Sound on DVD


Everything You Ever Needed To Know About Dolby Digital and DTS But Were Afraid To Ask

Disclaimer: If you are looking for an article debating the relative sonic merits of Dolby Digital vs DTS, you won't find that here. This article is designed to give you an understanding of what the terms Dolby Digital and DTS mean, and to help you to decide on whether or not you need these features in your next DVD-related purchase.

    DVD brings with it both the most advanced visual and the most advanced aural experience of any consumer product to date. However, the advances on the audio side can be a little confusing and a little overwhelming if you are not familiar with all of the technical ins-and-outs of the format, as they are quite significantly in advance of all other previous mainstream consumer audio technologies.

    Let's start by giving you a bit of background information. The Compact Disc, which the vast majority of you will be familiar with, provides two channels of audio running for a maximum time of 73 minutes. That's simply not long enough for a movie. Additionally, movies often use more than 2 channels of audio, frequently using up to 6 channels. In addition to up to 6 channels of audio, a DVD needs to fit the movie onto the disc as well. Without going into the nitty gritty technical details of the process, it became obvious during the development of the DVD specification that it would be necessary to invent methods of storing audio on DVD which took up less room than the audio took up on CD, so as to leave room for the video as well.

    When the standards were being set for DVD, four different audio storage methods made their way into the format's specification, with a fifth one being added in later. Currently, only two of these storage methods are commonly in use. Most of these audio storage methods involve compression of the audio data so that it takes up less physical room on the DVD.

Dolby Digital

    Dolby Digital is by far the commonest audio format used on DVD. Almost every DVD produced has a Dolby Digital audio track on it in one form or another.

    Dolby Digital is popular for a number of reasons, not the least of which is the fact that it is an extremely compact way of storing up to 6 channels of audio. In the world of DVD authoring, disc space is always at a premium, and so anything that saves space is looked upon favourably. Dolby Digital works by compressing the original audio and discarding unnecessary audio information. It manages to compress audio by the most impressive ratio of 12:1.

    A common misconception about the term Dolby Digital is that this term automatically means that 6 channels of audio information is present on a DVD. Dolby Digital is simply the method used to compress and carry the audio signal, and can carry anything from one mono channel to two stereo channels to six channels of surround audio.

    Purists argue that Dolby Digital is not transparent. That is, they argue that you can hear the difference between a Dolby Digital compressed audio signal and the original uncompressed audio. Based on my personal listening experiences, Dolby Digital still manages to sound very, very good indeed and is certainly more than adequate for the vast majority of DVD applications.

Linear PCM

    At its heart, this is the same format as that used on Compact Disc, however the DVD specification allows for higher quality audio and more channels than just the two available on Compact Disc. The big problem with Linear PCM is that it takes up a lot of physical room on a DVD, thereby leaving less room available for video programming. However, it is not compressed in any way, and so it does tend to sound the best of all of the competing audio formats.

    Linear PCM soundtracks tend to be found on music DVDs, usually as two channels of audio. To give you an idea of the space requirements of Linear PCM audio, consider the fact that two channels of Linear PCM audio takes up 8 times the space of two channels of Dolby Digital audio.

MPEG Audio

    MPEG audio is by far the most misunderstood of all of the DVD audio formats. Fortunately, it is not very popular, and will most likely fade away into historical obscurity. However, as there are a significant but small number of DVDs out in the Australian marketplace that utilize MPEG audio, I will discuss it here.

    The first point of confusion that arises when discussing MPEG audio on DVD is that it is often mistakenly thought to be the same as MP3 audio. These are not the same, nor are the two formats compatible with each other, a situation made all the more confusing by the gradual appearance of DVD players on the Australian marketplace that can play both formats.

    The second point of confusion that arises when discussing MPEG audio on DVD arises because of the fact that the video compression algorithm for DVD is also called MPEG. MPEG video compression and MPEG audio compression are two totally separate entities, related in name only.

    MPEG Audio is much like Dolby Digital in that it is a method of storing multi-channel audio compactly on a DVD. It has the capacity to store up to 8 channels of compressed audio. As for Dolby Digital, MPEG audio compression is not transparent, and relies on the discarding of unnecessary audio information.

    In the early days of DVD in Australia, MPEG audio was quite popular. These days, it is almost unheard of on any new release, and for all intents and purposes is a dead format in this country.


    Discussing DTS is always a thorny issue with DVD aficionados, as they seem to be divided into two rabid camps; those that proclaim the superiority of the DTS format and those that claim that they cannot hear any difference between DTS and Dolby Digital. I plan to steer clear of any subjective quality opinions on this topic, rather concentrating on the cold, hard, necessary facts.

    Like Dolby Digital and MPEG Audio, DTS is a method of compressing audio on DVD down to a more manageable size. However, unlike Dolby Digital and MPEG Audio, DTS does not offer anywhere near the level of compression that these other two formats do. DTS typically works at a compression ratio of 3:1, far less than the Dolby Digital compression ratio of 12:1. DTS protagonists claim that this lower compression ratio results in better sound, which sounds like a plausible argument. DTS deriders claim that the ratio of compression is not a valid yardstick when comparing audio compression algorithms, which also sounds like a plausible argument.

    Unfortunately, the only real way that this argument could be settled conclusively would be for double blind tests to be carried out comparing original uncompressed audio with Dolby Digital and DTS versions of this same audio. Sadly, both Dolby Digital and DTS seem extremely reluctant to allow these sorts of tests to be carried out, for obvious commercial reasons.

    DTS is not currently a popular sound format on DVD, for a number of reasons. Firstly, as intimated previously, DTS soundtracks take up a lot more room on a DVD than does a Dolby Digital soundtrack - 4 times as much, in fact. This leaves less room for other things on the DVD such as multiple languages and subtitles. Secondly, not all DVD players support DTS audio. The DVD specification did not initially include DTS audio, meaning that many older DVD players completely ignore its presence on a DVD. Most current generation DVD players recognize DTS audio, so the level of DTS compatibility is gradually increasing. In a survey I recently conducted, 30% of respondents had the capacity to listen to DTS audio in their home theatre setups.


    When the DVD audio specification was first written, Sony got their theatrical SDDS sound system into the specification, even though they had no plans to market a home version of this system at the time. They still have no plans to market a home version of the SDDS sound system, but the potential does exist for SDDS to become another competing audio format should Sony ever decide to pursue this course.

What Do I Need To Buy To Listen To DVDs?

    With all of these confusing and competing sound formats, it gets hard to figure out what you actually need to listen to DVD. DVD is a great method of getting into home theatre, since you don't necessarily need a whole lot of new audio equipment in order to take the plunge into the format. However, the more sophisticated your sound system, the better your DVDs will sound.

TV speakers

    At a very basic level, you can listen to DVD via your TV's speakers, whether they be either mono or stereo, so long as your TV has the appropriate audio input connectors. Pretty much any TV with an AV input will have the appropriate RCA connectors.

Stereo Speakers

    Moving up a level in sophistication, you can hook the audio output of your DVD player up to your stereo system. This will almost invariably provide better sound quality than that available through your TV's internal speakers.

Simulated Surround Sound

    Many DVD players have a feature which simulates the effect of surround sound through two speakers. Such features go by names such as Tru-Surround and Spatializer N-2-2. None of these sound anywhere near as good as a true surround sound setup does, but they can give surprisingly good results whilst you are saving up for your complete surround sound setup.

Surround Sound Speakers

    We are now moving into the realm of truly high quality home theatre sound. To get true surround sound, you will need at least 4 speakers, and preferably 5. You will need two at the front of your home theatre setup and two at the back to provide surround sound. You will also need a speaker between the two front speakers to act as a centre channel, although you can get away without this speaker in the short term if you are really strapped for cash. If you can afford it, buying a matched set of 5 speakers is a great idea. One of the problems with using a mish-mash of speakers for surround sound is that they will all sound slightly different, making it easy to tell which speaker a sound is coming from. This has the effect of destroying the surround sound illusion, reducing it to listening to the sounds emanating from five speakers situated around you instead of providing you with the illusion of space that a properly matched set of speakers will provide.

    Finally, a speaker that you can get away without, but one which will really enhance the sound of your home theatre is a subwoofer. A subwoofer adds a solid bottom end to the sound of the rest of your speakers, whilst decreasing their individual workloads. A subwoofer will really bring out those big on-screen explosions and let you rattle your house's foundations in a most satisfying way.

Surround Sound Processor

    Now we get into the most complex area of home theatre sound, and the area where you can most easily be led astray by bad advice. The surround sound processor is the part of your home theatre that actually generates the signals that drive your 4, 5, or 6 speaker setup.

    There are three main configurations that you can choose from for your surround sound processor, depending on your budgetary capacity and your sonic requirements.

5.1 Ready / Dolby Digital Ready Receivers
    The cheapest configuration is a so-called "5.1-ready" or "Dolby Digital-ready" surround sound processor. This is also by far the most complex configuration to explain properly and hence the configuration that you are most likely to be bamboozled into buying even if it is not the most appropriate for your requirements.

    This type of surround sound processor is not Dolby Digital compatible, despite the misleading labelling and despite what the very persuasive salesperson would have you believe. These processors are often quite cheaply priced, making them a seemingly attractive option for the first-time buyer. You can certainly hook six speakers up to them, and they will provide Dolby ProLogic decoding which will provide basic surround sound, but they will not give you the full benefit of any of the digital surround sound formats previously discussed since they do not come equipped with any digital audio inputs or any digital audio processing capability

    What these receivers do provide is a 5.1 analogue input channel. You can plug an already-decoded surround sound signal into this 5.1 input, and the receiver will pass this through to its amplifier stage without further processing. So, if your DVD player has a Dolby Digital or DTS decoder built in, you can plug the output from this into the input of this type of receiver and get Dolby Digital or DTS surround sound that way.

    The biggest problem with this type of receiver is that they are relatively inflexible and relatively unexpandable. Once you have used up the 5.1 input channel, you cannot plug any other digital surround device into it. With the imminent introduction of Digital TV with its Dolby Digital sound, this would seem to me to be a crippling disadvantage of this type of receiver. When you also take into account the fact that DVD players with inbuilt surround sound decoders are more expensive than those without, "5.1-ready" receivers seem to me to be a false short-term economy. I personally feel that no one should purchase such a receiver, as it leaves you with no room to expand in the future. The only possible exception to this rule might be someone who already owns a DVD player with an inbuilt Dolby Digital or DTS surround sound processor who is really strapped for cash and even then you should think long and hard about this option, as many DVD players really don't have the best in-built surround sound decoders to begin with.

Digital Surround Sound Receiver
    This is the type of surround sound processor that will suit the majority of budding home theatre enthusiasts. These receivers accept digital audio inputs from appropriately-equipped devices such as DVD players, and will decode one or more of the surround sound formats discussed at the beginning of this article. They have inbuilt power amplifiers, and can be connected directly to your 4, 5, or 6 surround sound system speakers. These days, decent quality versions of these receivers can be had for less than $1,000.
Standalone Surround Sound Processor
    As the name suggests, these are surround sound processors only. They have no power amplification built-in and simply take digital audio inputs and decode them. They are the most flexible of all of the surround sound processors but also tend to be the most expensive as they are usually targeted at the higher end of the home theatre market.

Which Surround Sound Formats Should My Setup Support?

    Finally, I will answer the $64,000 question - which surround sound formats should you look for when buying a surround sound receiver and/or DVD player? The short answer to that question is that you should make sure that the equipment that you buy supports Dolby Digital, DTS and Linear PCM. Following is the reasoning behind this statement.

    Dolby Digital decoding is absolutely essential, as the majority of DVD software out there utilizes this surround sound format.

    Linear PCM is universally supported by all digital surround sound receivers and DVD players and is also essential, as many DVDs carry this type of sound. Additionally, you will be able to plug your CD player's digital audio output into a receiver that supports Linear PCM decoding and get potentially higher quality sound from your CDs.

    DTS is not a popular format for DVD software at this stage, however this is likely to change in the near future. DTS will never become the dominant DVD audio format since Dolby Digital has far too much of a lead over DTS at this point in time, but DTS will progressively become a more and more significant audio format as time progresses, and thus is one that it is worth being compatible with now, rather than having to upgrade later. The additional cost of having DTS support is now minimal, so cost should not be a significant factor in this decision. Indeed, it is becoming progressively harder and harder to find a current model digital surround receiver or DVD player that doesn't support DTS.

    MPEG Audio is a dead digital audio format in Australia. Very few DVD titles exist with this as the sole or primary audio soundtrack, and virtually no new titles carry this surround sound format even as a secondary soundtrack. You can achieve satisfactory playback of MPEG audio encoded titles by plugging the analogue stereo audio output of your DVD player into an analogue input on your digital surround sound processor for those very few titles that require this. Some DVD players convert MPEG audio into Linear PCM audio which allows MPEG Audio to be heard through your digital surround sound processor even if it doesn't directly support MPEG Audio. Accordingly, it is not essential that your digital surround processor supports this sound format.

Closing Thoughts

    I hope that this article has given you a greater understanding of the various audio formats on DVD, and what you do and don't need to play them back. DVD is a wonderfully scalable technology which caters for many budgets and many sound setups and allows you to progressively expand your audio setup as your budget and other constraints allow. This article should equip you with the knowledge you need to make your next purchasing decision wisely so that you don't end up making an expensive mistake.

© Michael Demtschyna
30th May 2000