It is also said to be directionless. In a monophonic system, the replayed near-reflected sound and reverberated sound become sounds coming from one direction; in a stereo system, the near-reflected sound and reverberated sound can be reproduced, allowing the listener to feel the original sound field. The sound environment has a good sense of space, envelopment and presence.

4. The difference between stereo and binaural sound

Assuming that when the waveforms of the left and right channels of the dual channel are in the same phase, there is no stereoscopic effect. The effect is the same as that of the mono channel, except that the sound image is in the middle of the two sound sources, which is called dual channel, not stereo. . When the waveforms of the left and right channels of the two-channel sound are exactly the same, artificially creating a certain phase difference can create a wide sound field and give it a three-dimensional feel. The artificial phase difference is fixed, so this kind of stereo is called false stereo. Or called virtual stereo. True stereo sound is two completely different waveforms in the two channels, with different phase differences at every moment. These differences have been determined in the early recording.