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 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 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.
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.
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.
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.
There are three main configurations that you can choose from for your surround sound processor, depending on your budgetary capacity and your sonic 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.
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.
© Michael Demtschyna
30th May 2000