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

What Is Dolby Digital?

Dolby Digital is a digital audio encoding system designed for DVD, Blu-ray Disc, Ultra HD Blu-ray Disc, and, in some cases, cable TV 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 or 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 Disc, and Ultra HD Blu-ray Disc players can pass Dolby Digital signals using a technique called bitstream, to properly equipped receivers for decoding.

Dolby Digital Channel Configurations

Here are the channel options when using Dolby Digital:

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, or AC3.

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

What Is Dolby Digital EX?

The Dolby Digital EX format is similar to Dolby Digital. It’s used in the same way, but it adds a third surround channel speaker behind the listener, making it a 6.1 channel system. The channels are represented by six speakers (left, center, right, left surround, center back, and 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 another source content, that contains 6.1 channel EX encoding and your receiver doesn’t have an EX decoder, the receiver defaults to Dolby Digital 5.1. The extra EX information is sorted and distributed (or mixed) within a 5.1 channel sound field. This means that the sixth (center back) channel information is placed into both the left and right surround channels as a mono signal. This creates a phantom rear center backchannel without the presence of a physical rear center speaker.

This isn’t 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. However, you still hear the sound that was originally encoded for the center backchannel.

What Is Dolby Digital Plus?

Dolby Digital Plus is a high-definition digital-based surround sound format that supports up to eight 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’s compatible with standard Dolby Digital-equipped receivers. This means that on a 5.1 channel receiver, you 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 used in the Blu-ray Disc format. It’s compatible with the audio portion of the HDMI interface and is used in streaming and mobile audio applications. Dolby Digital Plus is built into the Dolby Audio platform for Windows 10 and the Microsoft Edge browser.

In addition to the Dolby surround sound formats discussed above, there are two higher-end Dolby audio formats: Dolby TrueHD and Dolby Atmos.

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