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HOME THEATER AUDIO CODECS
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A GUIDE TO HOME THEATER AUDIO CODECS
Cables Needed: RCA analog stereo cables.
DOLBY DIGITAL (AC-3) SURROUND SOUND (DD)
The Dolby Digital surround sound format provides up to five discrete full frequency (from 20Hz to 20,000Hz) channels (front left, front right, center, surround left, surround right), plus an optional sixth channel for Low Frequency Effects (LFE). The low frequency effects channel contains only low bass frequencies (3Hz to 120Hz).
Dolby Digital offers a maximum bit rate of 640kbps. Both Blu-Ray and HD-DVD players are required to support DD at its maximum bit rate.
Cables Needed: Toslink (Optical) or Coaxial S/PDIF (Sony/Philips Digital Interconnect Format), HDMI, and Multi-Channel Analog Cables (see footnote).
DIGITAL THEATER SYSTEMS (DTS) DIGITAL SURROUND
Both Blu-Ray and HD-DVD players are required to support DTS at its higher 1.5Mbps bit rate.
Cables Needed: Same as Dolby Digital.
COMPARISON OF DOLBY DIGITAL AND DTS DIGITAL
Both DD and DTS use lossy data reduction techniques for soundtracks in order to minimize the limited space available on a DVD. Dolby Digital can be encoded in 192Kbps (reserved for 1.0 or 2.0 soundtracks and generally lower quality), 384Kbps (better quality), 448Kbps (used on the majority of DVD 5.1 soundtracks), and up to 640Kbps. DTS can be encoded in 754Kbps (the most commonly used), or a maximum rate of 1.5Mbps (very seldom seen). Theoretically, the less compression used in the encoding process, the better the sound quality will be. However, Dolby and DTS use different compression techniques, and their bit rates are not directly comparable to one another. While 448Kbps Dolby Digital encoding is better than 384Kbps Dolby Digital encoding, 754Kbps DTS Digital encoding is not necessarily better than 640Kbps Dolby Digital encoding.
DOLBY DIGITAL EX (THX SURROUND EX) AND DTS EXTENDED SURROUND (DTS-ES)
In November 2001, Dolby Laboratories began to license the Dolby Digital EX (jointly developed by Lucasfilm’s THX division and Dolby Laboratories. Because the surround back channel is not a discrete channel, the correct way to refer to these two formats is “Dolby Digital 5.1 EX Matrix” and “DTS 5.1 ES Matrix”. It would be misleading to refer to them as 6.1-channel or 7.1-channel formats.
DTS-ES Discrete 6.1: A true 6.1-channel format
DTS-ES optionally supports a discrete full-bandwidth surround back channel, independent from the surround left and surround right channels. This is called DTS-ES Discrete 6.1.
DOLBY PRO-LOGIC II & DOLBY PRO-LOGIC IIx
Dolby Pro-Logic is very disappointing when you play a CD or stereo album through it. For this reason, Dolby Laboratories introduced Dolby Pro-Logic II (DPL II). It creates 5.1 discrete channels (5 channels are full-bandwidth) from stereo CDs, old Dolby Surround movies, Laser Discs, and DVDs that were not mastered for 5.1. Pro-Logic IIx, an enhancement over DPL II, converts any stereo or 5.1-channel audio input to 6.1-channel or 7.1-channel output. There are usually two or three modes: Music, Movies, and Games.
DTS Neo:6 is equivalent to Dolby Pro Logic II and IIx. It can convert stereo and matrix content (music or movie) to 5.1 or 6.1 full-bandwidth discrete channels.
DTS 96/24 is a new and enhanced version of DTS Surround and allows encoding of 5.1 channels of 24-bit, 96kHz audio on the DVD-Video format. Prior to the introduction of DTS 96/24, it was only possible to deliver two channels of 24-bit, 96kHz audio on DVD-Video.
DOLBY DIGITAL PLUS (DD+): DD+ is a lossy format that uses a more efficient compression technique at data rates from 96Kbps to 6 Mbps, resulting in better sound quality. Although DD+ can support up to 7.1 discrete channels, the majority of Hollywood movies are only mixed for 5.1. Toslink (Optical) or Coaxial S/PDIF cannot carry a DD+ signal and will automatically play the standard Dolby Digital AC-3 track instead. HDMI cable is needed for transmission of DD+. If the player decodes DD+ to PCM, any version of HDMI cable can transmit the signal. If the player transmits the DD+ signal in bitstream, HDMI 1.3 connection is needed. Multi-Channel Analog Cables can also be used
DTS-HD HIGH RESOLUTION AUDIO (DTS-HD HR) Similar to Dolby Digital Plus, DTS-HD High Resolution is an improved version of the previous DTS Digital formats. It is a lossy format that delivers up to 7.1 channels of sound with sampling frequencies from 48kHZ up to 96 kHz and 24 bit depth resolution. It runs between 1.5Mbps to 6Mbps. Note that DTS-HD HR is sometimes referred to as DTS-HD, which is misleading. Its quality is between DTS-HD Master Audio and the older DTS Digital 5.1 and DTS-ES. Same as DD+, except if Toslink (Optical) or Coaxial S/PDIF cables are used, the player will send the standard DTS Digital Surround signal to the receiver.
AUDIO CODECS FOR HIGH DEFINITION DISCS
A BRIEF INTRODUCTION TO PCM, LOSSY, AND COMPRESSED LOSSLESS AUDIO
The audio on a Blu-Ray or HD-DVD disc is stored in either uncompressed linear Pulse Code Modulation (PCM) or the compressed Dolby TrueHD and DTS Master Audio algorithms.
PCM is a procedure to represent an analog signal in digital form. Its accuracy is dependent upon the Sampling Rate and Sample Size.
Sampling Rate or Sampling Frequency is defined as the number of times samples are taken per second to convert an analog signal to digital. A higher sampling rate (e.g., 96kHz or approximately 96,000 samples per second) allows for higher frequencies to be represented.
Sample Size or Quantization is the number of bits used to represent the analog audio signal each time it is sampled in the analog-to-digital conversion process. A higher bit number allows a more accurate representation of the amplitude of the audio signal, resulting in better dynamic range.
The Bit Rate or Data Rate is the number of bits-per-second that can be processed. It is calculated by multiplying (sampling rate) x (sample size) x (number of channels).
Currently, the very best listening experience to end-users comes with Linear PCM coding on a disc. Unfortunately, 6 channels or more of high-resolution sound take up way too much bandwidth even on a high definition disc format. Using lossy perceptual compression codecs, such as MPEG, Dolby Digital, and DTS, is one solution. Perceptual lossy compression techniques throw away the least significant bits of the audio input. Theoretically, they represent detail that is impossible to hear, or at least difficult to hear. Unfortunately, a lossy codec compresses content such that the result, when decompressed, is not exactly the same as the original master.
Unlike perceptual lossy data reduction, a lossless codec compresses the data without losing any of it when it is decompressed. The result, when decompressed, is exactly the same as the original, with no compromises. Meridian Lossless Packing (MLP), developed by the British high-end audio manufacturer for DVD-A is the original compressed and lossless techniques for recording high resolution audio on a disc. MLP is licensed by Dolby Laboratories and enables up to six channels of 96 kHz/24 bit audio, or two channels of 192 kHz/24 bit audio onto a DVD-Audio disc. Dolby TrueHD, used in Blu-Ray and HD-DVD is based on MLP, and adds 8 or more full-range channels at higher bit rates. DTS Master Audio uses a different compression algorithm.