What are Surround Sound Headphones?

Dolby Surround: A sound that encodes back-effect channels into a stereo channel. A decoder is required for playback to separate the surround signal from the encoded sound. Dolby Surround is the consumer version of the original Dolby Multichannel analog movie sound format (Dolby Analog and Dolby SRSpectral Recording). When making a Dolby surround soundtrack, the four channels of audio informationleft, center, right, and single surroundare encoded on two soundtracks in matrix coding. Stereo program sources such as videotapes and TV shows will carry two soundtracks into millions of homes, and after decoding by Dolby Pro Logic technology will restore the original four-channel surround sound.

Dolby Surround

Dolby Surround Sound Overview

Dolby directional surround sound has been popular as a standard audio system for commercial theaters and home theaters for many years. Dolby digital surround sound has been adopted by emerging video and audio carriers such as digital TV DTV and digital video DVD, making Dolby surround sound almost Become synonymous with contemporary multi-channel audio reproduction technology. However, the powerful Dolby company did not stop there. It has followed the trend of rapid development of audio and video technology, and has developed a number of new surround sound coding technologies for different application backgrounds, such as Dolby E, Dolby. Pro Logic II and Dolby Headphone are the three new surround coding formats recently introduced by Dolby.

E Dolby Surround Dolby E

Dolby E for digital TV

Although Dolby Digital encoding technology has been identified as the audio compression specification for US-made digital TVs, there are still many technical issues that need to be resolved if it is to be used for digital TV transmission. First, digital TV transmission must comply with the AE3 standard jointly developed by the Audio Engineering Society (AES) and the European Broadcasting Union (EBU). According to this standard, the TV sound signal is transmitted using an AES / EBU cable. This cable can only transmit two-channel audio signals, while the Dolby Digital is 5.1 channels, so either the existing TV broadcast transmission must be changed. Line architecture, or to establish a new digital television audio transmission method. Secondly, during the propagation of DTV signals, because they are continuously transmitted or received, they may be subjected to multiple encoding-decoding processes. In order to enable the end user to receive audio programs of the same quality as the sender, this multiple encoding-decoding process should not cause degradation of the sound quality. Dolby Digital is designed for a single encoding-decoding. Its goal It is because the sound quality is optimized under the condition of low bit rate, so it cannot meet the above requirements. In addition, Dolby Digital's audio frames are not synchronized with the video frames, so when the image content is changed during the editing process, changes in sound quality are difficult to avoid. In order to solve the above problems, Dolby has developed a new digital audio compression technology called "Dolby E".

Dolby E is a professional-grade audio coding system designed for digital TV broadcast transmission and post-production. Using Dolby E technology, one AES / EBU channel can transmit up to 8 channels of high-quality audio digital streams, and it can also load Dolby Digital control data signals (often called metadata, including Dolby Digital audio Channel mode, dynamic range type, and output level automatic adjustment parameters to ensure that users can correctly play Dolby digital programs). With its advanced encoding algorithm and high data transmission rate, Dolby E can ensure that the audio signals of digital TVs go through up to 10 cycles of encoding-decoding or other editing processes without causing sound quality degradation. Dolby E's audio can be well matched with video signals of various formats, and will not cause blind sound, sound glitches or other distortions due to video interference. Dolby E's audio frames and video frames completely correspond in time sequence, which can realize accurate integrated audio and video editing. More importantly, by using this new encoding technology, existing broadcast television systems can propagate multi-channel digital television audio signals without large-scale transformation, thereby greatly reducing system costs.

The main application of Dolby E is in the middle of digital TV stations, and programs are transmitted from one local TV station to another via satellite. When transmitted to the transmitting terminal, Dolby E will be converted into PCM audio, and then encoded into Dolby digital signals and transmitted to millions of households. Therefore, consumers cannot directly receive Dolby E signals at home, but can only receive 5.1-channel Dolby digital signals. To avoid confusion between the two, Dolby E is commonly referred to as a transmission coding system, and Dolby Digital is referred to as a transmission coding system.

The standard data transmission rate of Dolby E is 1.92Mbits / sec (for 20bit / 48kHz audio signals). The typical working mode is "5.1 + 2", which means that 5.1-channel surround sound signals are transmitted using six channels, and the other two Each channel carries a two-channel matrix-encoded signal (such as Dolby directional surround sound) or a two-channel stereo signal. Can also work in other modes, such as 5.1 + 1 + 1 mode with two mono signals, 3 × 2 mode with three stereo signals, and 6 × 1 with six mono signals Mode, etc. Dolby's current version supports audio signals with a frame rate of 29.97fps (NTSC format, PAL format is 25fps), 20bit word length, and 48kHz sampling frequency. Future versions will support 25fps, 16bit, or 24bit audio signals.

The first Dolby E surround encoder DP571 and Dolby E surround decoder DP572 have been launched, the former is priced at $ 5,395 and the latter is priced at $ 3,995. The frequency response of DP572 is 20Hz 20kHz (0.25dB, distortion is less than 0.01% (1kHz) or 0.02% (20Hz 20kHz), and the dynamic range is greater than 110dB.

II Dolby Surround Dolby II

Audio and video compatible Dolby Pro Logic II

Since the introduction of Dolby Pro Logic surround sound in 1987, it has been popular all over the world for more than ten years, but with the rapid development of audio and video products, its weaknesses have gradually begun to show. On the one hand, it is developed for the audio-back technology of the home theater, which cannot well take into account the playback of pure music; on the other hand, its surround channel is mono, narrow bandwidth, and separated from the front channel The degree is not large enough, making its surround sound effect less than ideal. The new Dolby Digital Surround Sound format introduced by Dolby overcomes the latter defect, but the former problem still exists to a certain extent, and listening to Dolby Digital Surround Sound must use a new format encoded program source. A large number of Dolby Pro Logic coded programming sources cannot enjoy this result. To this end, in June 2000, Dolby introduced an improved decoding technology for Dolby directional surround sound, and named it "Dolby Pro Logic II". Dolby claims that using this technology to replay traditional Dolby directional surround-sound movies or two-channel stereo music programs can achieve effects similar to Dolby Digital 5.1-channel surround sound.

Similar to Dolby Digital and DTS, Pro Logic II can also provide the output of 5 channels plus subwoofer. The difference is that its 5 main channels are not true and completely independent channels, but each The separation between channels is much better than traditional Pro Logic. For Pro Logic II, the separation between all channels can reach more than 40dB, the surround channel is dual-channel, and there is no special limit on the bandwidth; and the separation between adjacent channels of Pro Logic is generally 30dB The left and right surround channels are mono and the bandwidth is only 7kHz.

Another significant improvement in Dolby Pro Logic II is the ability to play back two-channel stereo music program sources such as CDs in surround sound. In this way, music playback can be more natural and realistic, full of liveliness, and the range of use is greatly expanded. It can be widely used in non-surround sound program sources such as TV audio, car audio, computer audio, and headphone audio. In fact, the early engine of this technology was for pure music surround sound playback, so it fully excavated the three-dimensional spatial information hidden in the two-channel stereo signal, which obviously enhanced the sound.

Technical Description

The sense of space, directivity, and sharpness expands the sound field range, and users can get a good sense of hearing even if they are not in the optimal position. In addition, the circuit implementation of the Dolby Pro Logic II processor is relatively simple and economical, and it is also compatible with Pro Logic decoding playback.

Table 1 compares Pro Logic and Pro Logic II. Table 2 shows the three working modes of Pro Logic II. Among them, the main difference between the movie mode and the Pro logic mode is that it has no limitation on the bandwidth of the surround channel, and it provides stereo surround sound output. The common feature of the two is that the surround channel has a certain delay relative to the front channel. The purpose is to use the Haas effect to enhance the positioning of the front channel signal.

Table 1 Comparison of Dolby Pro Logic and Pro Logic

Table 2 Working mode of Dolby Prologic

The characteristics of music recordings and movie soundtracks are significantly different. The goal of home theater is to reproduce the complex multi-channel environment in the movie scene in the home environment, so the setting of the playback sound field parameters is relatively strict, and the playback effect of music often varies from person to person. To this end, the movie mode in Pro Logic II is preset with various parameters, while the music mode provides different options, and users can choose according to their preferences. These options are:

Scale control: Continuously adjust the front and back positions of the sound field to help users find the best speaker balance based on the music content they choose to listen to.

Center Width Control: Adjust the center sound image so that it is positioned exactly where the center speaker is. With this control adjustment function, well-balanced left, middle, and right sound fields can be produced, especially for car drivers or front-row spectators with very limited positions. In the home environment, the center width control can effectively improve the coordination between the center channel and the left and right main channels, and control the width or height of the sound field.

Panorama mode: It is used to expand the front stereo sound field to produce the sound return 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 treble attenuation caused by the reflection or absorption in the room, which can make the music sound more natural and soft, and the sound field has a moderate range extension. In addition, music playback requires that sound from all speakers reach the user's ears at the same time, so the surround delay function and the auto balance function are turned off in the music mode.

The world's first Dolby Pro Logic II product is the RFQ5000 surround sound processor designed specifically for car audio by Rockford Fosgate. It has a digital coaxial input terminal and 5 analog line output terminals, and its working modes include movie, music, panorama, Pro Logic and stereo pass-through. Only a few months after the introduction of Dolby Pro Logic II, the large-scale listing of its products still needs the realization of special-purpose chips.

Headphone Dolby Surround Headphone

Catering Dolby Headphones for Personal Theater

A wandering world traveler, who wants to enjoy DVD movies on the way, without a TV set and surround sound equipment. How is this possible? An AV enthusiast has newly purchased a home theater and wants to let go of the volume and enjoy the charm of Dolby Surround, but his wife is noisy, the elderly want to rest, and the child wants to learn. A college student wants to look at the DVD alone to relax his nervousness while studying, but the collective dormitory, library, and classroom cannot find a personal space that does not interfere with others' work or study. What should I do? In order to meet personal desires and requirements for watching home theater programs on different occasions as mentioned above, in recent years, "personal theater" has emerged as the times require, and Dolby has also catered to this demand and specially developed a surround sound coding called "Dolby Headphone" The format allows users to listen to Dolby Surround programs with stereo headphones.

When you listen to music with a pair of good stereo speakers, you can get a sense of space and location similar to listening to music in the concert hall. This reproduction of the spatial sound field is actually the result of the combination 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 effects"; the second is that you Its own physiological structure, mainly the auricle and its surrounding head and shoulders, is often called the "auricle effect". If you listen to the same music with a pair of stereo headphones, the above two factors no longer work, so the spacious sense of space disappears, and you get a thin sound image, only the width (from one ear to the other Ears) without a sense of depth. In particular, the sound image that should appear in front of you is now in the center of your head. This "in-the-head" effect is also an important reason for the poor presence of headphones and the fatigue of long-term listening. Since it is so difficult to reproduce two-channel stereo with headphones, it is obviously more difficult to use it to reproduce multi-channel surround sound.

In response to the above problems, Dolby Headphone uses advanced digital signal processors to take advantage of the latest research results of physiological acoustics and environmental acoustics to simulate "environmental effects" and "auricular effects", thereby generating a strong sense of surrounding and positioning. sense. It uses ordinary stereo headphones to produce a surround sound effect formed by five speakers (front left, center, right, and rear left surround and right surround) in a large enough room. Of course, both the speakers and the room are Virtual.

Dolby Headphone can simulate listening effects in three different rooms. DH1 refers to a room with a small volume and relatively "dry" (referring to shorter reverberation time). It is suitable for watching movies and listening to pure music. This is a designated reference room for Dolby Headphones. It is applicable to 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 larger room with acoustic characteristics much like a concert hall or commercial cinema. DH2 and DH3 are options for Dolby Headphones. Users can choose the appropriate listening mode based on the content of the program and their interests. In addition, Dolby Headphone does not require any settings or adjustments by the user, making it very convenient to use.

Dolby Headphone is used for a variety of purposes, both CD, MD, MP3 and other pure audio carriers to listen to music, or Dolby Surround, Dolby Digital, PCM or MPEG encoding format to listen to surround sound effects. There are also a variety of products that can load Dolby Headphones, such as cable and satellite TV set-top boxes, VCR recorders, DVD-Video and DVD-Audio players, personal and desktop computers, game consoles, car audio, and civil aviation cabin audio. .