Surround sound technology has been a part of the broadcast world since TV in the U.S. went stereo in the mid-1980s. The decoding format at the time was Dolby Surround, a matrixed encoded three-channel (L,R,S) audio soundtrack, a descendent from the film industry that is capable of being transported on a two-channel, stereo infrastructure. This proved to be a terrific advancement for TV stations, requiring only a minimal amount of upgrading to implement and brand their stereo stations with surround capabilities.
Much has happened since this innovation. Currently, there are two primary surround sound formats from Dolby Labs for the film, consumer electronics and television industry - one of which is Dolby Digital, the exclusive standard for ATSC Digital Television in the U.S. The other is the Dolby Surround format. Each of these has evolved to include added features as explained below. Digital Theater Systems (DTS) has multiple film and video formats and SDDS (Sony Digital Dynamic Sound) has a competing film format as well.
Formats for TV and film Dolby Surround: Loosely termed 3 Stereo, this passive surround decoding technology was first introduced to the upstart home theater market in the early to mid-1980s. This technology lacks an active steered center channel but provides a mono surround channel for listeners. Dialog information is created by the left/right speakers and reproduced as a phantom center channel image. As long as the left/right audio signals are of equal level and the listener is in the correct viewing position, dialogue comes from the center of the home theater system, hopefully the same place the image is located. The rear channels incorporate a form of Dolby B noise reduction and have a limited frequency response of 100- to 7000Hz. Consumer Dolby Surround gear has been supplanted by Pro Logic equipment.
Dolby Pro Logic expands on passive surround by providing the listener with a steered center channel speaker output designed to draw dialog and effects to the screen of the program. This 1987 innovation allows the audience to sit anywhere in the room and always localize the dialog and center effects information as coming from the picture, as long as the center speaker is installed above or below the screen.
Pro-Logic II expands on standard Pro-Logic by offering additional movie and music modes to the decoder offering a more discreet-like experience to the rear channels for the listener.
Dolby Digital: In the early 1990s, Dolby Digital was introduced to the motion picture industry and subsequently started to hit the consumer video marketplace about five years later. This new technology brought digital reproduction with it and eliminates matrixing and the frequency response limitations of passive surround and Pro Logic. It's capable of up to six discreet digital channels, five of which can be full frequency. The system is based on audio bit-reduction technology. It uses perceptual coding and auditory masking to eliminate information in the original audio that is undetected by the human ear. (See Production Clips, BE February 1999.) This encoding and decoding process uses the Dolby AC-3 lossy bit-reduction algorithm. This process significantly lends itself to video technologies like DVD-V, LD and DTV where the image and ancillary data require massive amounts of bandwidth from the comprehensive transport stream.
Dolby Digital Surround EX adds an active steered rear center channel to a 5.1 soundtrack. It does this by matrix decoding of the left and right surround channels but does not require any additional channel infrastructure before decoding to do so.
DTS: Digital Theater Systems' first soundtrack was introduced in 1993. 5.1-channel DTS technology is used in motion picture, DVD-V, -LD and -CD formats. DTS' perceptual coder uses a lossy algorithm and operates at a higher bit rate than Dolby AC-3. Because of this process, DTS claims more accurate reproduction. However, the increased bandwidth required over AC-3 prohibits use of DTS in some applications.
In June, a new DTS-ES program introduced DTS-ES Discreet, Matrix 6.1 and DTS Neo:6, all of which add an additional channel to existing rear channel surround reproduction.
SDDS: Sony Dynamic Digital Sound is a motion picture-only format introduced in 1994. It boasts eight channels of discreet digital sound. The bit-reduction process is a lossy type utilizing the Sony ATRAC algorithm, the same technology used for Sony MiniDisc.
ATSC audio: How does it fit in? Dolby Digital is the consumer name for products incorporating Dolby AC-3 encoding and decoding and, as stated above, is the standard for ATSC audio. Dolby Digital, however, does not necessitate 5.1-channel sound, as it may be misunderstood to do so, although it may be configured for 5.1 channels of audio. Dolby Digital programs may be recorded or transmitted in mono, stereo, matrixed surround (LT- RT) and a variety of multichannel configurations. A 3/2 (three front, two rear) channel setup and an additional low-frequency enhancement channel (LFE) is the maximum number of discreet channels possible, totaling 5.1.
This 5.1-channel audio payload is carried at a 384kb/s ATSC rate and has a frequency response for the main five channels of about 20- to 19kHz. The LFE is band limited to 120Hz.
The low-frequency enhancement channel may contain special discreet audio program content (additional effects like booms and crashes) and is not the same as the subwoofer's role. Therefore it's different than and in addition to the other low frequency signals that are contained as part of the main channels. Also, the LFE content is not downmix compatible. This means that if a DTV listener's monitoring equipment is not capable of 5.1-channel reproduction, an automatic downmix of the audio is usually available on the consumer decoder. This output supplies a Pro Logic-, stereo- or mono-compatible signal and is available at the digital audio output connection of the device and/or the analog RCA type connectors. Any low-frequency information contained in only the LFE will not be reproduced at this output. It is therefore necessary to make certain that all or some of any critical low-frequency LFE information is redirected to the main channels during a 5.1 mix or remix by the broadcaster, program supplier or re-mixer.
Mixing Surround Sound: NTSC vs. ATSC Analog world: Mixing in Dolby Surround is an effective way to enhance an audio program with little additional equipment impact on the stereo TV infrastructure. A mixing console capable of at least four output buses and a suitable monitoring system is necessary, along with a surround encoder and decoder for creating multichannel programs. If properly mixed, a four-channel program mixed left-center-right-surround (LCRS) can be matrix encoded and the broadcast will be surround-, stereo- and mono-compatible. This composite-matrixed program will connect up within the plant on a standard stereo pair and can be decoded by the audience on Dolby Surround, Dolby Pro Logic and Dolby Digital consumer equipment.
The digital domain: Along with issues like 5.1 capability, the LFE and bit rates, it's also important to note that the DTV mixing engineer must establish the proper limiting of sound sources to prevent distortion. Unlike NTSC broadcasting, there may be no safeguard downstream to correct for errors. Increased dynamics are a goal, but proper levels must be maintained at the console (especially if it's digital) to avoid distortion from too hot a signal. Even metadata, as described below, can't repair a faulty signal once it's recorded. Levels can easily be monitored on any contemporary digital meter, especially the ones reading VU and peak simultaneously. Headroom should be used but cannot be completely used up. The digital path is not as forgiving as its analog counterpart and DTV's dynamics may be wide open right into the listener's living room.
The metadata element Unlike NTSC analog audio, ATSC digital audio is accompanied by metadata information that is always present with the Dolby AC-3 bitstream. This data is used to adjust the audio at the listener's receiver/decoder. Metadata supplied by the producer or broadcaster allows the audio to remain in a fairly unprocessed form throughout the television plant and through transmission. Its intent is to both automatically adjust certain parameters like dialog levels and to allow the audience to select other pre-established settings like dynamic range choices. Other metadata establishes the amount and configuration of the channels; i.e. two-channel, 5.1 or Dolby Surround encoded, to name a few of the remaining settings.
Dolby E Dolby E was developed to handle transporting multichannel audio within a digital television plant limited to a two- or four-channel AES audio infrastructure. It provides the necessary link at the distribution stage, the process in between contribution/production and emission of audio signals. Similar to AC-3/Dolby Digital, Dolby E has metadata capabilities compatible with the ATSC system. However, Dolby E's algorithm works at a higher bit rate than Dolby Digital and is capable of eight channels of audio encoded on a single AES pair including the metadata. Its signal is timed to the TV plant and is frame-compatible with video. Its milder codec is capable of about 10 cascades without noticeable degradation to the sound.
Dolby E provides a solution for transporting 5.1 channels of audio plus additional LT, RT or SAP channels along with metadata. It does this without the station having to expand its baseband infrastructure beyond an AES pair.
Four Channel Left-Center-Right-Surround (LCRS) program mixing at the professional level and LCRS (Surround and Pro-Logic) monitoring at the consumer level have blazed the trail for today's cutting edge 5.1 channel audio.
Viewers will continue to demand bigger and better quality audio programming to accompany their DTV pictures. Audiences are purchasing more and more multichannel home theater equipment, clearly stating their desire for outstanding audio soundtracks to support the amazing TV pictures DTV is capable of.
