Dolby Digital and DTS
Dolby Digital is the venerable and familiar audio format, also known as AC-3, used on conventional DVDs; it’s also a base level audio standard for Blu-ray discs. It works just like it did on DVDs in sound schemes that range from 1.0 to 5.1, but offers a higher maximum bit rate of 640 kbps on Blu-ray, and sounds just like Dolby Digital Plus encoded at that same bit rate. (As you’ll see in the section on Dolby Digital Plus, however, its maximum bit rate is considerably higher, though seldom used.)
All Dolby logos courtesy of Dolby Laboratories.
All Blu-ray players must support Dolby Digital. When players can’t deliver higher bit rate Dolby encodings, they always default to Dolby Digital playback. Blu-ray requires that any time a high bitrate version of Dolby is encoded on a disc, it will also include a plain-vanilla Dolby Digital track as well. Thus if decoding of another higher bitrate form of Dolby audio doesn’t succeed, you will still be able to play back a Dolby Digital 5.1 audio track, with its maximum 640 Kbps bit rate.
Typical Dolby Digital audio tracks include the following information:
- The Audio Codec is Dolby Digital.
- The Audio Channels (sound scheme) can appear as 1.0 (rare, only for older movies), 2.0 (likewise, though not as old), or 5.1 (most typical).
- Audio Fidelity data does not appear for this encoding, but the Dolby Digital specification indicates that it is always 16 bits, and can be 32 kHz, 44.1 kHz (the typical audio CD frequency), or 48 kHz (typical for DVDs and Blu-ray discs).
- Audio Bit Rate values are usually 640 kbps (most 5.1 discs), but may be either 192 kbps (1.0) to 448 kbps (2.0 and some 5.1). 448 kbps is the maximum bit rate for conventional DVDs.
See Table 1 for information about sound schemes, SPDIF, and HDMI handling for Dolby Digital. Both Dolby Digital and DTS (covered in the next section) use what’s called “lossy compression” to create audio tracks. In the simplest of terms this means that audio information gets lost, and some sound fidely compromised (as compared to the original studio master recording) during the process of performing the data manipulations and reductions necessary to squeeze these types of audio tracks into their recorded forms. According to some sources, Dolby Digital can achieve compression ratios as high as 12:1, though ratios in a range 4:1 to 6:1 are more common. Dolby Digital is also a constant bitrate (CBR) encoding, which means it operates at the same bitrate at all times, regardless of the compressed audio material it is handling at any given moment.
DTS is another venerable and familiar sound format that originated for use on conventional DVDs. Like Dolby Digital, it is a compressed format that supports sound schemes that range from 1.0 to 5.1, where 5.1 is by far the most commonly used on Blu-ray discs. DTS supports a maximum bit rate of 1.5 Mbps, and Blu-ray players are generally more than equal to the task of handling this kind of data rate. All Blu-ray players must support transmission of a DTS bitstream across a digital link, and be able to decode at least two channels internally; that said, the vast majority of Blu-ray players can decode 5.1.
All DTS logos courtesy of DTS, Inc.
The original 1997 DVD specification did not mention DTS, so older DVD players do not recognize DTS audio tracks. All modern DVD players can decode DTS internally, or pass it on to an external decoder through a digital link of some kind, but it is often reduced to a bit rate of 768 kbps. Higher bit rate DTS encodings are built around a DTS core with a maximum bit rate of 1.5 Mbps, so that if extensions fail to decode, base level DTS audio still remains available for processing or forwarding. If any DTS encoding is present on a Blu-ray disc, you can count on obtaining base-level DTS audio at a minimum level from a Blu-ray player.
A typical DTS audio track data will include the following information:
- The Audio Codec is DTS.
- The Audio Channels (sound scheme) can appear as 2.0 (rare, only for older movies), 3.0 (likewise, though not as old), or 5.1 (most typical).
- Audio Fidelity data does not appear for this encoding, but the DTS specification indicates it can be either 16 or 24 bits deep, and can be recorded at 48 kHz (typical for DVDs and Blu-ray discs) or 96 kHz.
- Audio Bit Rate values are usually 1.5 Mbps (for most 5.1 Blu-ray discs), but may be at 768 Kbps, a typical value for most conventional DVDs that include DTS encoded audio material.
See Table 1 for information about sound schemes, SPDIF, and HDMI handling for DTS. According to some sources, DTS can achieve compression ratios as high as 10:1, though ratios around 4:1 are more common. DTS is another constant bitrate (CBR) encoding, which means it operates at the same bitrate at all times, regardless of the structure of the audio material it is handling at any given moment. This arrangement tends to favor data reduction at the cost of audio quality, just as the use of a lossy compression algorithm does likewise.