Multichannel Speaker Formats

Audio has made a giant step forward since the introduction of the DVD. Hopefully this article will shed some light onto the various formats.

DTS for Music:

This is a six channel format that can be delivered on CD, DVD.

If you have read any of our home theater related reviews before you will know that we find the audio enhancements of DVD at least as impressive if not more so than the video. The audio formats available for DVD video discs are Dolby Digital, MPEG2 (only standard format in Region 2, that is Europe) and PCM. Optional formats are DTS and SDDS. That they are optional means a DVD movie using one of them must also include one of the standard formats. Worth noting that many DTS discs only includes a 2-channel Dolby Digital soundtrack not a 5.1 one. Now this doesn’t mean you really need to own a DTS decoder since in almost every case a DD 5.1 version of the movie also exists. It’s just that you should avoid buying the DTS edition if you don’t own a DTS decoder.

You may also see some movies using Dolby Surround or Dolby Pro Logic soundtracks which are not really separate formats but a Dolby Digital or PCM stereo track that’s Dolby Surround / Pro Logic encoded.

Here follows an overview of the formats mentioned above. The technical details are taken from the official DVD FAQ, Dolby’s, DTS’s and Sony’s site:

Dolby Surround includes three channels. Two for the front with a left and a right speaker and one channel for the two rear speakers used for surround sound, which also has a limited frequency response of 100Hz to 7000 Hz.

Dolby Pro Logic is an enhancement to Dolby Surround and uses a four-channel mix of left, center, right and surround channels, which has the same limited frequency response as Dolby Surround. These channels are used for left, center and right speakers up front with one channel used for rear speakers.

Both Dolby Surround and Dolby Pro Logic are mixing technologies that add a rear channel and, in the case of Dolby Pro Logic, also a center channel onto a 2-channel signal. This means they can be played back on any stereo (or even mono system) and that they, unlike the other formats discussed, below work fine in the analog domain. The main disadvantages compared to Dolby Digital and DTS are of course the mono and frequency limited surround channels but in addition there is also some leakage between the channels not present with the digital formats that have 6 discrete channels.

All Dolby Digital decoders can decode Dolby Surround/Pro Logic content, this includes the software DVD players. If you still only own a Dolby ProLogic /Surround decoder/receiver it’s interesting to know that most DVD players (including software DVD players) can down-mix the Dolby Digital 5.1 tracks to a Dolby Pro Logic one. In the PC market it can be interesting to know that the Movie Mode on the Live/Audigy and QSound’s QMSS also take advantage of Dolby Pro Logic content to some extent even though neither one is an actual Dolby Pro Logic decoder. We cover this in more detail in a separate article.

With the introduction of Dolby Digital many may consider Dolby ProLogic as interesting as Soundblaster compatibility. To some extent that may be true, but far more people like to watch old movies with a Dolby ProLogic soundtrack that was released longtime before Dolby Digital 5.1 become common or even existed. In addition you may have plenty movies on VHS tapes and watch (analog) TV shows where Dolby ProLogic is the only option. We shouldn’t forget either that there are also a large number of games, especially console games, that use Dolby Surround/Pro Logic for both interactive and non-interactive soundtracks.

Dolby Digital provides up to six separate channels of crystal clear digital surround sound. The six discrete channels are used in left, center and right speakers up front along with separate channels for two rear speakers and the LFE (low frequency effects) channel often connected to the subwoofer. All the main channels (left front, right front, center, left surround and right surround) covers the entire audible range (20 Hz – 20,000 Hz) but many decoders have a small mode for all or some (usually surround and center) of the speakers where they filter away the low frequencies and send them to the subwoofer. The LFE track (the ".1") makes it possible to reproduce low bass effects with stunning impact (twice as loud as the other channels).

All DVD players can decode and down-mix Dolby Digital 5.1 to stereo so you never need to worry that you won’t get any audio if the movie only feature a Dolby Digital soundtrack. Far from all DVD players offer 5.1 output though and without that feature you need an external decoder to take full advantage of the DD 5.1 sound track. Virtually all decoders and 5.1 systems with a decoder integrated support Dolby Digital even if they may also support DTS.

You should be aware that not all movies using a Dolby Digital soundtrack actually offer 6 channels. The channel combinations available are (front/surround): 1/0, 1+1/0 (dual mono), 2/0, 3/0, 2/1, 3/1, 2/2, and 3/2 with the LFE channel being optional in all combinations. The by far most common are Dolby Digital 2.0 (may be Dolby Surround encoded) and Dolby Digital 5.1. You can often see quite clearly on the DVD Movie box which format it uses, but to what extent a DD 5.1 movie truly uses the surround and LFE channels varies quite a bit. We will cover this in more detail in the performance section.

It’s interesting to know that despite the fantastic sound movies with a Dolby Digital track offer, Dolby Digital actually uses a lossy compression technology named AC-3. The AC-3 decoder in the Dolby Digital sound system eliminates the data people cannot hear while trying to maintain all the data that people can listen to from the original PCM with a sample rate of 48 kHz at up to 24 bits. The bitrate can vary between 64 kbps to 448 kbps, with 384 being the normal rate for 5.1 channels and 192 being the normal rate for stereo. For more details check out Dolby’s official Dolby Digital FAQ

Dolby Digital Surround EX adds a third surround channel to the Dolby Digital format. This third channel is reproduced by rear-wall surround speakers, while the left and right surround channels are reproduced by speakers on the side-walls. Among the benefits are more realistic flyover and fly around effects and a more consistent surround field throughout the auditorium. You can find more info on Dolby's site.

Currently very few decoders support it and 6.1 speaker systems are uncommon. In fact I’m not aware of anyone in the multimedia market. The only sound card I know that claims 6.1 output for DVD movies are Hercules GameTheater XP (see review), Audiotrak's Maya 7.1 and Maya 7.1 Gold. Other soundcards using the same chip as the GameTheaterXP (CS4630) may also offer this with future drivers. The only software DVD player that support Dolby Digital Surround EX and DTS-ES is PowerDVD XP 4.0 (see press release). Interesting to note that Cyberlink offers a patch for $12 for the PowerDVD 3.0 version bundled with GameTheaterXP that will enable Dolby Digital Surround EX support. There is no patch available for the PowerDVD 3.0 bundled with Maya 7.1 Gold, AudioTrak instead says you need to upgrade to PowerDVD 4.0 XP if you want to take advantage of rear center channel. There are as you probably know several musician focused cards offering plenty of in/outs but as far as we know none of them offer 6.1 output with any software DVD player,

MPEG 2 is, just like Dolby Digital, a multi-channel digital audio format using lossy compression from original PCM format with sample rate of 48 kHz at 16 or 20 bits. The variable bit rate for MPEG2 format is 32 kbps to 912 kbps, with 384 being the normal average rate. It offers the same channel combinations as DD 5.1 with the addition of 5/2 format or 7.1 as most refer to it. It adds left-center and right-center channels. I’m not aware of any DVD movie that features a 7.1 sound track though.

The MPEG-1 format is also an officially supported format unlike MPEG-1 Layer 3 more commonly known as MP3. Many recent DVD players support MP3 though, not as a track on a DVD disc, but as MP3 audio files that are stored on a CD.

The MPEG2 format was originally supposed to be the most common on DVD movies in Europe with Dolby Digital only being optional but for various reasons Dolby Digital has become the standard in Europe, just as it is elsewhere, with just a few movies using the MPEG2 format. I’m not aware of any software DVD player, decoder or speaker system in the multimedia market which support the MPEG2 format even though several include an MP3 decoder.

The major advantage of the MPEG2 format is its backwards compatibility with MPEG 1 hardware due to the surround channels being an extension stream matrixed onto the MPEG-1 stereo channels. The MPEG-1 system will only see the 2 stereo channels and for that reason playback without issue.

Linear PCM (LPCM) is uncompressed (lossless) digital audio, the same as you find on CDs and most studio masters. Where they differ is CDs are limited to 16 kHz/16 bits and 2-channels while on DVDs it can be sampled at 48 or 96 kHz with 16, 20, or 24 bits/sample and up to 8 channels. The maximum bit rate is 6.144 Mbps, which limits sample rates and bit sizes when there are 5 or more channels but in practice few if any DVD movie feature more than a 2-channel LPCM track. DVD players are required to support all the variations of LPCM, but many down-sample it to 48 kHz and 16 or 20 bits. It’s debatable if higher sample rates or bit rates offer an audible advantage.