Surround sound is an integral part of the home theater experience, and with that, there are lots of surround sound formats in use, depending on your audio system's capabilities, speaker layout, and content.

Probably the most used are formats that are part of the Dolby Digital family. In this article, we discuss three: Dolby Digital, Dolby Digital EX, and Dolby Digital Plus. These formats are commonly used on DVDs and streaming content and are also present as a supplemental selection in Blu-ray and Ultra HD Disc content.

What Dolby Digital Is

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, for TV broadcast and streaming content. This system provides efficient transfer for audio signals that may be composed of 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 in use have a built-in Dolby Digital decoder and all DVD, Blu-ray Disc, and Ultra HD Blu-ray Disc players are equipped with the ability to 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 how many channels it has. Dolby Digital may also be referred to asDD, DD 5.1, AC3

• Monophonic: Represented by one or two speakers. However, 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 adds a third surround channel that is placed directly behind the listener. The means that it is a 6.1 Channel system.

The six channels are represented by six speakers (left, center, right, left surround, center back, right surround), and a subwoofer (.1.).

This means that the listener has both a front center channel and, with Dolby Digital EX, a rear center channel. If you are losing count, the channels are labeled: left front, center front, right front, left surround, right surround, subwoofer, with a surround back center. A home theater receiver with a Dolby Digital EX decoder is required to access the full 6.1 channel experience.

However, if you have a DVD, or other source content, that contains 6.1 channel EX encoding and your receiver does not have an EX decoder, the receiver will default to Dolby Digital 5.1. What it does is sort out the extra EX information and distribute/mix it within a 5.1 channel sound field.

Usually, this means that the 6th (center back) channel is placed into both the left and right surround channels as a mono signal, which in turn creates a 'phantom' rear center back channel without having to have a physical rear center speaker.

This is not as accurate as a setup that includes 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 back channel.

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.

In addition, Dolby Digital Plus contains 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 employed by Blu-ray Disc format. Dolby Digital Plus is compatible with the audio portion of the HDMI interface, as well as being applied 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.

Although Dolby Digital Plus has its own specific label designation, in many applications, Dolby Digital 5.1 and 6.1 (EX) are often referred to as just Dolby Digital.

No matter which format in the Dolby Digital family discussed above you have access to, the goal is to provide a room-filling surround sound listening experience that enhances the home theater viewing experience or a fuller audio experience from a compatible home theater receiver, PC, or portable device.

However, depending on your content and components, there are additional Dolby home theater audio formats to be aware of including Dolby TrueHD and Dolby Atmos.