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The Basics of Surround Sound Formats Explained

Wed 09 February 2011 | Technical articles | Back to article list

In this weeks article we take a small diversion from our historical HDMI journey to talk Surround Sound formats. Most AV Receivers available these days have a long list of decoding options. So which is which?

Dolby Prologic

Dolby was the first company to really get to grip with surround sound using its “Dolby Surround” format for full size cinemas. Later they also launched the “Prologic System” for homes that was the first step toward widely available domestic surround sound.

Perhaps the most important technical aspect of all the Prologic formats (including the later Prologic II and IIx) is that only two channels of audio carrier are required to move it around. That’s significant because it means that any carrier of stereo audio could be used, with suitably encoded audio and a decoder at the playback end, to generate a (limited) surround field. Broadcast TV, VHS tape and Laserdisc were all popular carriers of early Prologic soundtracks.

The number of surround channels available with Prologic was limited at first, as was the audio bandwidth. The three front channels (Left, Centre & Right) were more or less full bandwidth but the two rear channels (Left and Right Surround) where both bandwidth and frequency limited. They also were actually the same mono signal! In addition, there were no special arrangements for low frequency effects (LFE). To carry more channels a digital system was needed and again Dolby were first to offer a solution…

Dolby Digital (AC-3)

The first appearance of Dolby Digital (or AC-3 as it was first know, AC-3 being the lossy digital compression format used) was on Laserdisc. Special players with digital outputs were required and the “clunkiness” of the whole system meant it only became popular at the more “enthusiast” end of the home cinema market. However, the rise of DVD in the late 1990s / early 2000s with its ability to carry the Dolby signals as standard kick started the home cinema market and with it came digital surround sound.

The Dolby Digital system offered a (up to) 5.1 channel system from the start. The front three channels remain but now the two surround channels offered a full bandwidth signal and were properly discrete. To complete the mix the “.1” channel offered a place to put low frequency information that could be replayed using a dedicated subwoofer.

DTS (Digital Theatre Systems)

The introduction of the DTS system started in cinemas with its use on the original Jurassic Park film. Rather than trying to squeeze the whole digital sound track on small pieces of magnetic strip between the films sprocket holes (which is how the Dolby system works) DTS uses a separate CD and player to store the audio which was synced to the film reels for playback. The extra space available on the CD (and the arguably superior DTS coding system) gave rise to a significant improvement in sound quality.  It wasn’t long before DTS soundtracks started to appear on DVDs alongside the Dolby soundtrack and domestic AV receivers that could decode the format soon followed.

6.1 and 7.1 Systems

It’s never long after a new technology is released that someone, somewhere that wants to improve on it. While 5.1 systems do a great job in the living room the makers of Star Wars Episode 1 (The Phantom Menace) decided more surround channels were needed in the cinemas to convey the sense of movement their film needed in sound. “Dolby Digital Surround EX” brought the answer and, rather than needing a complete redesign, the extra “Centre Surround” channel was encoded into the existing surround channels so no change in carrier was needed.

Soon a whole raft of 6.1 channel systems (often referred to as “7.1” systems) were born. The “7.1” term refers to the fact that installations often feature two Centre Surround speakers, one each toward the left and right corners. DTS also offered a 6.1 channel system, DTS Surround ES, together with a 6.1 system with a fully discrete Centre Surround channel – DTS Discrete 6.1.

High Resolution Surround Sound

With the capacity of DVD discs “maxed out” the next steps in improved surround sound had to wait for the arrival of a higher capacity carrier. After a short battle with the opposing HD-DVD format BluRay became the high definition videodisc of choice and with it came higher capacity for sound data. The format developers were quick to use this to good effect.

Both Dolby and DTS came forward with two formats one lossy and one lossless, each. Dolby gave us Dolby Digital Plus and Dolby TruHD while DTS offered DTS-HD and DTS Master Audio. While these offered yet more potential resolution and dynamic range their data hungry nature also meant a new connection between source and decoder was needed. Lucky HDMI was perfect…

Next week we return to our historical look at HDMI to see which version can be used for connecting which surround sound format.



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