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Digital sound in high definition
DIGITAL sound in high definition is making waves. High-definition audio, or HD audio, the new formats are doing for music what high-definition TV is doing for television, recording it in higher detail and playing it back at a higher fidelity, or a more life-like way.
HD audio formats allow music to be recorded and played on the multiple channels of surround-sound systems such as Dolby Digital 5.1, which has six channels compared with the two channels of traditional CD stereo recordings.
According to Prof. Peter Otto, technical director of the Department of Music at UC San Diego, “The most interesting thing about HD audio is the idea of music recorded for and played back in high-definition surround sound. Think about being in a concert hall. You hear the music directly from the instruments, but you also hear a lot of reflections. The wood and other surfaces in the hall are designed to reflect the sound. It gives the music an immersive quality. With HD surround, you can re-create those reflections.”
There are two main HD audio competitors: DVD-Audio and Super Audio CD. Intel is working on a third format that would be used on PCs. DVD-Audio uses the same type of disc as DVD-Video, but it stores high-resolution audio files instead of video. DVD-Audio files are higher quality than the sound files on a DVD movie, which are CD quality. DVD-Audio was developed by a DVD industry consortium. Super Audio CD, or SACD, was developed by Sony and Philips. The format stores high-resolution sound on CDs.
Digital versus CD audio
Surround sound: CD audio has two channels — left and right — to produce stereo in the left and right speakers. HD audio has six or more tracks to produce surround sound in home theater systems with six or more speakers.
Sampling rate: A sound wave is measured as it moves in two directions: up and down, and across a distance (typically shown as left to right on a graph). In digital audio, sample measurements are taken at a number of points as the wave moves in those two directions. If a single wave were measured at five points, and the dots were connected by straight lines, it would create a very choppy record of the actual shape of the wave. If that same wave were sampled 500 times, the sample points would be closer together and would create a smoother recording of the actual shape of the wave. An increase in sample points improves the quality of a digital recording. Sampling rate refers to the number of samples per second. CD audio is recorded at the rate of 44,100 samples per second. High-definition audio increases the number of samples per second. DVD-Audio, for instance, typically records sound at the rate of 96,000 samples per second.
Sampling precision: For each sample point, digital audio stores two numbers representing the height and left-to-right position. The number of bits per sample determines how precisely those numbers are recorded. For instance, a sound wave might have a height of slightly more than five on a scale of zero to 10. A low bit sample might store the sample as 5.2. A higher bit sample might record that as 5.29786, which is more precise. Using more bits reduces the sampling error, as it is known in digital audio. CD recording uses 16-bit samples.
“DVD-Audio uses 24-bit samples. Unfortunately, lax recording techniques can negate the technical advantages of the new formats. A very well-produced CD can sound better than a poorly produced SACD or DVD-Audio recording,” says Prof. Atkinson.
The rivalry between DVD-Audio and SACD already has sparked debate among sound engineers and amateur audiophiles. Some see the competition as the latest sequel to the Sony Betamax versus VHS war, with the loser eventually disappearing.
There is one significant difference in this battle. Some manufacturers are making universal high-definition audio players that will play both formats, along with DVD-Video. Consumers with a universal player are able to buy recordings in either format, so some experts think both can survive. As the competition plays out, consumers will face some difficult choices. Initially, SACD was a stereo format, meaning it was designed to be played over two speakers. Music fans will need to buy a multi-channel SACD player, if they want to experience the surround-sound capabilities. Some SACD discs can be played on ordinary CD players. Those discs have two layers: (a) SACD layer (b) standard CD layer. The discs will produce CD-quality sound when played on ordinary CD players. DVD-Audio discs typically will play on several types of players. The discs contain a Dolby Digital track that will produce CD-quality sound when played on a standard DVD player. It will yield surround-sound if played through a surround-sound receiver. The discs also include a high-resolution stereo track that will play only on DVD-Audio players. A final, high-resolution surround-sound track will produce the full effect when played on DVD-Audio players connected to surround-sound systems. While there are differences, the two formats have some similarities. Most people who have listened to one or the other agree they sound better than CDs, although some think most of the improvements in sound occur at inaudible frequencies, so the difference is imperceptible to human ears.
High-definition audio is an improvement over CD audio because it records more detailed information from the original sound waves. It is somewhat comparable to the improvement between the image in a high-resolution photograph and a lower-resolution photograph. A high-resolution camera records more information and produces more pixels, or dots of color, in the photograph. The pixels in the high-resolution photo are also smaller and more precise.
Think of a sound wave as an arch, with two curving sides and a rounded top. An analog recording stores a representation of the entire wave. A digital recording cannot take in the entire wave; instead, it measures sample points of the wave. Going back to the camera comparison, a low-resolution camera would produce a fuzzy picture because the camera had taken a small number of sample points. If the resolution were increased, essentially increasing the sample points, there would be a more accurate recording of the image. If a sound wave were recorded as five points on a graph, it would make a very rough representation of that sound. If it were recorded as 1,000 sample points, it would be much closer to the original, smooth curve of an analog wave.
CD audio is recorded at the rate of 44,100 samples per second, which creates a very good representation. HD audio increases the number of samples per second. DVD-Audio typically records sound at the rate of 96,000 samples per second. There is a second difference. Using the camera analogy again, high-resolution photos have smaller, more precise pixels. High-definition audio uses more precise numbers to more accurately record the sample data. The precision is known as the sample bit size. CDs are recorded with a sample bit size of 16. DVD-Audio records at a bit size of 24.
While audiophiles debate the merits of the two competing technologies, consumers are warming to both formats, according to market research analysts. In a year or so, a new generation of computers will support HD audio and give further momentum to the higher-resolution sound formats, they say.
The writer
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