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 technologies Most DTS consumer codec techniques are documented in ETSI. DTS Digital Surround, DTS-ES, DTS 96/24, and DTS-HD are recorded in ETSI TS 102 114. DTS:X is recorded in ETSI TS 103 491. DTS Digital Surround On the consumer level, DTS is the oft-used shorthand for the DTS Coherent Acoustics (DCA) codec, transportable through S/PDIF and part of the LaserDisc, DVD, and Blu-ray specifications. This system is the consumer version of the DTS standard, using a similar codec without needing separate DTS CD-ROM media. As with standard CD players, DVD and Blu-ray Disc players cannot decode audio from DTS audio CDs. DTS is related to the aptX audio coding format, and it is based on the adaptive differential pulse-code modulation (ADPCM) audio data compression algorithm. In contrast, Dolby Digital (AC-3) is based on the modified discrete cosine transform (MDCT) compression algorithm. Both music and movie DVDs allow delivery of a DTS audio signal, but DTS was not part of the original DVD specification (it was added later in 1997), so early DVD players do not recognize DTS audio tracks at all. The DVD specification was revised to allow optional inclusion of DTS audio tracks. The DVD title must carry one or more primary audio tracks in AC-3 or LPCM format (in Europe, MPEG-1 Audio Layer II is also an allowed primary track format). The DTS audio track, if present, can be selected by the user. Subsequent DVD players now decode DTS natively or pass it through to an external decoder. Nearly