This question has been asked in many different ways by people just entering the Home Theater hobby. The basic idea is the same; why do DVD soundtracks (encoded in Dolby Digital or DTS) sound so much better than music CDs on the same 5.1 system? When asked on popular Home Theater and Hi-Fi message boards, the question invariably results in technical specifications being bandied about, like sampling and bit rate comparisons between a DVD’s Dolby Digital soundtrack and CD’s PCM. Right about then you can count on some smarty-pants to step in and tell everyone that DVDs don’t sound better, in fact they sound worse than CDs because Dolby Digital uses a lossy compression algorithm.
So what does it all mean? And what does it mean when a movie soundtrack from your favorite DVDs simply sounds better than your CDs?
Here’s a quick breakdown of the numbers:
Dolby Digital, DTS and CDs digitize analogue sound by sampling it. CD samples 16bit words at 44.1KHz. Dolby Digital and DTS both sample at 48KHz/24bits. These seem like bigger numbers until you take into account that they compress them just like zipping your Word documents to send in an email. What you’re listening to (and watch) on your DVD has to be uncompressed on the fly as you watch the movie which isn't good for high fidelity sound.
The numbers may show significant differences in how the soundtracks are created but to really hear the difference between a 16 and a 24 bit word you’d have to make careful comparisons under controlled conditions. It’s likely your DVD soundtracks will sound better (or worse) because of other more important concerns than bit and sampling rates. Probably the most important contribution to the fidelity of any digital recording is not the technology used in producing a recording but its implementation. A well made recording will sound excellent, and movie soundtracks are made to really kick it in at the right moment for the movie, to draw your attention. The other important factor is the number of discreet channels on the DVD. Dolby Digital and DTS soundtracks are recorded in 6 channels (or five and one tenth, 5.1). This is pleasurable to our ears because of a principal called psychoacoustics. Humans are programmed to perceive depth and subconsciously localize sound. That’s why Stereo sounds cool compared to mono and why 5.1 (DVD) sounds so good compared to Stereo(CD). So, don't despair if you're wondering why DVD soundtracks in fact sound better to you, they do to most people. Don't worry about the numbers just enjoy the sound.
If you were to send your Dolby Digital sound to just one speaker with some high end gear and a keen ear, then compare to the exact same recording on CD, I think the CD would probably sound “better“. If you think you can hear the difference it behooves you to try DVD Audio or SACD, both are relatively new high sampling rate uncompressed multi-channel music formats. Two-channel music will never sound the same way again. As much as I love the sound of DVD-Audio I ask myself; how much of this is just acoustic placebo? Maybe there really are people with golden ears who can hear the difference, but when I believe I am one of them I pop my audio test DVD in and bring my hearing back down to Earth. As a man approaching middle age (and I consider my hearing good) I can barely discern sounds below 34Hz or above 14kHz. This is not a formal hearing test by any means but most adults are far from the 20hz to 20Khz, the so called range of human hearing. It's possible any sound quality gains from a 96kHz/24bit sampling rate on DVD Audio is lost on my ears, which have endured a lot of live rock and roll in their day.