Multichannel audio in broadcast applicationsBy Kimio Hamasaki
One of the principal aims of broadcast technology development is the enhancement of presence, which makes viewers feel as if they were at the actual location where the program was shot. TV technology has progressed toward high resolution, from black-and-white to color and on to HDTV. Audio technology also has progressed, with spatial impression being improved from mono to stereo and on to multichannel.
Figure 1. The loudspeaker arrangement of current multichannel audio systems based on ITU-R BS.775-1, where the 5.1 surround sound system consists of six loudspeakers (left, center, right, rear left, rear right and Low Frequency Enhancement). 3-2 and 3-1 have no Low Frequency Enhancement, and 3-1 has only a mono surround signal.
Multichannel audio opens up a new dimension of sound production in television. It also confers substantial advantages in sound reproduction, such as spatial impression and sound source localization in the horizontal plane.Pioneering multichannel audio in Japan
Multichannel audio in broadcasting is not new in Japan; indeed, it has been taking place since 1984, when various experimental HDTV productions using 3-1 quadraphonic surround sound began to be produced. Since 1992, the public broadcaster NHK has been broadcasting many HDTV programs using this format, and in December 2000, digital broadcasting via broadcast satellite was launched. This supports 5.1 and 3-2 multichannel sound programs. (See Figure 1.) It also has three attractive features for the viewers: namely high-quality video images by HDTV, the high quality and sense of presence of 5.1 multichannel surround sound, and data broadcasting. These features are expected to lead to a rapid increase in the number of viewers of digital broadcasts.Figure 1. The loudspeaker arrangement of current multichannel audio systems based on ITU-R BS.775-1, where the 5.1 surround sound system consists of six loudspeakers (left, center, right, rear left, rear right and Low Frequency Enhancement). 3-2 and 3-1 have no Low Frequency Enhancement, and 3-1 has only a mono surround signal.
Digital broadcasting via broadcast satellite in Japan is called ISDB (Integrated Services Digital Broadcasting) or BS Digital Broadcasting (Broadcast Satellite Digital Broadcasting). In this digital broadcasting system, video is encoded as MPEG-2, and audio is encoded as MPEG-2 AAC (Advance Audio Coding). The latter was designed to reduce the bit rate of the MPEG-1 audio coding scheme for the equivalent quality of audio, and it supports various audio reproduction configurations such as mono, 2-channel stereo, 3-2 multichannel audio and 5.1 multichannel audio.
The MPEG-2 AAC system has three profiles: Main profile, Low Complexity (LC) profile and Scalable Sampling Rate (SSR) profile. The Main profile enables encoding of the highest sound quality at the same bit rate as MPEG-1. The LC profile, which is used for digital broadcasting in Japan, can be decoded with less complexity than that of the Main profile, although there is a slight degradation of audio quality. The SSR profile allows decoding according to several audio bandwidths.
MPEG-2 AAC also can achieve near-CD sound quality at half the bit rate of MPEG-2 BC (Backward Compatible) - an extension of MPEG-1 coding to multichannel audio that has compatibility with MPEG-1 2-channel stereo decoding. For instance, 5.1 surround sound can be coded at 320kb/s with near-CD sound quality, and 2-channel stereo can be coded at 128kb/s using the Main profile, or at 144kb/s using the LC profile with near-CD sound quality.Multichannel in theory
Multichannel audio programs via BS Digital Broadcasting are produced and broadcast by commercial broadcasters in Japan. These programs include various productions such as music, drama, documentary and sports. They attempt to let viewers experience the benefits of 5.1 surround sound compared with ordinary two-channel stereo, and trials have shown that by proper use of a center loudspeaker, it is possible to achieve the following advantages:
- Fix the sound image in the frontal area.
- Widen the area for optimal listening positions.
- Make the audio cues coincide with the visual cues.
Proper use of rear loudspeakers offers the following improvements:
- Reproduction of spatial impression.
- Creation of sound images in the rear space.
For ordinary viewers, the difference between 5.1 and 2-channel stereo is much more noticeable thanks to the rear loudspeakers than the center loudspeaker. For most productions, therefore, the rear channels are intended to enhance the surround sound feeling or to create some sound images in the rear space, although trials have shown that it is not always easy to achieve these effects sufficiently.Multichannel in practice
There are several issues concerning 5.1 recordings and productions. Of these, the most important concern is how to use a center loudspeaker and rear loudspeakers properly, and what kinds of microphones are needed and how to arrange them for practical 5.1 recording. Indeed, some engineers and academics already have proposed practical microphone arrays for 5.1 surround sound recordings such as OCT-Surround, INA 5 and Hamasaki-square, especially for classical music recordings.
While some broadcasting stations are trying to broadcast programs with 5.1 surround sound in Japan, not many programs have actually been produced. This is often because producers and directors have little experience of hearing attractive multichannel audio themselves, so they are not that interested in producing it. Other discouraging factors are that 5.1 production is expensive compared with 2-channel stereo, it is time-consuming, and it requires specialized facilities.
For anyone who is new to multichannel audio, it is essential to demonstrate well-produced examples in order to arouse their interest in 5.1 surround sound. While cost issues may depend on the experience of the engineers and directors involved, various practical recording and mixing techniques can be used to shorten the production time.
Although multichannel audio productions are increasing, 2-channel stereo recording still will be necessary for every production in the future. Therefore, it also is important to solve the problem of how to manage both 2-channel stereo and 5.1 surround sound recordings simultaneously by one engineer, rather than by two engineers separately.Compatibility issues
BS Digital Broadcasting in Japan provides a useful feature for compatibility between 2-channel stereo and 5.1 surround sound. When a broadcaster airs a multichannel audio program, a TV receiver can reproduce either the original 5.1 surround sound or 2-channel stereo, which is down-mixed from the original 5.1 surround sound. For this down-mixing, the mixing ratio of each component (left, center, right, rear left, rear right) to create 2-channel stereo must be chosen carefully in order to maintain the sound impression as faithfully as possible. In the BS Digital Broadcasting system, a broadcaster can select the ratio of down-mixing in the domestic receiver according to the content of the program.
The recording of 5.1 productions also is an issue for broadcasting because most digital videocassette recorders have only four digital audio tracks. In Japan, some broadcasters intend to use a new digital HDTV VCR with eight linear PCM audio tracks in order to maintain high sound quality, while other broadcasters are applying the Dolby-E encoded system to the ordinary digital VCR with four audio tracks, in order to record 5.1 surround sound.Future of multichannel audio for broadcasting
Initially, it was difficult to get viewers interested in listening to multichannel audio, and during the early stages, some viewers reported serious problems such as being unable hear commentaries or vocals on the center channel -- often because they did not have a center loudspeaker, or because of an incorrect balance among channels. Therefore, in addition to explaining the advantages of 5.1 surround sound, it is important that broadcasters properly educate viewers about how to receive and listen to 5.1 surround sound programs, and how to set the loudspeakers.
Thanks to the growing popularity of DVD-video and recent 5.1 surround sound broadcasts such as the World Cup Soccer Games and music programs by famous performers, viewers have more opportunity than ever to listen to multichannel audio, and the situation is now changing. SACD and DVD-audio disks with multichannel audio are also in the marketplace, and it has been reported that the number of viewers in Japan who have equipment for listening to 5.1 surround sound has been increasing.
As some of the problems and issues concerning multichannel audio are expected to be solved soon, the benefits of surround sound to the broadcaster will be to increase the number of viewers of digital broadcasts. In the future, the research of multichannel audio will realize the 'super-surround' audio-visual system that enables the creation of super high-definition and 3-dimensional audio-visual broadcasting, which would be difficult via other media such as the Internet or telecommunication systems.Kimio Hamasaki is a senior research engineer at NHK Science & Technical Research Laboratories.