Although dolby digital coding technology has been determined as the audio compression standard of American digital TV, there are still many technical problems to be solved before it is applied to digital TV transmission. First of all, the transmission of digital TV must conform to AE3 standard jointly formulated by Audio Engineering Association (AES) and European Broadcasting Union (EBU). According to this standard, TV audio signals are transmitted through an AES/EBU cable, and only two channels of audio signals can be transmitted, while dolby digital is 5. 1 channel. Therefore, it is necessary to change the existing TV broadcasting line architecture or establish a new digital TV audio transmission mode. Secondly, in the propagation process of DTV signal, it may go through many encoding and decoding processes because it needs to be sent or received continuously. In order to enable the end user to receive audio programs with the same quality as the sender, this multiple encoding-decoding process should not lead to the deterioration of sound quality. However, dolby digital is designed for single coding-decoding, and its goal is to achieve the best sound quality at low bit rate, so it cannot meet the above requirements. In addition, the audio frames and video frames in dolby digital are not synchronized, so when the image content changes in the editing process, the change of sound quality is inevitable. In order to solve the above problems, Dolby Company has specially developed a brand-new digital audio compression technology called Dolby E. ..
Dolby E is a professional audio coding system specially designed for digital TV broadcast transmission and post-production. Using Dolby E technology, an AES/EBU channel can transmit up to 8 channels of high-quality audio digital streams, and at the same time, it can also load dolby digital's control data signals (usually called metadata, including dolby digital's channel mode, dynamic range type, output level automatic adjustment parameters, etc. To ensure that users can correctly play dolby digital programs). Dolby E, with its advanced coding algorithm and high data transmission rate, can ensure that the audio signal of digital TV can go through as many as 10 cycles of coding and decoding or other editing processes without causing deterioration of sound quality. Dolby e's audio can well match video signals of various formats, and will not cause blind sound, burr or other distortion due to video interference. Dolby e's audio frames and video frames are completely corresponding in time sequence, which can realize accurate integrated editing of audio and video. More importantly, using this new coding technology, the existing radio and television system can spread multi-channel digital TV audio signals without large-scale transformation, thus greatly reducing the system cost.
The main application of Dolby E is the transmission between digital TV stations, which transmits programs from one local TV station to another via satellite. Dolby E will be converted into PCM audio when transmitted to the transmitting terminal, and then encoded into dolby digital signal, which will be transmitted to thousands of households. Therefore, it is impossible for consumers to receive the Dolby E signal directly at home, and they can only receive the dolby digital signal of 5. 1 channel. In order to avoid confusing the two, Dolby E is usually called the transmission coding system, while dolby digital is called the transmission coding system.
The standard data transmission rate of Dolby E is 1.92Mbits/sec (for 20bit/48kHz audio signal), and the typical working mode is "5. 1+2", that is, 6 channels transmit 5. 1 channel surround sound signals, and the other 2 channels transmit 2-channel matrix coded signals (such as Dolby directional logic surround sound). It can also work in other modes, such as "5. 1+ 1" mode for two mono signals, "3×2" mode for three stereo signals and "6× 1" mode for six mono signals. Dolby's current version supports audio signals with a frame rate of 29.97fps (NTSC system and 25fps in PAL system), a word length of 20bit and a sampling frequency of 48kHz. Future versions will support 25fps, 16 bit or 24 bit audio signals.
The first Dolby E surround sound encoder DP57 1 and Dolby E surround sound decoder DP572 have been launched. The former costs $5,395, while the latter costs $3,995. The frequency response of DP572 is 20 Hz ~ 20 kHz (0.25 dB), the distortion is less than 0.0 1%( 1kHz) or 0.02% (20 Hz ~ 20 kHz), and the dynamic range is greater than 1 10dB. Dolby Pro Logic II is compatible with audio and video music.
Dolby directional logic surround sound has been popular all over the world for more than ten years since 1987 came out. However, with the rapid development of audio-visual products, its weaknesses have gradually emerged. On the one hand, it is developed for the sound reproduction technology of home theater, which can not take into account the playback of pure music; On the other hand, its surround sound channel is mono, with narrow bandwidth and insufficient separation from the front channel, which leads to its unsatisfactory surround sound effect. The new format of dolby digital surround sound introduced by Dolby Company overcomes the latter defect, but the former problem still exists to a certain extent. To listen to dolby digital surround sound, the program source encoded by the new format must be adopted, so a large number of Dolby directional logic encoded program sources available to consumers cannot enjoy this achievement. Therefore, Dolby Company introduced the improved decoding technology of Dolby directional logic surround sound in June 2000, and named it "Dolby Pro Logic II". Dolby claims that by using this technology to replay traditional Dolby directional logic surround sound movies or two-channel stereo music programs, the effect similar to dolby digital 5. 1 channel surround sound can be obtained.
Like dolby digital and DTS, Pro Logic II can also provide the output effect of five channels plus subwoofer, but the difference is that its five main channels are not really completely independent channels, but the separation between channels is much better than traditional Pro Logic. For Pro Logic II, the isolation between all channels can reach more than 40dB, and the surround sound channel is dual-channel, so there is no special limitation on bandwidth. However, the isolation degree of the adjacent channels of Pro Logic is generally around 30dB, and the surrounding channels are mono, with a bandwidth of only 7kHz.
Another important improvement of Dolby Pro Logic II is that it can play back a two-channel stereo music program source (such as CD) in the form of surround sound. This can make music play more natural and realistic, full of sense of scene, and greatly expand the scope of use, and can be widely used in non-surround sound program sources such as TV stereo, car stereo, computer stereo and earphone stereo. In fact, the early development motivation of this technology is for the surround sound playback of pure music, so the three-dimensional spatial information hidden in the two-channel stereo signal is fully tapped, which obviously enhances the sense of space, directivity and clarity of the sound, expands the sound field, and allows users to get good hearing even if they are not in the best position. In addition, the circuit implementation of Dolby Pro Logic II processor is relatively simple and economical, and it is also compatible with the decoding and playback of Pro Logic.
Table 1 compares Pro Logic and Pro Logic II. Table 2 shows three working modes of Pro Logic II. Among them, the main difference between movie mode and Pro logic mode is that it has no limitation on the bandwidth of surround sound channel, and it provides stereo surround sound output. The similarity between the two is that the surround channel has a certain delay relative to the front channel, in order to enhance the positioning sense of the front channel signal by using Haas effect.
Table 1 Comparison between Dolby Pro Logic and Pro Logic Ⅱ
Table 2 Working mode of Dolby Prologic
There is a clear difference between music recording and movie sound. The goal of home theater is to reproduce the complex multi-channel environment in the movie scene in the home environment, so the parameters of playing sound field are strict, and the playing effect of music often varies from person to person. For this reason, the movie mode in Pro Logic II is preset with various parameters, while the music mode provides different options, which users can choose according to their own preferences. These options are:
Scale control: constantly adjust the front and back positions of the sound field to help users find the best loudspeaker balance point according to the music content they choose to listen to.
Central width control: adjust the central sound image to accurately locate it at the position of the central speaker. Using this control and adjustment function, a well-balanced left, middle and right sound field can be produced, especially for car drivers or front-row spectators with great position limitations, and the effect is very obvious. In the home environment, the central width control can effectively improve the coordination between the central channel and the left and right main channels, and control the width or height of the sound field.
Panoramic mode: used to expand the front stereo sound field and produce the echo effect of multiple surround sound speaker arrays.
When playing music, a high-frequency filter called Shelf is added to the surround channel to compensate for the high-pitched attenuation caused by room reflection or absorption, which can make the music sound more natural and soft, and the sound field expansion range is moderate. In addition, music playback requires all speakers to reach the user's ears at the same time, so the surround sound delay function and automatic balance function are turned off in music mode.
The world's first Dolby Pro Logic II product is the RFQ5000 surround sound processor specially designed for car audio by Rochford Fosgate. It has a digital coaxial input terminal and five analog line output terminals, and its working modes include movie, music, panorama, Pro Logic and stereo through. Dolby Pro Logic II has only been released for a few months, and the mass market of its products has yet to be realized by special chips. Dolby Headphones for Personal Cinema
How can a wandering wanderer enjoy DVD movies on the road without TV and surround sound equipment? An AV enthusiast bought a new home theater and wanted to turn down the volume and enjoy Dolby's charm. But what should he do if his wife is too noisy, the elderly have to rest and the children have to study? A college student wants to watch DVD alone to relax his nervous nerves after studying, but there is no personal space in the dormitory, library and classroom that does not interfere with others' work or study. What should I do? In order to meet the individual's desire and requirements of watching home theater programs on the above different occasions, in recent years, "Personal Cinema" came into being, and Dolby Company also catered to this demand, and specially developed a surround sound coding format called "Dolby Headphones" so that users can listen to dolby surround programs with stereo headphones.
When you listen to music with a pair of stereo speakers with good performance, you can get a sense of space and position similar to a concert hall. The reproduction of this spatial sound field is actually the result of the interaction of two factors. One is the acoustic characteristics of the room, including the reflection, scattering and absorption of sound by indoor objects, which can be called "environmental effect"; The second is your own physiological structure, mainly the auricle and its surrounding head and shoulders, which is often called "auricle effect". If you listen to the same music with a pair of stereo headphones, the above two factors no longer work, then the sense of spaciousness disappears, and you get a thin audio-visual image with only width (from one ear to the other) and no sense of depth. In particular, the audio-visual image that should have appeared in front of your eyes now appears in the center of your head. This "head-to-head" effect is also an important reason for the poor telepresence of headphones and fatigue after listening for a long time. Since it is so difficult to reproduce two-channel stereo with headphones, it is obviously more difficult to reproduce multi-channel surround sound with headphones.
In order to solve the above problems, Dolby headphones use advanced digital signal processor and the latest research results of physiological acoustics and environmental acoustics to simulate "environmental effect" and "auricle effect", thus generating a strong sense of encirclement and positioning. It can use ordinary stereo headphones to produce a surround sound effect consisting of five speakers (left, middle, right, rear left surround and front right surround) in a large enough room. Of course, the speaker and the room are virtual.
Dolby headphones can simulate the listening effect of three different rooms. DH 1 refers to a relatively dry room with a small volume, which is suitable for both watching movies and listening to pure music. This is the designated reference room of Dolby headphones, which is suitable for all devices equipped with Dolby headphones. The acoustic characteristics of DH2 are more lively and the reverberation time is relatively long, which is especially suitable for listening to music. DH3 is a big room, and its acoustic characteristics are very similar to those of a concert hall or a commercial cinema. DH2 and DH3 are Dolby headset options. Users can choose the appropriate listening mode according to the content of the program source and their hobbies. In addition, Dolby headphones do not need to be set or adjusted by the user, so it is very convenient to use.
Dolby headset can be used to listen to pure audio carriers such as CD, MD and MP3, or music in Dolby surround, dolby digital, PCM or MPEG encoding formats. Dolby headphones can be loaded into a variety of products, such as cable and satellite TV set-top boxes, video recorders, DVD-Video and DVD-Audio players, personal and desktop computers, game consoles, car stereos and civil aviation cabin stereos.