I previously wrote an introductory article about the difference between sound formats and upmixers. However, it isn’t always so clear cut.
For example, the original Dolby Surround in theatres used an analogue stereo track to encode 4 channels: Left, Right, Centre and Surround using Dolby Pro Logic encoding and decoding. So while the soundtrack was physically was stereo, the encoding of the other channels used phase matrixing to encode and then decode the additional channels. So we could say that Pro Logic is both a sound format and an upmixer.
With this in mind, let’s enumerate Dolby’s Sound Formats and Upmixers released over the years.
Dolby Stereo (Late 1970s onward) – Sound Format
The original Dolby Stereo system in commercial cinemas used a four-channel audio format that included three front channels (left, center, and right) and one rear channel. The rear channel was monophonic, meaning that it provided the same audio signal to all the speakers in the rear of the listening area. This four-channel setup is often referred to as 4.0 surround sound.
Dolby Surround used a process called matrix encoding to encode the four channels into a two-channel stereo signal that could be recorded onto conventional analog media such as the optical print on film soundtracks. This meant that movies and other content could be distributed with Dolby Stereo soundtracks without requiring a separate track for the surround sound channels. The content could also be played back as mono, stereo or surround sound by being backwards compatible with existing equipment.
To decode the surround information however, a special Dolby cinema decoder was required. This decoder would take the two-channel stereo signal and extract the four original channels of audio, which could then be played back through the appropriate speakers in the listening area.
Dolby Stereo was a significant improvement over the previous monaural and stereo sound systems, as it provided a more immersive and realistic audio experience. It became widely adopted in movie theaters, then later in home theater systems, and television broadcasts, and it paved the way for more advanced surround sound technologies such as Dolby Digital and Dolby Atmos.
Dolby Stereo is still used on all optical prints as a fall-back track should the digital signal fail due to dirt on or damage to the optical print. In such a case, the Dolby Cinema Decoder switches over to the Dolby Stereo track to play back the analogue-encoded Dolby Surround mix so the audience doesn’t miss the dialogue and surround effects.
Editorial Note by Roland Jutai: In all my cinema-going years, I only ever encountered such a switch-over to the analogue track by the decoder and it was on one of the Scream movies. The analogue track has a much higher noise floor than any of the digital soundtracks - such as Dolby Digital. So while it provided a great fall-back, it certainly highlighted just how much cinema sound improved since the 80s when Dolby applied its noise reduction technologies to Dolby Stereo and is the final product still used on analogue tracks.
Dolby Surround (1980s) – Sound Format & Decoder
When Dolby Stereo came to home audiences, it was marketed as Dolby Surround. Since processing power in home electronics wasn’t as great as Dolby’s cinema decoders, the original Dolby Surround decoders would normally only decode the surround channel information along with the Left and Right channels, leaving the centre channel information within the Left and Right channels. This basically meant that decoders would allow for L, R and two surround channels that were playing in mono.
Dolby Pro Logic (Late 1980s)
As processing power in home electronics was catching up, Dolby had the option of releasing the same type of matrix decoding for consumers as they were using in their Dolby Stereo matrix decoder in theatres. This decoder was called Dolby Pro Logic and could now decode the full 4 channels: Left, Right, Centre and Surround, and could be played back over a 5 channel system with two surround speakers playing in mono.
Dolby Digital (1992 – named Dolby Stereo Digital until 1995)
Dolby Digital is a digital audio compression technology that was introduced in cinemas with Batman Returns in the summer of 1992. It is used to encode and compress up to six discrete channels of audio (left, center, right, left surround, right surround, and LFE) into a single digital bitstream.
Dolby decided to use the area between the perforations on the side of an optical print to encode digital audio, which they called Dolby Stereo Digital (1992), and then later shortened to Dolby Digital (1995).
Dolby Digital is also sometimes referred to as AC-3 (Audio Codec 3), which is the technical name for the compression algorithm used to create the Dolby Digital bitstream. It has become a widely adopted standard in the entertainment industry, and is used in a variety of formats including DVDs, Blu-rays, streaming media, and digital broadcast television.
Dolby Digital can support bitrates from 32 kilobits per second (kbps) to 640 kbps, with higher bitrates providing better audio quality. The technology also includes a dynamic range control feature that allows content creators to adjust the volume of the audio to ensure that it can be heard clearly in different listening environments.
Dolby Digital runs at 320kbps on optical film, 384kps or 448kbps on DVD and 640kbps on Blu Ray. On Blu Ray, Dolby TrueHD includes a core 640kbps Dolby Digital bitstream for backwards compatibility.
It is normally abbreviated as DD 2.0, DD 5.1 and so on.
Dolby Surround EX / THX Surround EX (1999)
Dolby Surround EX was introduced in 1999 with Star Wars Episode I: A Phantom Menace as an extension of the Dolby Digital 5.1 surround sound format.
Dolby Surround EX adds an additional center rear channel to the traditional 5.1 channel setup by matrixing the information into the left and right surround channels and using a decoder similar to Dolby Pro Logic to recover the information during playback.
This allows for more precise placement of sound effects for sound information that travels back to front or side to side in the surround sound field.
To support Dolby Surround EX in the home, home theatre receivers also implemented Surround EX or THX Surround EX decoding, which would recover the Dolby 6.1 information from the 5.1 mix.
Dolby Pro Logic II (2000)
Dolby Pro Logic II is an enhanced version of the original Dolby Pro Logic technology. It is compatible with all Dolby Stereo and Dolby Pro Logic-encoded content and instead of creating mono surrounds, it can decode stereo sources into five channels: left, centre, right and both left and right surround for a stereo surround field.
The decoder improves over Dolby Pro Logic in frequency response and steering-precision as well, whereby all channels are full-range and it is compatible with bass management to create full 5.1 surround sound.
It also included several modes that are optimized for different types of content. For example, “Music mode” is designed to provide a spacious and natural-sounding listening experience for music, while “Movie mode” is optimized for movies and TV shows, and provides a more enveloping and immersive surround sound experience.
Content can be specifically encoded to be Pro Logic II compliant for more precise steering. In fact, Dolby released an SDK to certain partners to help them do this, such as Nintendo for their Gamecube and Wii consoles.
Dolby Pro Logic IIx (2003)
Dolby Pro Logic IIx can process stereo (2.0) and surround (5.1) content into 7.1 surround sound with two discreet surround back channels. It is compatible with all Dolby Stereo, Dolby Surround, Dolby Pro Logic, Pro Logic II and Dolby Surround EX content.
Dolby used its experience developing Dolby Surround EX with THX to improve on the original Dolby Pro Logic II decoder to decode stereo surround back channels. It is the preferable decoder to use for Surround EX content and it uses strategies included in THX Surround EX to ensure the surround soundstage doesn’t collapse into the back speakers with difficult content.
Dolby Digital Plus (2005)
Dolby Digital Plus (DD+) is an advanced audio coding technology developed by Dolby Laboratories. It is an enhancement of the original Dolby Digital audio format and provides improved audio quality with increased efficiency.
While DD+ supports up 15.1 channels, only 7.1 channels of audio were implemented for Blu Ray and streaming media. DD+ delivers high-quality audio with a bit rate of up to 6 Mbps. It also supports more advanced audio features like advanced audio coding, audio channel linking, and improved dynamic range control.
Dolby Digital Plus was used in HD DVD, the failed competitor fo Blu Ray, and still used for streaming services to provide higher-quality data.
DD+ uses advanced audio coding (AAC) to compress audio data. AAC is a lossy compression format that is designed to provide high-quality audio at lower bit rates.
Dolby TrueHD (2006)
Dolby TrueHD was designed for high-definition multimedia formats such as HD DVD and Blu Ray. The format uses MLP (Meridian Lossless Packaging) to compress up to 15.1 channels of audio, but generally only 7.1 channels are used on Blu Ray.
Dolby TrueHD is the first lossless format from Dolby that carries the studio master audio data bit for bit.
Dolby Pro Logic IIz (2009)
Dolby Pro Logic IIz is the first Dolby format to use height channels in the form of front heights above the front left and right channels. It is an evolution of Dolby Pro Logic IIx matrix decoder and a bit of a prelude to Dolby Atmos, which was in development already when this came out.
The Pro Logic IIz decoder is backwards compatible with all stereo, 5.1 and 7.1 content and extracts 9.1 channels of information with two height channels. In today’s nomenclature we would say it is 7.1.2 – as 9.1 and 11.1 was used when the technology was first introduced for matrix-decoded content.
Dolby Surround 7.1 (2010)
Dolby Surround 7.1 was a theatre-only sound format that was offered as a free upgrade for cinemas supporting Dolby Surround EX.
It was delivered to cinemas supporting digital projection only as the soundtrack could be delivered in the digital cinema equivalent of Dolby TrueHD 7.1 and decoded digitally by the Dolby Cinema Processor. I do not believe Dolby Surround 7.1 ever appeared on optical print, where Surround EX was the only possible extension to Dolby Digital. This is supported by the fact that Dolby Cinema Processors list Dolby Surround 7.1 as D-Cinema audio only. This means Dolby Digital on optical print was never extended beyond Surround EX.
Dolby Atmos (2012)
Dolby Atmos is an object-based audio format whereby 9.1 or 7.1 channel-based bed layer is complemented by mono sound-objects that can move in 3D space and are not tied to channels. Dolby Atmos is the first sound format to support dedicated overhead speakers, even though those channels are not object-based.
The commercial cinema version of Dolby Atmos supports 9.1 as the bed layer with 118 simultaneous objects being steered across up to 64 speakers.
The home version of Dolby Atmos can use 24 floor-level and 10 overhead speakers, but it is normally limited by how many speakers a particular home theatre processor supports. However, more speakers might be possible in the future with a “Dolby Atmos Pro” version, like DTS did with DTS:X Pro.
Brave was the first movie in the theatre to use Dolby Atmos. While the first home release on Blu Ray was Transformers: The Age of Extinction.
In the home, Dolby Atmos can be delivered as either a Dolby TrueHD or a Dolby Digital Plus extension. Any processors that doesn’t support Atmos simply ignores the extension data and only plays back the base layer. Any processor that supports Atmos extracts the sound objects from the data stream and steers – or renders – them in 3D space based on the speaker layout.
Dolby Surround (2012)
Dolby Surround Upmixer (DSU) or just Dolby Surround for short is Dolby’s latest version of surround upmixing technology to expand stereo, 5.1, 7.1 and 9.1 content to support ANY speaker configuration including any number of overhead channels. It is a complementary technology to Dolby Atmos in a sense as it can expand channel-based content to utilise all the speakers in a Dolby Atmos installation.
DSU 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 theatre receivers and processors. While it is possible to license both the Pro Logic decoders separately along with DSU, only Yamaha did so on their CX-A5100 processor. Other companies decided not to pay for dual licensing and therefore the Pro Logic upmixers are now a relic of history.
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