question: How does Dolby Surround and Pro Logic work if it's just stereo audio?
comment: Prologic works by flipping the phase on the audio meant for the rear channel. The decoder works like this: * any audio this is present on both the left and right channel and is in phase, is routed to the center speaker. * any audio that is present on both the left and right channel but is out of phase is routed to the rear speaker. * any audio that is only on the left stays on the left. * any audio that is only on the right stays on the right. EDIT: If you don't know what phase means. Audio is air moving back and forth. Speakers make audio by moving back and forth. Audio is encoded as voltage that swings from positive to negative. So when we say that audio is on both the left and right channels but 'out of phase' that means one speaker is pushing the air while the other speaker is pulling the air. In practice you can have the left and right be out of phase and you won't really notice. But the ProLogic decoder can tell and thus can route the out of phase audio to the rear speaker.
passages: Section::::Dolby Pro Logic.
  
  In 1987 the decoding technology was updated and renamed "Dolby Pro Logic".
  
  A Pro Logic decoder/processor "unfolds" the sound into the original 4.0 surround—left and right, center, and a single limited frequency-range (7 kHz low-pass filtered) mono rear channel.
   In addition to 5db of noise processing, the surround channel is slightly delayed, so that any front channel sounds that leak into the surround channel arrive at the listener after the front channels. This takes advantage of the Haas effect - audio that is present in the front speakers but delayed in the surround speakers will have the psychoacoustic effect of emanating from the front of the sound stage.
  
  "Dolby Surround and Dolby Pro Logic decoders are similar in principle, as both use matrix technology to extract extra channels from Dolby Stereo stereo-encoded audio.
   Dolby Pro Logic
  
  Dolby Pro Logic is a surround sound processing technology developed by Dolby Laboratories, designed to decode soundtracks encoded with Dolby Surround. Dolby Stereo was developed by Dolby in 1976 for analog cinema sound systems. The format was adapted for home use in 1982 as Dolby Surround when HiFi capable consumer VCRs were introduced. It was replaced by the improved Pro-Logic system in 1987.
  
  The term "Dolby Surround" describes the encoding technology or matrix-encoded soundtrack, whereas Pro Logic refers to the decoding technology and processor.
   "How does it work?"The Dolby Surround upmixer is based on phase and gain relationships of elements in the signal, but importantly employs wideband functionality that analyzes and processes multiple perceptually spaced frequency bands in the signal. The benefit is a finer-grained analysis of the source content prior to steering. The result, we believe, is a more accurate soundstage.
  
  When employed with overhead or Dolby-enabled speakers there is sense of additional spaciousness or what I call "air."
   "What's the best speaker configuration for Dolby Surround?"It is not limited to a specific speaker count. It can be employed in a 5.1.2 Dolby Atmos system, a 24.1.10 Dolby Atmos system, or any speaker configuration in between."
  
  Section::::Software encoding and decoding.
  
  BULLET::::- The liba52 decoder library for AC3 and A52 digital sound optionally exports Lt/Rt stereo sound compatible with Pro Logic decoders.
  
  BULLET::::- HandBrake and FFmpeg are capable of downmixing Dolby Digital AC-3 5.1 to Lt/Rt stereo tracks compatible with Dolby Pro Logic I & II decoders.
   By the late 1980s integrated-circuit manufacturers were working on designing integrated-circuit matrix decoders. A typical early example is the SSM-2125 from PMI. The SSM-2125 is a complete Dolby Stereo matrix decoder (except for the surround delay) on a single chip, it allowed domestic decoders which used the same logic system found in professional decoders to be marketed to the consumer. These decoders were thus given the name Dolby Pro Logic.
  
  Section::::Dolby Stereo 70 mm Six Track.
   In 2000, Dolby introduced "Dolby Pro Logic II" (DPL II), an improved implementation of Dolby Pro Logic created by Jim Fosgate. DPL II processes any high quality stereo signal source into five separate full frequency channels (right front, center, left front, right rear and left rear). Dolby Pro Logic II also decodes 5 channels from stereo signals encoded in traditional four-channel Dolby Surround. DPL II implements greatly enhanced steering compared to DPL, and as a result, offers an exceptionally stable sound field that simulates 5 channel surround sound.
   Because of the limited nature of the original DPL, many consumer electronics manufacturers introduced their own processing circuitry, such as the "Jazz", "Hall", and "Stadium" modes found on most common home audio receivers. DPL II forgoes this type of processing and replaces it with simple servo (negative feedback) circuits used to derive five channels. The extra channel content is extracted using the difference between the spatial audio content between two individual channels of stereo tracks or Dolby Digital encoded 5.1 channel tracks and outputs it appropriately. In addition to five full range playback channels, Pro Logic II introduced a Music mode which includes optimized channel delays, and adds user controls to—for example—adjust apparent front sound stage width.
   BULLET::::- Dolby Digital EX: introduces a matrix-encoded center rear surround channel to Dolby Digital for 6.1 channel output. This center rear channel is often split to two rear back speakers for 7.1 channel output.
   Dolby Digital EX is similar in practice to Dolby's earlier Pro-Logic format, which utilized matrix technology to add a center surround channel and single rear surround channel to stereo soundtracks. EX adds an extension to the standard 5.1 channel Dolby Digital codec in the form of matrixed rear channels, creating 6.1 or 7.1 channel output.
  
  Section::::Versions.:Dolby Digital Surround EX.
   "Dolby Pro Logic IIz" expands on Pro Logic IIx with the addition of a height component, creating front height channels above the front left and right speakers, expanding a 5.1 or 7.1 system to 7.1 Height or 9.1. It identifies spatial cues in low-level, uncorrelated information, such as ambience and effects like rain or wind in the side and rear surround channels, and directs it to the front height speakers. The channels it adds are matrixed, not discrete.
  
  Section::::Dolby Surround (2014).
   BULLET::::- Dolby Stereo – first cinema analogue surround sound system
  
  BULLET::::- Dolby SR - professional analogue recording on magnetic tape
  
  BULLET::::- Dolby Surround - renamed Pro Logic in 1987
  
  BULLET::::- Dolby Pro Logic – consumer version of the Dolby Stereo analogue surround sound system
  
  BULLET::::- Dolby TrueHD – lossless codec for HD DVD and Blu-ray Disc
  
  BULLET::::- Dolby E - allows 6 to 8 channels of audio to be compressed into an AES-EBU digital audio stream
  
  BULLET::::- DTS (sound system) – formerly Digital Theater Systems
  
  BULLET::::- Home cinema
  
  BULLET::::- Loudspeaker
   The terms Dolby Stereo, Dolby Surround and Lt/Rt are all used to describe soundtracks that are matrix-encoded using this technique.
  
  Section::::Dolby Pro Logic II.
   BULLET::::- Dolby Surround, Dolby Pro Logic, Dolby Pro Logic II, Dolby Pro Logic IIx, and Dolby Pro Logic IIz: these decoders expand sound to a greater number of channels. All can decode surround sound that has been matrixed into two channels; some can expand surround sound to a greater number of speakers than the original source material. See the referenced articles for more details on each decoder.
  
  BULLET::::- Audistry: sound enhancement technologies
  
  BULLET::::- Dolby Volume: reduces volume level changes
   When Dolby Labs and DTS introduced technologies to add a rear center surround channel, these technologies found their way into AV Receivers. Receivers with six amplifiers (known as 6.1 receivers) will typically have both Dolby and DTS's technologies. These are Dolby Digital EX and DTS ES.
  
  Dolby introduced Dolby Pro Logic II to allow stereo sources to play back as if they were encoded in surround sound. DTS introduced a similar technology, . These decoders have become common on most current receivers.
   BULLET::::- Dolby Headphone: an implementation of virtual surround, simulating 5.1 surround sound in a standard pair of stereo headphones.
  
  BULLET::::- Dolby Virtual Speaker: simulates 5.1 surround sound in a setup of two standard stereo speakers.
   With the arrival of Dolby Digital, the CP line had to be discontinued. It was replaced with the DC line, namely the "DC-1" [1996] and the "DC-2". It was at this point that the company introduced its revolutionary surround processing algorithm "Logic 7". Logic 7 was notable for generating a convincing soundfield from seven loudspeakers when presented with either a stereo or 5.1 input.
  
  After a while, Lexicon added the "MC-1" to its lineup of the DC-1 and DC-2, and the MC-1 became its new flagship.
   A Pro Logic decoder/processor "unfolds" the sound into the original 4.0 surround—left and right, center, and a single limited frequency-range (7 kHz low-pass filtered) mono rear channel—while systems lacking the decoder play back the audio as standard stereo.
  
  Although Dolby Surround was introduced as an analog format, all Dolby Digital decoders incorporate a digitally implemented Dolby Surround Pro Logic decoder for digital stereo signals that carry matrix-encoded Dolby Surround. One of the first was the MSP400 surround sound receiver and amplifier by RCA for their high-end Dimensia brand. It was released in 1987 for the Digital Command Component System.
  
  Section::::Dolby Surround.
   In contrast, Dolby's competing EX codec, which also boasts a center rear channel, can only handle matrixed data and does not support a discrete sixth channel; it is most directly comparable to DTS-ES Matrix.
   Dolby Headphone is incorporated into the audio decoders packaged with surround headphones including:
  
  BULLET::::- Razer Thresher 7.1
  
  BULLET::::- Razer Thresher Ultimate
  
  BULLET::::- HyperX Cloud Revolver S
  
  BULLET::::- Astro Gaming A40 System
  
  BULLET::::- Astro Gaming A50 System
  
  BULLET::::- Logitech G430
  
  BULLET::::- Logitech G35
  
  BULLET::::- Logitech G930
  
  BULLET::::- Logitech G933
  
  BULLET::::- Logitech G633
  
  BULLET::::- Plantronics GameCom Commander
  
  BULLET::::- Plantronics Gamecom 777
  
  BULLET::::- Plantronics Gamecom 780
  
  BULLET::::- Plantronics GameCom 788
  
  BULLET::::- Plantronics RIG 500E
  
  BULLET::::- Turtle Beach Systems Ear Force DXL1
  
  BULLET::::- Turtle Beach Systems Ear Force X41
  
  BULLET::::- Turtle Beach Systems Ear Force X42
  
  BULLET::::- Turtle Beach Systems Ear Force Recon 320
   The Dolby Stereo Matrix is straightforward: the four original channels: Left (L), Center (C), Right (R), and Surround (S), are combined into two, known as Left-total (LT) and Right-total (RT) by this formula:
  
  where "j = 90° phase-shift"
   Section::::DTS technologies.:DTS Neo:X.
  
  DTS Neo:X reconstructs 2.1, 5.1, 6.1, or 7.1 sources to 11.1 front height and width channel systems. Dolby's Pro Logic IIz's system adds only front height channels to the 7.1 configuration. Neo:X also matrix downmixes 11.1 sources to 5.1 or 7.1 channel systems.
  
  Section::::DTS technologies.:DTS 96/24.
   All Dolby TrueHD-enabled Blu-ray Disc players are capable of decoding Dolby TrueHD audiotrack to an arbitrary number of channels more suitable for player output. For example, all Dolby TrueHD-capable players can create a 2-channel (stereo-compatible) mix from a 6-channel source audiotrack.
   The Mono SR simplified noise reduction format for cinema was developed by tomandandy for the film in 2001–2002. It is an open source audio format that maintains the simplicity of monaural sound when motion picture delivery requirements include Dolby Digital noise reduction.
  
  Section::::Releases and versions.
  
  Multiple versions of the film exist, as cuts were made so it could receive less restrictive ratings in the U.S. and other areas.
   Dolby reintroduced the "Dolby Surround" terminology in 2014 to refer to something entirely new. The term now refers to a new upmixer whose "... purpose is to enable Atmos receivers and speaker configurations to serve non-Atmos signals." "Dolby Surround" is a complete replacement for Pro Logic that upmixes stereo and multi-channel inputs to play over Atmos configurations. It is:"...part of the Dolby Atmos bundle of technologies. It is an upmixer designed to function with traditional channel-based layouts, as well as Atmos enabled layouts that include overhead or Atmos-enabled speakers. It processes native stereo, 5.1, and 7.1 content.
  