DTS (sound System) - History

History

Work on the format started in 1991, four years after Dolby Labs started work on its new codec, Dolby Digital. The basic and most common version of the format is a 5.1-channel system, similar to a Dolby Digital setup, which encodes the audio as five primary (full-range) channels plus a special LFE (low-frequency effects) channel for the subwoofer.

Note, however, that encoders and decoders support numerous channel combinations, and stereo, four-channel, and four-channel+LFE soundtracks have been released commercially on DVD, CD, and Laserdisc.

Other, newer DTS variants are also currently available, including versions that support up to seven primary audio channels plus one LFE channel (DTS-ES). These variants are generally based on DTS's core-and-extension philosophy, in which a core DTS data stream is augmented with an extension stream which includes the additional data necessary for the new variant in use. The core stream can be decoded by any DTS decoder, even if it does not understand the new variant. A decoder which does understand the new variant decodes the core stream, and then modifies it according to the instructions contained in the extension stream. This method allows backward compatibility.

DTS's main competitors in multichannel theatrical audio are Dolby Digital and SDDS, although only Dolby Digital and DTS are used on DVDs and implemented in home theater hardware.

One of the DTS Inc.'s initial investors was film director Steven Spielberg, who felt that theatrical sound formats up until the company's founding were no longer state of the art, and as a result were no longer optimal for use on projects where quality sound reproduction was of the utmost importance. Spielberg debuted the format with his 1993 production of Jurassic Park, which came slightly less than a full year after the official theatrical debut of Dolby Digital (Batman Returns). In addition, Jurassic Park also became the first home video release to contain DTS sound when it was released on LaserDisc in January 1997, two years after the first Dolby Digital home video release (Clear and Present Danger on Laserdisc), which debuted in January 1995.

In theatrical use, a proprietary 24-bit time code is optically imaged onto the film. An LED reader scans the timecode data from the film and sends it to the DTS processor, using the time code to synchronize the projected image with the DTS soundtrack audio. The multi-channel DTS audio is recorded in compressed form on standard CD-ROM media at a bitrate of 882 kbit/s. The audio compression used in the theatrical DTS system (which is very different and completely unrelated to the home Coherent Acoustics-based DTS Digital Surround format) is the APT-X100 system. Unlike the home version of DTS or any version of Dolby Digital, the APT-X100 system is fixed at a 4:1 compression ratio. Data reduction is accomplished via sub-band coding with linear prediction and adaptive quantization. The theatrical DTS processor acts as a transport mechanism, as it holds and reads the audio discs. When the DTS format was launched, it used one or two discs with later units holding three discs, thus allowing a single dts processor to handle two-disc film soundtracks along with a third disc for theatrical trailers. The DTS time code on the 35mm print identifies the film title which is matched to the individual DTS CD-ROMs, guaranteeing that the film cannot be played with the wrong disc. Each DTS CD-ROM contains a DOS program that the processor uses to play back the soundtrack, allowing system improvements or bug fixes to be added easily. Unlike Dolby Digital and SDDS, or the home version of DTS, the theatrical DTS system only carries 5 discrete channels on the CD-ROMs. The .1 LFE subwoofer track is mixed into the discrete surround channels on the disc and recovered via low-pass filters in the theater.

DTS and Dolby Digital (AC-3), DTS's chief competitor in the cinema and home theater market, are often compared due to their similarity in product goals. In theatrical installations, AC-3 audio is placed between sprocket holes, leaving the audio content susceptible to physical damage due to film wear and mishandling. DTS audio is stored on a separate set of CD-ROM media, whose greater storage capacity affords the potential to deliver better audio fidelity. However, the separation of print film and audio track has both pluses and negatives. AC-3 (and SDDS) reside entirely on the 35 mm film itself, simplifying distribution by eliminating an extra (optional) deliverable. But DTS's CD-ROM media is not subject to the usual wear and damage suffered by the film print during the normal course of the movie's theatrical screening. Disregarding the separate CD-ROM assembly as a potential point of failure, the DTS audiopath is comparatively impervious to film degradation, unless the film-printed timecode is completely destroyed.

In the consumer (home theater) market, AC-3 and DTS are close in terms of audio performance. When the DTS audio track is encoded at its highest legal bitrate (1509.75 kbit/s), technical experts rank DTS as perceptually transparent for most audio program material (i.e., indistinguishable to the uncoded source in a double blind test). Dolby claims its competing AC-3 codec achieves similar transparency at its highest coded bitrate (640 kbit/s). However, in program material available to home consumers (DVD, broadcast, and subscription digital TV), neither AC-3 nor DTS typically run at their highest allowed bitrate. DVD and broadcast (ATSC) HDTV cap AC-3 bitrate at 448 kbit/s. But even at that rate, consumer audio gear already enjoys better audio performance than theatrical (35 mm movie) installations, which are limited to even lower bitrates. When DTS audio was introduced to the DVD specification, studios authored DVD movies at DTS's full bitrate (1509.75 kbit/s). Later, movie titles were almost always encoded at a reduced bitrate of 754.5 kbit/s, ostensibly to increase the number of audio tracks on the movie disc. At this reduced rate (754.5 kbit/s), DTS no longer retains audio transparency.

AC-3 and DTS are sometimes judged by their encoded bitrates. DTS proponents claim that the extra bits give higher fidelity and more dynamic range, providing a richer and more lifelike sound. But no conclusion can be drawn from their respective bitrates, as each codec relies on different coding tools and syntax to compress audio.