Many years ago, I listened to some multi-channel demonstrations recorded in halls and churches in the multi-channel production studio of a fax production company in Hong Kong. At that time, I was shocked by the replay effect of the symphony and the organ, the realism was coming, and the sound stage was huge, as if returning to the scene, which was an effect that could not be achieved by two-channel stereo. In the home environment, people often use two-channel stereo sound (commonly known as "hi-fi speakers") to enjoy** instead of using multi-channel speakers. The reasons for this are complex. To put it simply, this is largely limited by cost, environment, publicity, and personal perception. For multi-channel audio, the public's understanding of it is the surround sound in the home theater, which is used to reproduce the shocking sound effects of aircraft cannons and flash floods, which is different from the two-channel stereo used to enjoy the beautiful **. For stereo, the word stereo is called stereophonic, or stereo. The basic meaning is "three-dimensional, three-dimensional". Therefore, the term stereo is applied to sound systems with more than two channels or speakers, including quad-channel, surround, and binaural sound systems. Therefore, multichannel sound is a form of stereo sound in its own right. There are two main ways to make stereo sound, the first is to pursue real and natural stereo sound, capture the live sound through an array of microphones, and any natural reverberation that exists, and finally be reproduced by multiple speakers to get as close as possible to the real sound effect of the scene. The second is called artificial stereo, or pan-stereo, which uses multiple speakers to replay mono, and artificially changes the relative amplitude of the signal sent to each speaker, so as to produce a sense of sound panning and movement, and can also achieve a complete artificial sound field effect. It can be seen that the former is suitable for ** recording, and the latter is suitable for film sound production. From this it can be seen that the use of multiple speakers to replay** can be close to the real effect of the scene. Each channel and the associated speakers create an independently localizable sound source, and the interaction between multiple channels creates a virtual sound source. In contrast, the commonly used Hi-Fi speaker uses two speakers to play stereo is a compromise, which is enough for the family, and multi-channel sound replay is the ideal way to play stereo sound. 
The main contents of the topic include:First: Bell Labs, the origin of mankind's exploration of multichannel replaySecond: Detailing the center channel Third: Virtual center sound image and real center sound imageFourth: Surround channel and sky channel Fifth: Four channels, the first use of surround sound in the domestic field Sixth: **Audio and SACD, promoting multichannel again on digital audio carriers**Seventh: Blu-ray era **Multichannel in the Documentation Period**Eighth: Partial Multichannel Pure **Software&**Documentation**Nine: Conclusion
Bell Labs, the origins of humanity's exploration of multichannel replay
In the '30s, Harvey Fletcher, a physicist at Bell Labs, and a number of engineers began working on stereo recording and replay. The approach is to assume that a large number of microphones are used to capture the sound, and then a large number of speakers are used to reproduce the sound. This is done by forming a microphone array in front of the orchestra, using more than 80 microphones, each of which feeds information to a corresponding speaker, which is placed in the same position (as the corresponding microphone) in a separate listening room, thus replicating the array wave. As for the reflected sound, it is not collected, because there is also reverberation in the listening room. The possibility of simplification was then studied with practical experience. In the end, they came to the conclusion that replay with two channels could be accepted by a single listener, but if it was a group of listeners, at least three channels were needed, i.e., the front left, center, and right channels, so there was a Bell three-channel stereo system. Because of this research, physicist Harvey Fletcher is known as the "father of stereo". In 1933, Bell Labs demonstrated a three-channel stereo system that transmitted the Philadelphia Orchestra's performance live from Philadelphia to Constitution Hall in Washington, D.C., via the ** line. By 1938, Bell Labs had expanded the number of channels to a three-channel stereo system, which was later supposed to be used in the Disney film Fantasia, also known as fantasound. In 1940, Bell Labs' three-channel stereo system was demonstrated in Carnegie Hall, using three huge loudspeakers, the sound of which was recorded optically on three tracks of motion picture film, and a fourth track to control the volume to enhance dynamic range. According to a historical report, the sound emitted by the speakers is as high as 100 decibels, and the effect is fascinated and even a little scary because the sound is too loud. Also present at the demonstration was the famous composer Rachmaninov, who commented that "it is too loud and lacks a sense of **". 
Physicist Harvey Fletcher, known as the "father of stereo sound".
3 huge speakers used in front of the Fantasound sound system
Installation of a fantasound sound system, which is a surround sound speaker installed around the perimeter of the theater
To be continued