DVD Audio Formats
Now that home theaters and home entertainment centers seem to be here to stay, I guess it is safe to say that the old fashion single speaker televisions are relegated to the museum. As such, it appears to be the right time to examine the plethora of audio options and different audio formats that have moved from the theaters into our homes. Most of us now have speakers placed all over the room and with the newer formats coming out even more speakers are on the horizon. We have turned over the design of our living rooms to these speakers and center our lives over the best position to listen to the sounds from our favorite films and television shows. Even the modest speakers found in our home computers have given way to speaker arrays surrounding our desks. So sit back, grab a cup of coffee and letís run through the various audio formats common to those little boxes next to our television sets.
The most basic sound format would logically be monaural, simple one channel sound. Well, forget logic here, true mono sound is extremely rare in home theater today, most of the time the receiver will actually provide two channel mono if it detects a mono audio source. Rather than just pumping the sound through the center speaker a typical home theater receiver will split the signal equally and send it to the front speaker pair. This provides a somewhat fuller sound field for the listener. So, technically modern mono sound is better placed with Dolby audio than anywhere else.
Next in line is pulse code modulation or PCM. This is a defined standard set for DVDs but its not always found on all discs. It provides a two channel audio and is found primarily on DVDs mastered from older two channel sources, especially concerts and transfers of records. Basically the way this format works is the nice smooth sine wave curve of sound is sampled, the amplitude and frequency at a particular moment in time is recorded digitally. Older PCM standards allow for the amplitude to be broken down into five ranges. The current pulse is assigned the nearest value when sampled. Currently, this has been expanded to a ten value range providing for a much truer reproduction.
Now we come to Prologic. This is a method devised by Dolby labs to provide a feel of surround sound from a two channel source. The rear speakers actually are mono that is they have the same information sent to the rear left and rear right but when heard in conjunction with the front speakers and considering the room is rarely symmetrical, the effect is you hear a rather full rear audio field. With more and more people using computers and portable DVD players to enjoy DVD and other programming, Dolby Laboratories has come up with advancements on their standard Prologic. The new system is called Prologic 2 (Prologic II). With this system there is an enhanced feeling of a true surround field utilizing as few as two speakers. There are two modes for Prologic 2 Virtual Speaker, Reference Mode where the distance between the front speakers sets the distance for the two virtual rear speakers. Then there is Wide Mode where the virtual rear speakers will appear to be farther apart creating a fuller sound stage in your listening area.
With the previous audio formats we dealt with basically two channel audio that can fool the audience into the perception of a surround field. Now we come to the true surround sound experience. Speaker manufacturers love these formats since it requires speakers all around you. For six channel audio, the current standard, you need two speakers in the front, an addition center speaker, two rear speakers and a sub woofer. The front and rear speaker should be full frequency models capable of reproducing the complete audible spectrum. The center speaker needs to only reproduce dialogue while the sub woofer carries the ultra low sounds of special effects. Dominating the market currently for six channel format used in DVDs and cable/satellite systems is Dolby 5.1 audio. Dolby started out as a noise reduction system about thirty years ago. Back then a lot of people had Phillips cassette tapes, where the tape speed was a noise prone 1.75 inches per second, compared to the 15 to 30 inches per second of quality tape recorders. Dolby encoded the signal and processed it to greatly reduce that annoying hiss. Now, they concentrate on the faithful reproduction of the sound they provide for theaters. Each of the six channels is highly compressed and encoded individually, one for each channel. Dolby Digital also decides how the bits are distributed among the various channels from a common bit pool. This technique allows channels with greater frequency content to demand more data than sparsely occupied channels, for example, or strong sounds in one channel to provide masking for noise in other channels. Dolby Digital's sophisticated masking model and shared bit pool arrangement are key factors in its extraordinary spectrum efficiency. Furthermore, where other coding systems have to use considerable (and precious) data to carry instructions for their decoders, or to carry the same audio in separate channels for compatibility reasons, Dolby Digital can use proportionally more of the transmitted data to represent essential audio, which means inherently higher sound quality. Out of about 29,000 DVD titles there are about 5,800 that employ Dolby 5.1 audio tracks.
There is an alternative six channel discrete audio format, DTS or Digital Theater Sound. This format was developed for theatrical use by Digital Theater Systems Inc. in California it made its appearance June 11, 1993 with the release of Jurassic Park. It has subsequently been adapted for use in home theater systems. The six speakers are laid out exactly in the same manner as Dolby 5.1; two front speakers, a center front speaker, two independent rear speakers and a sub woofer. There are some pretty important differences; DTS is not stored as compressed as Dolby 5.1. This means a better throughput, almost double of 5.1 but the cost is it takes up a lot more bandwidth (space) on the DVD disc. This limits the number of extras on a disc with DTS. The current trend seems to be discs with both DTS and Dolby 5.1 on the same disc, often with a second disc for the extras. In the case of DTS, a technique called "forward-adaptive bit allocation" is used. Using this technique, the allocation of data to each sub-band is predetermined exclusively by the encoder. This information is explicitly conveyed to the decoder along with the actual bits to be used. Forward-adaptive bit allocation's primary advantage is that the psychoacoustic model used resides exclusively within the encoder. Because the model is encoder-based, extremely complex psychoacoustic coding algorithms can be used (as decoder processing ability isn't a limiting factor). Forward-adaptive bit allocation also allows psychoacoustic model modifications and improvements to be passed directly on to installed decoders, essentially "future-proofing" DTS decoders from premature obsolescence. With all this technical jargon behind us the question remains, "what does this mean to me as I sit and listen to a DTS DVD?" The answer is very subjective. There are those that collect DVDs that swear by DTS. The lesser compression does provide for less compression artifacts and therefore a clearer sound filed. Based on my personal experience and those of many other people that have done a side by side comparison between DTS and Dolby 5.1 I can say that the perception provided by DTS is that of a fuller sound stage. It is richer and seems fuller than the same sound track in Dolby 5.1. It also appears to have better backfill, the sound of the rear speakers are more connected, the sound appears to come smoothly from the rear rather than being able identify discrete sounds from each of the two rear speakers. This format has not caught on as well as many people believe. Of the 29,000 DVD titles there are about 780 six channel DTS DVDs. Of the studios that have committed to producing DTS discs Image is in the lead followed by Universal, DVD International, Red Distribution and Columbia/Tri-Star. Speaking of Columbia, they have produced a special line of DVDs, Superbit. With this mastering format there is less compression used for the audio and video tracks in order to maximize the quality of the reproduction. They almost all have DTS and Dolby 5.1 as well as anamorphic video. The lack of full compression does have a trade off; there is no room for the extras. In more recent releases of Superbit titles a second disc with the extras has been included. Currently there are 24 Superbit titles out there.
Seven Channel Formats
What is the future of home theater sound? Some may consider moving up from six to seven channels. Both Dolby labs and DTS have started to push their new seven channel audio formats with studios for inclusion on DVDs. For Dolby the new format is called Dolby EX, for DTS it is DTS ES. With each the technical specification as similar to the six channel counterparts with the inclusion of a center rear speaker. Practically this means handing over your room design more than ever to your home theater. To properly enjoy seven channel sounds you canít have your couch against the rear wall, you need the space to place the center rear speaker. You also have to get a seven channel enabled receiver or amplifier which, needles to say, is more expensive than the six channel counterpart. Before you go ahead and rearrange your living room and invest in a new receiver consider there are currently only 89 seven channel DVDs released. Among the studios that are dedicated to this new format are Anchor Bay, Warner Brothers, Fox, Universal and New Line. Most studios reserve this format for big, action packed block busters like the Lord of the Rings and the new Star Wars flicks. Unlike Dolby EX where the rear center channel is typically only discrete, DTS ES can be either matrix or discrete. This means that for DVD player and receivers that can process a separate, seventh channel you get better separation for the rear center channel. Whether the disc is encoded for discrete rear center or not a matrix coding is always provided so for those 'older' DTS systems the rear center channel is not lost. So matrix emulates the seventh channel for six channel systems while discrete provides the same unique track the other six channels use.
So, while we have come a long way from the little speaker mounted in the cabinet of our television sets the journey is far from over. Many people now prefer to watch and hear a movie at home rather than the theater, no disturbing cell phones, no over priced snacks and you can pause the film when Ďnature callsí. In any case, listening to your favorite movie has become an important part of the lives on many people.
I maintain a complete database of all region one DVDs and have provided lists of several of the formats discussed above. If you want to look up a favorite film to see if it is in the format you want you can check it out with these links: