Guide to Surround Sound Decoding
Surround sound has become an essential element of our entertainment, but understanding the different ways in which it's delivered can be complicated. Here, we'll try to break down the basics and provide a guide to common surround formats and what differentiates them from one another.
What's a surround sound format?
Representing high-quality sound digitally requires the capture and transmission of huge amounts of audio information. This information consumes a tremendous amount of storage space (on Blu-ray, DVD and video-game discs) or bandwidth (for television or Internet broadcasts).
Since disc storage and bandwidth have physical limits, digital audio is usually encoded in one or several different formats, using a process called data compression to compact the information down to a more manageable size. The encoded audio must then be decoded by your home theater equipment before it can be fed to your speakers and turned into sound (that's why surround sound formats are sometimes referred to as "codecs," shorthand for encoder/decoder).
So, fundamentally, in order to enjoy surround sound encoded in any format, your system must include at least one component (usually a home theater receiver or Blu-ray player) with a surround sound decoder that supports that format.
Understanding data compression
Data compression for surround sound applications comes in two flavors:
- In "lossy" compression, a complex series of algorithms is employed to identify portions of the audio signal that are deemed least essential to the human listening experience. These less critical sonic cues are discarded in the encoded version, resulting in a significant reduction in file size with minimal perceived impact to the sonic result when decoded. Lossy surround codecs vary in quality based on numerous technical factors, including:
- Sampling frequency — Higher sampling rates translate to wider frequency response (that is, the range between the highest highs and the lowest lows that a system can reproduce).
- Bit depth — Greater bit depths translate to wider dynamic range (that is, the range between the loudest and quietest sounds that a system can resolve at a given level of amplification).
- Bit rate — This represents the amount of digital information a system can process in a given amount of time. A system's bit rate essentially defines its sonic limits; sampling frequency and bit depth of a soundtrack can be adjusted in the authoring process to produce a data combination that can be processed within the limits of the system's bit rate.
Identifying available formats and decoders
- on media packaging to identify the format(s) encoded on a disc, and
- on the front panels of receivers and Blu-ray players to identify decoders supported by each device.
The tabs below list and describe the most common formats in ascending order of quality, with relatively comparable Dolby and DTS technologies listed side by side.
• Lossy compression formats
The worldwide standard in audio encoding — featured as an audio option on every DVD, Blu-ray Disc and HDTV broadcast — Dolby Digital delivers up to five discrete channels of full-range audio. Front left, front right, front center, surround left and surround right channels are augmented by an LFE (low frequency effects) channel (the ".1" in "5.1-channel") for an enveloping audio experience.
Offered as an audio option on select DVDs and all Blu-ray Discs, DTS (formally known as DTS Digital Surround) delivers up to 5 discrete channels of full-range audio; the decoder includes a "bass management" facility that channels low-frequency information to a separate subwoofer channel. DTS offers a slightly higher bit rate and the ability to operate at a superior 48kHz sampling frequency (vs. 44.1kHz for Dolby Digital), and some users prefer it over Dolby Digital.
Dolby Digital EX
An extension on the original Dolby Digital 5.1-channel surround codec, Dolby Digital EX is offered as an audio option on select DVDs and Blu-ray Discs. EX encoding adds the capacity for a centrally located rear surround channel, matrix-encoded into the left and right surround channels and extracted upon playback. This additional channel may be played back via either one or two additional speakers (in a 6.1- or 7.1-channel speaker configuration, respectively
A 6.1-channel extension on the original DTS Digital Surround 5.1-channel surround format, DTS-ES is offered on select DVDs. The ES extension (short for "Extended Surround") adds capacity for a fully discrete center-surround channel, positioned between the left and right rear speakers (DTS-ES Discrete 6.1). It can also support a non-discrete center-surround signal that is matrix-encoded (DTS-ES Matrix 6.1) into the left and right surround channels and extracted on playback (similar to the analogous Dolby Digital EX).
(no comparable Dolby format)
DTS 96/24 offers a higher quality level for multichannel audio than DTS Digital Surround. Offered on select DVDs and Blu-ray Discs, it delivers up to five discrete full-range channels at a higher bit rate than DTS; the decoder includes a "bass management" facility that channels low-frequency information to a separate subwoofer channel. DTS 96/24's improved 96kHz sampling frequency and 24-bit depth enable it to capture more audio information than DTS.
Dolby Digital Plus
An enhanced version of the original Dolby Digital surround format, Dolby Digital Plus is offered as an audio option on select Blu-ray Discs and HD broadcasts. It supports up to seven discrete channels of high-bit rate full-range audio, plus an LFE (low frequency effects) channel. The higher bit rate and bit depth of Dolby Digital Plus let it retain more audio information than Dolby Digital, and thus reproduce a more accurate representation of the original soundtrack.
DTS-HD High Resolution (DTS-HD HR)
Offered as an audio option on select Blu-ray Discs and HD broadcasts, DTS-HD High Resolution is an enhanced version of the original DTS Digital Surround format. It supports up to seven discrete channels of full-range audio; the decoder includes a "bass management" facility that channels low-frequency information to a separate subwoofer channel. DTS-HD HR's higher data rates and more efficient compression deliver significantly improved sound quality over DTS or DTS 96/24.
• Lossless compression formats
Offered as an audio option on select Blu-ray Discs and typically deliverable only via an HDMI connection, Dolby TrueHD is capable of an unlimited number of audio tracks, but the Blu-ray audio standard limits its practical use to a maximum of 7.1 channels.
Note: While many Blu-ray players are equipped with Dolby TrueHD decoding capability, others feature "Dolby TrueHD digital out." Players in the latter group must be connected via HDMI to a receiver with Dolby TrueHD decoding to realize the benefits of Dolby TrueHD.
DTS-HD Master Audio (DTS-HD MA)
Offered as an audio option on select Blu-ray Discs, and typically deliverable only via an HDMI connection, DTS-HD Master Audio delivers up to 7.1 channels of surround sound at a super-high bit rate, adding a pair of side-surround channels to the standard DTS 5.1 configuration for seamless spatial effects.
Note: While many Blu-ray players are equipped with DTS-HD decoding capability, others feature "DTS-HD bitstream output." Players in the latter group must be connected via HDMI to a receiver with DTS-HD decoding to realize the full benefits of DTS-HD. However, older receivers equipped only with DTS Digital Surround decoding will still benefit from the faster-bit rate encoding of DTS HD Master Audio, delivering audio that's noticeably superior to standard DTS playback.
• Uncompressed audio
Linear PCM (LPCM)
Linear PCM, sometimes notated as "LPCM," is an uncompressed audio format — essentially, an exact digital copy of the original studio master recording. Offered as an audio option on select Blu-ray Discs, and typically deliverable only via an HDMI connection, LPCM can deliver up to eight channels of audio at up to 192kHz sampling frequency and a depth of up to 24 bits per sample. Its maximum bit rate of 27.6MBps exceeds those of both Dolby TrueHD (18MBps) and DTS-HD Master Audio (24.5MBps).
Audio encoded in LPCM is free from any "compression artifacts" (audible distortions of the original signal caused by the loss of data in the compression process), but this in itself is not a guarantee of superior audio quality. Because both uncompressed audio and high-definition video require huge amounts of the finite available storage space on a disc, a balance between the two must be struck in the authoring process. Producers of Blu-ray Discs may choose to reduce sampling frequency, bit depth or both in the LPCM encoding in order to avoid shortchanging the picture quality of the disc