Dolby Digital, Dolby Digital EX, and Dolby Digital Plus

Providing better audio quality for movie and TV viewing

Surround sound is an integral part of the home theater experience, and there are lots of formats in use. The most familiar formats are part of the Dolby Digital family. Below we discuss three: Dolby Digital, Dolby Digital EX, and Dolby Digital Plus.

Dolby Digital Logo
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Dolby Digital

Dolby Digital is a digital audio encoding system designed for use on DVDs, Blu-ray, and Ultra HD Blu-ray Discs, and, in some cases, TV broadcast and streaming content. This format provides efficient transfer for audio signals that may have one, or more channels, that can be decoded by a home theater receiver or AV Preamp/Processor with a Dolby Digital decoder and distributed to one or more speakers.

Almost all home theater receivers have a built-in Dolby Digital decoder. All DVD, Blu-ray, and Ultra HD Blu-ray Disc players can pass Dolby Digital signals via a technique referred to as bitstream to properly equipped receivers for decoding.

Dolby Digital is often referred to as a 5.1 channel surround system. However, the term "Dolby Digital" refers to the digital encoding of the audio signal, not the number of channels it has. Dolby Digital may also be referred to as DD, DD 5.1, AC3

Dolby Digital Channel Options

  • Monophonic: Represented by one or two speakers. With two speakers, both reproduce the same sound so the sound will appear to come from the space between the speakers.
  • 2-Channels: Represented by two speakers — one on the left and on the right front of the listening position.
  • 4-Channels: Represented by four speakers — Two placed on the left and right front of the listening position, and two on the left and right, and slightly behind the listening position.
  • 5.1 Channels: Represented by five speakers (left, center, right, left surround, right surround), and a subwoofer (.1)

Dolby Digital EX

The Dolby Digital EX format is used in the same applications as Dolby Digital but adds a third surround channel speaker placed directly behind the listener, making it a 6.1 Channel system. The channels are represented by six speakers (left, center, right, left surround, center back, right surround), and a subwoofer (.1). This means there are both front and rear center channels. A home theater receiver with a Dolby Digital EX decoder is required to access the full 6.1 channel experience.

If you have a DVD, or other source content, that contains 6.1 channel EX encoding and your receiver doesn't have an EX decoder, the receiver will default to Dolby Digital 5.1. The extra EX information is sorted and distributed/mixed within a 5.1 channel sound field.

This means that the 6th (center back) channel information is placed into both the left and right surround channels as a mono signal, which creates a "phantom" rear center backchannel without the presence of a physical rear center speaker.

This is not as accurate as having a dedicated rear center channel speaker as the output level can't be adjusted independently of the left and right surround channels, but you are still able to hear the sound that was originally encoded for the center backchannel.

Dolby Digital Plus

Dolby Digital Plus is a high definition digital-based surround sound format that supports up to 8-channels (7.1) of surround decoding. The channel distribution is as follows: front left, front center, front right, left surround, right surround, left surround back, right surround back, and subwoofer.

Dolby Digital Plus also includes a standard Dolby Digital 5.1 bitstream that is compatible with standard Dolby Digital-equipped receivers. This means on a 5.1 channel receiver you will hear a 5.1 channel mix of the soundtrack, rather than a 7.1 channel mix. The surround back left and right channels are folded into the left and right surround channels.

Dolby Digital Plus is one of the several audio formats that can be used in the Blu-ray Disc format. It's compatible with the audio portion of the HDMI interface, as well as being available for use in streaming and mobile audio applications. Dolby Digital Plus is also built into the Dolby Audio platform for Windows 10 and the Microsoft Edge browser.

The Bottom Line

No matter which format in the Dolby Digital family discussed above you have access to, each provides room-filling surround sound for home theater or a fuller audio experience from a PC or portable device.

In addition to the Dolby surround sound formats discussed above, there are two higher-end Dolby audio formats you may encounter. For all the details, read our companion articles on Dolby TrueHD and Dolby Atmos.