DTS Coherent Acoustics Coding (CAC) maps discrete 6-channel, 24-bit encoded data onto the 16-bit PCM digital audio stream, which is found on either a DVD or compact disc. The DTS CAC signal is passed via the digital output (either coaxial or optical) present on many DVD and CD players. Any optical medium playback device with a digital output can pass along the DTS signal to a DTS processor and/or A/V Receiver amplifier. Virtually all DVD players will playback both Dolby Digital and DTS. In turn, depending on the encoding, DVD discs have the capacity to playback both an AC-3 (Dolby Digital) and DTS soundtrack.
While Dolby Digital uses a perceptual coding scheme to reduce the bit rate, DTS uses compression technology and the CAC algorithm to do pretty much same thing. The compression ratio is 3.75:1 of a 24-bit PCM digital audio stream with an 8 times oversampling rate. It has a typical data rate of 1,411 kb/s (much higher than Dolby Digital). It performs transparently by coding 24-bit data at a bit-rate lower than 16-bit linear PCM. The Coherent Acoustic Coding algorithm is a scaleable digital coding methodology, which operates on a multirate filterbank. It has been designed to filter the audio signal into frequency bands, which match the critical perceptual bands of the human ear. Within each frequency band the signals are re-quantified at a variable resolution. This is determined by the available bit-rate and an analysis of the long/short periodicity of the audio signal in each frequency band. According to DTS, this allows an efficient sharing of the limited number of quantisation bits without any transient pre-echo distortion. Further, by coding the spectral analysis to extend and include all channels, the re-quantisation routines are fed from a common bit-pool. As well, DTS feels that this optimizes the coding performance and audio quality of each individual channel in a multichannel format delivering a full-bandwidth for each. Essentially, this allows 6 channels of transparent quality 24-bit recorded material at 48 KHz with less digital compression. While Dolby Digital uses a different approach of providing multi-channel sound with low bit-rates, the results are somewhat similar. Dolby Digital is an evolutionary approach building upon what has come before. DTS, on the other hand, is a revolutionary approach that was designed specifically for multi-channel sound from the beginning.
DTS has also spawned several variations including: DTS-ES 6.1 (both Matrix and Discrete), DTS NEO:6, DTS-96/24, and DTS Interactive.
So, as good as Dolby Digital and DTS are today, there are newer surround sound schemes that “push the envelope” even further in providing the listener with additional realism and a 360-degree experience recreating much more of a theater-going experience in either 6.1 or 7.1 digital surround configurations. These new flavors include: Dolby Digital EX, DTS-ES, Dolby Digital Plus, Dolby TrueHD, DTS-HD, and DTS-HD Master Audio.
In most Home Theater set-ups, smaller speakers can be used for the fronts (Left, Center, Right) and dipole speakers for the rear. Dipoles emulate the soundfield of an array of speakers. The use of a subwoofer allows you to be able to use smaller speakers, which fit better, in most living/family rooms environments. Back surround speakers in 6.1 or 7.1 set-ups can be one or two more dipole speakers for a complete 360-degree experience.
Dolby Digital EX
Dolby Digital EX takes the Dolby Digital 5.1-channel setup one step further with an additional center surround channel (reproduced through one or two speakers) for extra dimensional detail and an enveloping surround sound effect. Feature films originally released in Dolby Digital Surround EX (the cinema version) carry the encoded extra surround channel in their subsequent DVD releases, as well as onto 5.1-channel digital satellite and TV broadcasts. If your home theater system has a receiver or preamp/processor with Dolby Digital EX decoding and speakers to support 6.1 or 7.1 playback, you can hear Surround EX soundtracks as they were meant to be heard, with the increased realism created by the extra surround channel. As in the cinema, with regular 5.1-channel Dolby Digital playback no sonic information is lost (although you’ll miss out on the heightened realism). Current Dolby Digital Surround EX soundtracks contain a digital flag that can automatically activate the EX decoding in a receiver or preamp/processor.
DTS-ES 6.1, which stands for Extended Surround, builds on the DTS aural experience by utilizing 6.1 discrete channels of audio information with the help of a back surround channel matrix, which is encoded into the left and right surround channels. While other matrix 6.1 surrounds are input and decoded in the analog domain, DTS-ES is entirely decoded in the digital domain with 24-bit precision and full octave equalization. What this means is that DTS-ES 6.1 allows for a fully discrete back surround channel as opposed to the matrix-encoded back surround channel of conventional Surround EX-encoded soundtracks. DTS-ES translates back channel information into precise sound effect positioning and imaging with no sound bleed into the left and right surround channels, or for a collapsing of left and right surround information toward the back surround channel. There are two types of DTS-ES: Matrix and Discrete. The Matrix version derives a matrixed back channel from standard 5.1-encoded software. Discrete, on the other hand, is, as the name implies a discrete sixth channel of aural information encoded on the disc for a fully-discrete back channel.
Dolby Digital Plus
According to Dolby, Dolby Digital Plus is audio that completes the high-definition picture. Dolby Digital Plus is the next-generation audio codec technology for all high-definition programming and media. It combines the efficiency to meet future broadcast demands with the power and flexibility to realize the full audio potential of the upcoming high-definition experience. Built upon Dolby Digital, Dolby Digital Plus was designed for the delivery formats of the future, but remains fully compatible with all current A/V receivers. With Dolby Digital Plus, you get even higher quality audio, more channels, and greater flexibility. It features: Multichannel sound with discrete channel output, channel and program extensions can carry multichannel audio programs of up to 7.1 channels and support multiple programs in a single encoded bitstream. Plus outputs a Dolby Digital bitstream for playback on existing Dolby Digital systems, supports data rates as high as 6 Mbps, bit rate performance of at least 3 Mbps on HD DVD and up to 1.7 Mbps on Blu-ray Disc, and accurately reproduces what the director and producer intended, interactive mixing and streaming capability in advanced systems, and supported by HDMI (Version 1.2a and above).
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Dennis has been involved with Consumer Electronics forever it seems. His 25+-year career includes a 12-year tour of duty at Consumer Reports magazine, as well as stints as a product reviewer, market analyst, technical editor, and consultant for the electronics industry. He lives in Ossining, NY with his two children, one demanding cat and piles of A/V equipment.