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SHOW NEWS! AT Hi-Fi '98
Sony/Philips Direct Stream Digital (DSD)
The Direct Stream Digital (DSD) process was developed as an archival replacement for the aging analog tapes that store much of the world's great music. Sony has been archiving their catalog of recorded music in this format for some time now. DSD is very different from the linear PCM used on current CDs and represents a paradigm shift for recording a digital representation of an analog waveform.
Linear Pulse Code Modulation (PCM) digital signals from CDs and LaserDiscs (and some DVDs) utilize a 16-bit digital code to represent the amplitude of an analog waveform at a given instant in time. (Longer word lengths that code the signal with greater precision are used in the recording process, but the actual CD standard only supports 16-bit precision). Sixteen-bits can represent 65,536 different levels of amplitude. These digitally coded samples are taken at a rate of 44,100 times per second. That sample rate limits high frequency response to about 20kHz (half the sample rate minus some additional room to allow for filter roll-off). During playback, a digital-to-analog converter transforms these discrete, digitally coded samples back into an analog waveform. Converting from analog to digital and back again is a complex task that requires incredible precision for accurate results.
The Direct Stream Digital process samples the analog waveform at 2.8224mHz! That's 2,822,400 times per second or 64 times the rate used for Compact Disc. It's a 1-bit system so each sample is only 1-bit in length (it's either a "1" or a "0") rather than the 16 to 24 bits used in the linear PCM system. Each sample simply codes whether the analog waveform was rising or falling at the time that the sample was taken. A waveform that rises in amplitude between one sample and the next will be coded as a "1" and a full-positive waveform would be represented by all "1s." A falling signal will be coded as a "0" with a full negative waveform represented by all "0s." A signal at the base line between positive and negative going analog amplitudes will be coded as an alternating series of 1s and 0s. This digital coding method which symbolizes the rate of change of an analog signal rather than its specific amplitude at a given time is known as Pulse Density Modulation (PDM). The resulting digital signal is very analog in nature and requires no conventional D-to-A conversion for playback. All that is necessary is a low-pass filter to remove the sampling frequency and noise shaping to shift the frequency range of the resulting noise upward and out of the audio range. The result is a coding system with a claimed frequency response of DC to 100kHz and noise at -120dB (or better) throughout the audio band. This is sufficient to represent a 10kHz square wave with reasonable fidelityxsomething a 44kHz/16-bit CD cannot do. The DSD system is simple to implement but it is not conducive to DSP manipulation in the bit stream form. DSD-coded material will probably be converted to linear PCM for digital signal processing, if necessary.
The Super Audio CD is a single-sided, two-layer disc that looks like a regular CD and will play in all existing CD players. The high density layer that contains the DSD-coded material is semi-transmissive and invisible to the laser in a regular CD player. This layer can hold 4.7GB of dataxenough for 74 minutes of six-channel DSD audio plus text, graphics and a limited amount of video, plus a two channel stereo mix of the same material! Fitting all this data on the disc is accomplished by lossless compression using a Philips-designed coding method called Direct Stream Transfer (DST). DST achieves a 50 percent reduction in data rate with no loss of integrityxthe original signal is recovered bit-for-bit.
The high density DSD-coded layer requires a new Super Audio CD player for playback but the second Red Book layer will play in the machine you already have. The Red Book standard CD layer beneath the high-density layer offers 780MB of data capacity with the same quality that we get now from CD, but the Red Book layer is made by downconverting the DSD-coded recording. Downconverting the DSD-coded material to 16-bit/44kHz using Super Bit Mapping Direct™ technology provides improved sound quality when the Red Book layer is played in a regular CD player.
The SACD camp claims that single-bit DSD technology is superior to PCM, especially for studio work. But until new DSD receivers appear, the DSD signal must be converted to multichannel PCM before it can be fed to a digital audio receiver. SACDs have the distinct advantage of playing in CD players as well as in new SACD/DVD players. Unfortunately, the physical watermarking used for copy protection on SACDs requires new optical pickup circuitry, so the high-resolution layer won't be readable in the millions of DVD-ROM PCs on the market. And the big disappointment for those interested in video is that SACD adds only text and graphics.
The dual-layer, hybrid disc could be sold next year in stores as if it were a regular CD. Most people wouldn't know that the DSD-coded material was there and they wouldn't care, because most people are satisfied with the sound quality of CDs now. You and I, on the other hand, could start collecting music from a single inventory at CD retailers even before we have a player that can utilize the new higher-quality part of the disc. When we do upgrade to a new Super Audio CD player, the SACDs that we have collected will deliver higher sound quality from the second, high-density layer containing the DSD-coded material. Everybody seems to win with this proposal except for competing manufacturers who will fight it tooth and nail. That's assuming that Sony/Philips can actually make the discs, and that they will actually offer them for sale, and that any additional manufacturing cost will be absorbed so the retail price of CDs won't go up. And then again, the public may decide that the DVD disc is the proper medium for the next generation of music recordings and the whole Super Audio Compact Disc proposal may never fly.
The Sound Of DSD
Previous comparison demonstrations of the DSD technology that I have heard have been sourced from computer disk storage and were very impressive. DSD was clearly superior to both 44kHz/16-bit and 96kHz/24-bit linear PCM. Super Downconverted 44kHz/ 16-bit from DSD was audibly better than regular 44/16 CD in these demonstrations. But it's a long way from the computer to the actual CD disc and lots of quality can get lost in that journey.
The Marantz demonstration at this show was sourced from actual discs, but the Super Audio CD disc was single layer. Marantz used two players and two discs to compare a DSD-coded disc to a regular 16bit/44kHz CD. Marantz had a pair of big Dunlavy speakers that I found preferable to the Sony speakers used in the Sony demo. The audible difference between the Super Audio CD and regular CD in this presentation was not as great as in previous demonstrations from computer disk, but was still perceptible.
The Sony demo of DSD used a real dual-layer hybrid disc for part of the audio comparison. They loaded a dual-layer disc into a prototype Super Audio CD player and played it. Additional, newly recorded DSD material that we heard was sourced from computer disk. Both multichannel and two-channel material was demonstrated. That same Super Audio CD disc was then played in a boom-box portable to show compatibility. Apparently, they really can make the dual-layer discs, and a standard CD player (even a cheap one) really can "see" through the high-density layer.
Some current DVD players may not play the Red Book CD layer of the Super Audio CD because it is the innermost layer, and the player may become confused when it tries to read the high-density DSD-coded layer nearest to the outer surface. I'll bet that the current Sony machines will work and all future DVD players could be made to play the new discs.
DMP (Digital Music Productions), Mobile Fidelity Sound Lab and Telarc International announced support for the Super Audio Compact Disc format at the show. Of course, the record companies owned or controlled by Sony and Philips will be on board.
I think that the Sony/Philips Direct Stream Digital process has great potential for delivering high quality sound and high-end player manufacturers will be better able to fully exploit its capabilities, but there are a lot of questions yet to be addressed, and public acceptance of another format is at the top of the list. I can't wait to get some hardware and software to listen to using my own system. I'll keep you advised of the results, when and if this happens.
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