Dolby Surround Upmixer

What It Does

Dolby Surround Upmixer (DSU) is a post-decoding audio processing mode that expands 2-channel or 5.1 audio to play over systems with more speakers, such as 7.1 or 5.1.2 configurations. It is an evolution from Dolby Pro Logic IIz and replaces Dolby Pro Logic, Dolby Pro Logic II, Dolby Pro Logic IIx, and Dolby Pro Logic IIz on home theater receivers and processors.

The technology can process channel-based content to as many as 17 speaker locations at listener level and to up to 10 Dolby Atmos-enabled or overhead speakers. It was designed to enable Dolby Atmos receivers and speaker configurations to render non-Atmos source material across any speaker layout.

Technical Approach: Frequency-Domain Processing

Dolby Surround Upmixer operates in the frequency domain, processing multiple perceptually-spaced frequency bands for fine-grained analysis of the source signal. The technology analyzes and processes multiple perceptually-spaced frequency bands, accurately steering each individual frequency range to appropriate speaker locations.

The upmixer primarily focuses on drawing ambient and atmospheric sounds into the height speakers while leaving most discrete sound effects in the original ground channels. To maintain an accurate frontal audio image, the upmixer will not send upmixed audio to the left wide and right wide speakers or any speakers located between the left, center, and right speakers. Audio is not upmixed to the center surround speaker.

Height channels are treated as unified left and right stereo signals rather than as discrete individual channel locations. DSU decodes height information as a single stereo signal, using the entire left side of the room as one channel and the entire right side as the other.

Historical Evolution

Dolby Pro Logic, the original surround decoder, rendered mono surround information at 7 kHz and below. In 2000, Dolby introduced Dolby Pro Logic II (DPL II), which could decode stereo sources into five channels: left, center, right, and both left and right surrounds for a stereo surround field. All channels were full-range, improving frequency response and steering precision over the original format.

Dolby Pro Logic IIz was the first Dolby format to use height channels, in the form of front heights above the front left and right channels. Dolby reintroduced the "Dolby Surround" name in 2014 for a new upmixer designed to let Dolby Atmos receivers and speaker configurations render non-Atmos source material; it later shipped as part of the broader Dolby Atmos AVR feature bundle.

Dolby Surround vs. DTS Neural:X

Dolby Surround and DTS Neural:X are the latest generation upmixing processors in 2015+ AV receivers, offering surround performance upgrades for stereo and 5.1 mixes, as well as height channel atmospherics for stereo, 5.1, and 7.1 mixes.

In High-Def Digest's listening comparison, Dolby Surround produced cleaner reverb and more natural voice rendering, while DTS Neural:X sometimes showed warbling between front and rear height speakers and delivered more dramatic, less subtle panning of moving effects across height speakers.

The two formats take opposite philosophies: DTS Neural:X pulls sound effects upward while leaving ambient sounds below and applies increased volume to height speakers. Dolby Surround Upmixer does the opposite, drawing ambient and atmospheric sounds into the heights while preserving discrete effects in the ground plane at subtler levels. Dolby Surround excels at spatial recreation and voice placement with refined, delicate processing; DTS Neural:X tends to be louder and produces better panning of aggressive sound effects.

Speaker Configuration Constraints

Because DSU uses stereo height processing rather than discrete per-channel steering, its upmixing is constrained by the speaker layout. The upmixer will not apply upmixing to speakers positioned in the front soundstage between left, center, and right channels, nor to left-wide or right-wide speaker locations. The center surround speaker does not receive upmixed content.