How did stereophonic technology come about?

                               How did stereophonic technology come about?

The word stereo should be familiar to everyone, so how did this technology come into being and develop? Let's chat briefly today.

In 1931, Alan Dower Blumlein, a researcher and engineer of the EMI company, went to the cinema with his girlfriend to watch a movie, and suddenly realized that if the sound can be emitted from both sides at the same time, the effect of the sound and the sense of real experience it can bring can be enhanced. On December 14 of the same year, Blumlein applied for a patent titled "Sound Transmission, Recording and Sound Reproduction System and Related Improvements". In this patent application, he outlined his views and ideas, which was to create a sound reproduction system that was better than the monophonic system used at the time. This system, which Blumlein called "binaural sound" (Binaural sound), is what we call stereo (Stereo) today.

This ingenious concept was so epochal that many people at the time neither understood it nor realized its potential value.

EMI, not quite sure what to do with Blumlein's invention, set it aside for a few years. They had some foresight, however, and in 1931 the company opened a new studio on Abbey Road, London, and installed Blumlein's electronic recording system. EMI also made some of the first stereo recordings and films in the 1930s, but then shelved the technology.

It wasn't until 1958 that the world's first publicly released stereo record came out, and this was already 16 years after Blumlein's death, and EMI's related patents had also expired for 6 years. In 2017, Blumlein was posthumously awarded a Technical Grammy Award honor by the Audio Engineering Society of America.

The term "binaural" means that a person needs to have two ears when listening. Our brains use the difference between the sound reaching the left ear and the right ear to localize the sound source. This difference includes 3 types: intensity difference, time difference and frequency difference. For example, in real life, if the trumpet on our right makes a sound, the sound that reaches our right ear will be louder than the sound that reaches our left ear. This is because our head absorbs and reflects some of the energy of the sound as it travels to the left ear. In addition, since it takes a certain amount of time for the sound to travel from left to right (about 1 ms per 0.3 meters), this will also cause the sound heard by the left ear to be slightly later than the sound heard by the right ear. Finally, due to the weak diffraction ability of high-frequency signals in sound, they cannot go around our head like low-frequency signals, so relatively speaking, the left ear receives less high-frequency signals than the right ear.

In order to simulate the signal difference between the two ears in actual listening, we can use a Dummy Head (dummy head) with a microphone in the simulated ear when recording, or use computer calculations to generate signal differences (this is 3D sound method of implementation). No matter which of these two methods is used, if we use a headphone to replay the resulting binaural signal, we can get a good sense of positioning, and at the same time feel that the sound is coming from around us , including the left and right sides, the rear and even the up and down directions. That is to say, by providing the left and right ears with two different signals that are similar to the actual listening, we can let our brain obtain a listening experience similar to the actual listening experience.

However, if speakers are used instead of headphones to play, the effect we feel is difficult to be consistent with the actual listening results. Because when listening to a real sound source, if a sound is emitted from the position directly in front of the listener, the sound will reach the left ear and the right ear at the same time, and the sound heard by the left and right ears will have the same intensity and frequency components. But there is no such a central speaker in a two-channel stereo playback system, so in order to create a centrally located sound image, the left and right speakers must emit exactly the same sound. The central sound image produced by such reproduction without a center speaker is called the phantom center.

If we can make the sounds from the two speakers reach only the ears that are closer to them as shown in Figure a above, then the playback effect of this simulated real sound source will be very good. However, the sounds from the two speakers also arrive at the ear that is farther away from them after a short time, and the sound that arrives later is slightly lower in intensity for each ear than the sound that arrives earlier. , while missing some high-frequency components. This late arrival disturbs our brains and slightly affects the clarity of the sound imaging. In practice, the difference in localization clarity of the sound image produced by the left and right speakers can surprise you, especially in some poorly tuned surround sound studios.

The best listening experience is obtained when the listener is positioned at the center of a pair of properly tuned and placed speakers. Sitting in this position for mixing is a must for any mixer, but for most ordinary listeners, this is not the ideal position for listening to music. Although some stereo information is gained when listening from a non-emperor seat, more stereo information is lost, especially when we are off center.

This is why some friends have different feelings when listening to sound works with speakers and headphones. In addition, due to the different analysis capabilities of the playback devices themselves, the feelings they can bring are also very different. For example, many mobile phone speakers are actually mono speakers that lack low-frequency playback capabilities. With such external speakers, you can’t actually hear the frequency and location information of many sounds. If you want to hear the full stereo information in the sound, the easiest way is to put on headphones and experience it.

Today, although new sound technologies emerge in endlessly, stereo is still a very important sound technology, which brings us a richer audio-visual experience.