Electronics project notes/Audio notes - multichannel and surround
Binarual and headphones are two extra cases we forget to think about
- Dolby is a company
- Dolby Surround tends to refer to multi-channel matrix schemes (initially analog, later digital)
- Dolby Digital are digital codecs.
Sometimes the channel stuff and the codec stuff is strongly related (or implied by context), so we generally don't need to be picky about terms, but knowing the distinctions can be useful.
Dolby Stereo (1976) seems to group both:
- Dolby SVA - A matrix decoding scheme,
- from two channels on optical film
- to four: left, right, center, and a mono rear surround
- was used in cinemas, to give the option for encompassing sound based on existing film (avoiding separate multitrack and possible sync issues)
- playable on regular stereo as-is, and consumer SVA decoders existed
- soundtracks were sometimes SVA as a selling point, in that they were playable everywhere, but would would sound cooler on specific systems.
- Dolby Stereo 70mm - Noise reduction used on 6-channel magnetic tracks on 70mm film print.
- ...so not something any of us consumers would ever deal with
Dolby Surround (1982) is a marking used on players that used a simplified passive matrix decoder to play SVA(verify) into left, right, and surround (the center channel was fed equally from left and right, because that works out as phantom center)
(note: Dolby made things confusing in 2014, see below)
Dolby Pro Logic (1987)
- amounts to an expansion of what Dolby Surround does (still four channels)
- implemented with ICs
- does some gaining (and some frequency filtering?(verify)) depending on what's currently dominant, without changing total energy, which gives the perception of better separation (Dolby calls this 'steering')
Dolby Pro Logic II (2000)
- takes regular stereo or Dolby Surround
- produces five sensible full frequency channels (left front, center, right front, left rear, right rear).
- Basically, the gaining/steering is better than before, meaning it adds a reasonable 'in the middle of the sound' feeling to most any stereo input
Dolby Pro Logic IIx (2003)
- Can take regfular stereo, Dolby Surround, or 5.1.
- similar, can output 6.1 or 7.1
Dolby Pro Logic IIz (2006)
- similar, goes up to 9.1 height systems
Dolby Surround is, since 2014, used in a completely different meaning than before, basically referring to part of Dolby Atmos system.
Dolby Digital, a.k.a. AC-3 is just about storing audio, not about processing it.
- is a pretty decent codec in general
- and its early introduction (1986, before MP3) and 5.1 support made it a common choice for that to store 5.1
- it being supported (alongside DTS) in digital interconnects like TOSLINK helped too
DTS is a brand. When used to refer to digital audio, it typically refers to the DCA (DTS Coherent Acoustics) codec, as e.g. carried over SPDI/F.
DCA is a a compressed, digital surround format.
DTS Neo:6 is a similar idea to pro logic II: produces more channels from fewer.
DTS Neo:X is similar and goes up to 11.1 height systems