DVD Audio Part I Of II: DVD Audio CDs
DVD Audio Compact Disks Are Great
By Trevor Marshall
December 04, 2000
Now that SDMI has "officially" survived the attacks of hackers, the first of a new crop of DVD-Audio Compact Disks are becoming available. What is DVD-Audio? What can it do?
"It Was 20 Years Ago Today"
Well, 33 years ago, actually, and I was struggling through an engineering course at Adelaide University. I vividly remember one of my friends excitedly saying, "Trevor, you've got to come over to my place for lunch. I want to show you something that will blow your mind" (it was the 60's, and we really did talk about "blown minds" back then). Well, what I heard that lunch time did blow my mind, and it blew millions of other minds as well.
My first experience with Sgt. Pepper's Lonely Hearts Club Band was in a tiny student apartment that contained two huge speaker boxes, lovingly hand-crafted from particle board, each housing a twin cone 8" Wharfedale speaker. Although the earlier Beatles' LPs (such as Rubber Soul) had experimented with stereo recording, stereo had added nothing to the musical experience. This time, however, Lucy in the Sky with Diamonds and With a Little Help from My Friends, had a sound that was fresh and new, a sound that was at once persuasive and seductive. Stereo sound had arrived.
Sometime in the 1970's a system called "Quadraphonic Sound" was launched. It failed to reach critical mass, and disappeared. Why? Well, there was no content available that could take full advantage of the two extra rear channels. No Sgt. Pepper was ever released for Quadraphonic sound.
In the 1990's, Dolby Laboratories launched Pro-Logic and Dolby Surround, both of which are analog systems that extract center-channel and rear-speaker information from an (ostensibly) stereo data stream. But I am getting a little ahead of myself. Let's get back to basics.
(Dolby Labs' diagrammatic layout of a "typical" home entertainment area, showing the listening lounge, television, and five speakers.)
Basically, Dolby's analog schemes extract spatial information from a two-channel audio stream that has been mixed from the discrete multi-channel audio signals using a matrix encoder. If you are interested in reading technical details of the matrixing process, this document contains a comprehensive description of the latest Matrix surround-sound technologies.
When the DVD-Video sound-track specifications were drawn up, they dictated the main sound track on a video DVD would be either in Linear PCM (2 channels) or in Dolby Digital (1 to 5.1 channels).
Dolby Digital AC3
The AC3 Dolby Digital specification allows for five audio channels: front left, front center, front right, rear left, rear right, and sub-woofer. The sub-woofer derives its content from an analog low-pass filter of the main five speakers, and an additional (digital) Low Frequency Effects (LFE) channel. Hence the "5.1" moniker.
AC3 encoding can be performed with 32,44.1 or 48-KHz sampling rates, and the maximum output data rate is 640,000 bits per second (640 Kbps).
Contrast this with standard digital audio CDs, which have a 44.1-KHz sampling rate and two 16-bit PCM audio channels. This gives a data rate of 1411 Kbps.
Clearly, Dolby AC3 is a very efficient method of compression and encoding of a five-channel digital audio stream. It gives a 10:1 compression advantage over the raw PCM data. Not as much as an MP3 encoder achieves, but the compression methodologies are somewhat similar.
An AC3 file consists of data "frames," each of which is composed of blocks of data, together with a CRC integrity check.
A Real World Example
(Click on the thumbnail for a larger picture)
I took the AC3 audio track from the video DVD Yellow Submarine and loaded it into Sonic Foundry's Soft Encode, in order to examine the individual track content.
Ringo's voice "In the town where I was born..." leads in Channel 1, the front-center channel, and most of the musical accompaniment has been mixed into channels 0 and 2. You can see the amplitudes of channels 3 and 4 (the rear speakers) and channel 5 (the LFE subwoofer channel) are much lower, and that these speakers really only come into play about halfway through the song, when the sucking, gurgling submarine noise and "full speed ahead it is, Captain" are played through them.
You can also see that the AC3 frame size of this selection is 1792 bytes, and the audio is sampled at 48 KHz. The average data rate is 448 kbps, much less than the 1411 kbps of a normal audio CD.
There have been many DVDs that have used the AC3 Surround sound to good effect, but I want to mention one, specifically.
It is Queen's We Will Rock You. Recorded live in Montreal during 1983, this video was remastered last year into Dolby AC3 Surround Sound. The result must surely be the definitive use of digital surround sound to enhance the entertainment experience from that of the original video.
If you want to experience what Digital DVD technology can really achieve, then go down to your local retailer with the We Will Rock You DVD clutched firmly in your hand, wind up the volume to the same level it would be in the theater (well, maybe a few dB softer) and select tracks 23 and 24 (We Will Rock You and We are the Champions). You will find yourself part of the audience, enjoying the finale of one of the classic rock concerts of all time.
If you play the VHS videotape with a stereo version of the audio mix, it is flat and uninteresting in comparison with the DVD.
Even though it is fun to sit in a theater and listen to helicopters flying from back to front and side to side, I can think of no better demonstration of the potential of surround-sound technology than this Queen concert. I wait with baited breath for other spectacles, such as Orff's Carmina Burana and Handel's Messiah to be recorded into the DVD format.
So What Is All The Fuss About DVD-Audio?
Way back in the past, the DVD-video group decided on a specification for video DVDs. If you look at the directory structure of a DVD disk you will find two folders in the root, one named VIDEO_TS and one named AUDIO_TS. The AUDIO_TS folder is always empty, as the industry managed to decide what to put in the VIDEO_TS folder quite early on, but the dispute over what was to go in the AUDIO_TS folder, the so-called "DVD-audio" content, was only resolved quite recently.
I am reminded of the furor that broke out when the digital audio CD was first mooted: "Digital doesn't sound as good as vinyl" and "digital sounds harsh, it will never catch on." were some of the early complaints. Well, the audio CD did catch on. Now (nearly) everybody is using audio CDs to store audio content.
The purists argue, however, not only that Dolby Digital Compression causes a corruption of recorded sound, but also that the audio CD format itself is a corruption of the higher-precision digital formats used to capture and mix the content.
DVD disks offer more than six times as much data storage as audio CDs. In my humble opinion, the industry has been trying to figure out how to gobble up that extra data bandwidth before somebody realizes that his whole collection of audio CDs could be converted to MP3 format and stored on a single DVD-Recordable disk! What the industry has come up with is a marketing-driven push toward a DVD-audio standard that will obsolete all our audio CD players, and most of our DVD players and DVD-ROM drives.
DVD-audio allows up to six channels of PCM audio, 24 bits in accuracy, with sampling rates to 192 KHz. The audio CD pales by comparison, with two channels of PCM audio at a 44.1-KHz sampling rate with 16 bits of accuracy.
When Will They Ever Learn?
In Part II, I am going to examine DVD-audio in more technical detail, contrasting the manner in which surround sound will be processed. I will also look at some of the new hybrid DVD-audio-video disks. Finally, I will look at some software that lets you produce your own Dolby AC3 audio CDs, on a standard CD-R burner, that will play on standard DVD-video players.
(Continue on to, DVD Audio Part 2: Sounding Good By Trevor Marshall)
Trevor Marshall is an engineering management consultant, with interests ranging from RF and Hardware design to Linux internals, Internet infrastructure, MPEG, and Digital Video. He started his career in the '70s, designing the Maplin Electronic Music Synthesizers. When the Microcomputer came along, he got sidetracked into computer software, programming the 2650, 8080, Z80, Z8000, 8048, 8306, 6805, 80x86, and Power PC families. Along the way, he also picked up a little expertise in RF system design, biomedical engineering, and the printing industry. His web site is www.trevormarshall.com. Page 1 of 1
A list of all 75 papers Trevor has published is available at this link.
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