Sound for Picture V: Surround Sound

In previous location recording articles, we looked at why transparent audio is perhaps the most important aspect of any video production. This culminates during the post-production stage with the addition of realistic ambient sound that captivates the audience and reduces, or at best removes, awareness of their real-world surroundings.

As many of us have become accustomed to over the last decade, most feature films and indeed some television programming comes with more than just left and right audio channels. Surround sound mixing makes use of multiple audio channels to envelop the audience, making them feel as though they’re in the middle of the action. By placing the audience in the middle of the soundscape and enabling them to hear sounds coming from all around them, film makers are able to maximise ‘suspended disbelief’ – the ultimate goal of any sound track.

A basic surround sound system will comprise at least six speakers that literally surround the audience: centre (C), left (L), right (R), surround left (SL), surround right (SR) and low frequency effect (LFE). This simple setup is better known as a 5.1 system – 5 speakers and 1 bass unit. The LFE channel (the .1) came to be known as such because it typically handles 1/10 of the frequency range of the other speakers.

5.1 Surround Sound speaker placement

Further speakers, specifically rear centre fill, can be added to create a 6.1 system:

6.1 Surround Sound speaker placement

And two additional speakers can be added to a 5.1 system, either centre left (CL) and centre right (CR) or left surround (LS) and right surround (RS) to create a 7.1 system:

 

7.1 Surround Sound speaker placement (widescreen format on left)

Whilst there are virtual surround algorithms (such as the Sound Retrieval System, or ‘SRS’) that make use of two speakers and psycho-acoustic phase effects to emulate surround sound, we’ll focus in this article on the true surround sound formats.

Dolby Digital™

Dolby Digital, formally known as AC-3 (short for audio coding 3) is the

standard surround format for our home cinema systems. As you will undoubtedly have noticed, Dolby Digital is the standard format for DVD video. In fact, the scope of this format has gone beyond DVD alone and is now also part of the High Definition TV (HDTV) standard.

The Dolby Digital format is capable of providing up to 5 discrete channels of full frequency effects (from 20Hz up to 20kHz) plus an additional sixth channel dedicated to low frequencies (20Hz to 120Hz). It’s important to be aware that not all Dolby Digital systems will have 5.1 channels of audio, and those that do will be designated as such – e.g. ‘Dolby Digital 5.1′. In fact, Dolby Digital can have as few as one channel of audio (mono) designated as Dolby Digital 1.0. Dolby Digital is encoded on the film release print and must be licensed from Dolby Labs for a fee.

DTS Digital Surround™

DTS Digital Surround (or simply ‘DTS’) is a competing alternative to Dolby Digital. DTS, like its competitor, is a 5.1 surround sound format that is available in cinemas and as an optional soundtrack on some DVD movies for home viewing. Unlike Dolby Digital, DTS is not a standard soundtrack format for either commercial DVD releases or the HDTV format used in digital television broadcasting.

As you would expect from a competing format, there are a few pros and cons to consider when opting for DTS over Dolby Digital. Firstly, DTS uses higher data rates than Dolby Digital and therefore some argue that there is an audible improvement in sound quality. This higher data rate, however, requires more storage and, as such, more space is needed on the DVD to accommodate DTS than Dolby Digital.

Dolby Surround Pro-Logic™

Dolby Surround Pro-Logic first made an appearance in home cinema systems in the early 1990s and is still the standard for analogue television broadcasts since the Dolby Surround Pro-Logic signal can be encoded in a stereo analogue signal. As all DVD players down-mix the Dolby Digital information to the Dolby Surround Pro-Logic format and output the signal as a stereo pair, you can still watch DVD movies on an older Dolby Surround Pro-Logic receiver.

Dolby Digital EX™, THX Surround EX™ and DTS Extended Surround (DTS-ES)™

  

Dolby Digital EX isan extension of Dolby Digital 5.1 to include a surround back channel, whosecorresponding speaker is placed directly behind the audience. This allows audioevents to occur behind the audience, further enhancing the 360˚ surround sound and enveloping the audience to a greater extent. The THX Surround EX format is a joint venture by Lucasfilm THX and Dolby Laboratories and is the home cinema version of Dolby Digital EX. While Lucasfilm THX licenses the THX Surround EX format for use on receivers and pre-amplifiers, Dolby Laboratories currently license THX Surround EX under its own name (Dolby Digital EX) for consumer home cinema systems. The method by which Dolby Digital EX and DTS-ES encode the surround back channel is known as matrix encoding since the back channel is encoded and later decoded from the surround left and surround right channels. The surround back channel information is encoded into the surround left and surround right channels and, for this reason, is sometimes referred to as Dolby Digital 5.1 EX or DTS 5.1 ES. As this surround back channel is not a discrete channel, the system is still technically a 5.1 system.

THX

One misconception we need to banish about THX is that it’s a surround sound format. THX is essentially a measure of quality that can be found in conjunction with different surround sound formats such as Dolby Digital and DTS. THX standards are a set of criteria that dubbing stages and movie theatres adhere to in order to guarantee that what the audience hears during playback is as close to what the mixing engineer heard during mixdown as possible. In theory, any production mastered to the THX standard should sound the same in any THX qualified cinema in the world. As such, THX isn’t really a format of surround sound, it’s a measure of quality that brings a standardisation to the film-making community.

Stay tuned to Jigsaw Broadcast for my forthcoming feature on audio post-production systems. If you have any queries about anything you have read, get in touch with the Broadcast team on 03332 400 222 or take a look at our full broadcast range.

Rob H
Rob H
Call us: 03332 409 306

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