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SACD, DSD, DTS, AC-3, DVD-Audio, SuperAudio

I suspect many disc collectors these days feel they are drowning in alphabet soup—SACD, DSD, DTS, AC-3, DVD-Audio, SuperAudio — and too many numbers ranging from 44.1 to 96 kHz, from "one" to 16 to 32 Bit. Let’s try to line up the letters and numbers and make some sense.

Regular CD format is 2.0 stereo, 44.1kHz 16Bit digital sound. This means that the frequency response of the two channels on the disk is limited to 44,100 divided by two or 22,050 Hz; an attempt to record frequencies higher than this could result in the production of subharmonics, so they must be filtered out before mastering. In practice, the frequency limit is even lower than this, due to the published technical standard. This would not be so bad, except that to be sure everything above 22,050 Hz is removed, the laws of physics require that some distortion and attenuation of frequencies below this limit must necessarily occur. The 16Bit encoding of each sample restricts the usable dynamic range before noise and distortion become objectionable, although clever ways (e.g. Sony’s "Super Bit Map," Mobile Fidelity’s "Ultradisc UHR," or Keith Johnson’s "HDCD" [requiring a special player and recently acquired by Microsoft]) have been found to work around this. Some CDs are "matrix encoded" with surround sound much as were the old SQ Quadraphonic LPs, and many of these are labelled "Dolby Surround." Because the Dolby laboratories charge a royalty for the use of the technique and the name, and since not all matrix encoding routines are licensed, there are CDs, quite a few in fact, which play back on your 5.1 Dolby decoder player in realistic surround sound without any notice on the packaging. If you like surround sound (I do), this is always worth a try.

The SACD format appears to be bound to the DSD "Direct Stream Digital" "one bit" recording technique. Recordings originally made in other formats must be dubbed to DSD before the SACD can be mastered. That "one bit" of course cannot be directly compared to, say, "24Bit" or other descriptions of digital resolution. My genius recording engineer friends deplore DSD as bad news all around, although some of the DSD recordings have sounded very good to my ears. It has been suggested that Sony’s high royalty fees have contributed to the distaste with which some view DSD, and others suggest that the system itself is fine but economics have led to certain compromises in practice.

SACD is a popular format with manufacturers because Sony has built in a copy protection scheme which reportedly uses two lasers during playback, i.e., there is NO WAY you’re ever going to be able to play it on a computer DVD drive. Sony and Philips had not until recently licensed other disc manufacturers, so manufacturing capacity was very limited, although obviously now increasing steadily.

Here is a good case where the media giants’ lawyers may have been conned, if you find that cause to rejoice. Sony has sold everybody on the copy protection features of SACD, but as any engineer knows, a commercial pirate operation would have no difficulty whatever re-digitising the analogue signal and producing pirate DVD-Audio’s from SACD’s—and they’d sound pretty good. However, if Sony, et al, can "win the race" and make sure "nobody" buys DVD-Audio equipment, then the market for pirate copies would be so small they wouldn’t be worth tooling up for. So, they are currently engaged in a vigorous and expensive campaign to manipulate the market. Remember, these are the people who brought us the El-Cassette, the Betamax, Digital Compact Cassette (DCC), and the Mini-disc. Not all innovations result in improvements, but, of course, without innovation we would have no improvement.

Many digital music master recordings had originally been made using 48kHz sampling (24,000 Hertz high frequency response limit) at 24Bit depth. If the original CD "Redbook" standards had been established thus, we could have started out with high resolution sound. But the 44.1kHz/16Bit format was dictated by the need to have the disk player fit into a standard automobile dashboard cut-out, which required a 118mm (4.6 inch) disk and to be sure that Beethoven’s Ninth Symphony could fit on a single disk, which was at first was limited to just 74 minutes of music. Of course in no time the technology was stretched beyond "Redbook" standards to allow up to 83 minutes on a single disk, so we could have had the 48kHz after all, but by then it was too late.

With DVD-Audio, a single 118mm inch disk can hold an entire opera at 48/24 resolution, but so far the commercial disks have instead been partitioned to allow a regular CD length program to be presented in several different formats, perhaps in imitation of the hybrid SACD, and sometimes also including brief video programs as well. Since no DVD can be made to play on a regular CD player, DVD-Audio disks are made to be "compatible" with home DVD players instead, of which there seem to be a sufficient number now anyhow. And delay and controversy over formats has allowed formats to proliferate so that a dedicated DVD-Audio player must test each disk for dozens of data arrangements and can take up to three minutes to initialise, whereas my SACD player chirps merrily for three seconds and is ready to go, no matter how many buttons I’ve pushed.

DTS stands for "Digital Theatre System" and as the name implies was invented for surround sound films. It offers superior accuracy in surround sound and is often the best sounding track on any disk that offers it. Many home DVD players are labelled "DTS" but watch the fine print: if it says "digital output" it means that an accessory DTS decoder must be connected to the digital output jack; without the additional expense of that decoder you get only silence if you select that option.

Sometimes the surround channel tracks on SACD’s and DVD-Audio’s are actually derived electronically from the two track recording. Your player may be able to do this better than the engineer at the studio. If you are disappointed with the sound quality in "Surround" setting, try playing the two track version on one of your player’s enhanced surround settings. Sometimes this will result in superior sound. Some players have the licensed "Dolby Digital" or AC-3 setting, others have it only for DVD video playback, and some players have proprietary systems which are to all practical purposes identical to the Dolby Surround system in all but name. You won’t know ‘till you try.

There has been talk of "watermarking" high resolution audio disks, but despite advertising claims no one has been able to work out a means of achieving the aims of watermarking without degrading the sound quality unacceptably. At the best, watermarking would not make piracy impossible or even difficult, but would make it a little easier to detect and prosecute. At the worst all the advantage gained by moving to high resolution sound would be lost. Do not buy any labelled watermarked audio disk without listening to it first on high quality playback equipment to be sure it is listenable. If you buy a disk which has degraded sound quality and you discover or suspect it may be watermarked, return it to the store at once and demand a full refund of your money.

SACD’s generally play nothing on your television screen but the standard screen-saver and text track information, but when playing DVD-Audio’s you often get menus with colourful scenes which change with the selection, after you visit the audio set-up menu to select your preferred manner of playing. Most DVD-Audio disks sound best on the DTS setting when played on a standard DVD player, but there are exceptions to that rule; try playing any disk in all the modes it offers to find the optimum presentation.

Paul Shoemaker

25 March 2004

A lengthy Postscript Feb 2005 S

When most CDs are labelled ADD, that means that the manufacturing of the CD has begun with the analogue signal produced by the playback of the original session master tape. This signal began with the sound pressure wave in the air at the recording session which caused a microphone diaphragm to move in a replica of this pressure wave. That motion caused to be produced an electrical signal, a replica of the movement of the microphone diaphragm. That electrical signal was amplified electronically (for now we will assume that this amplification was perfect and without flaw) and then was used to produce a time-varying magnetic field in a recording head. A mechanical tape transport device presented a moving magnetically susceptible tape to this record head and received a replica pattern of magnetisation corresponding to this time-varying magnetic field in the record head. To begin any further processing, this tape is moved by a transport mechanism by a playback head and the varying magnetic field in the tape generates a varying magnetic field in the armature of the playback head which in turn generates an electrical signal in the wire coils of the playback head.

At this point, the original sound wave pattern has been remapped six times and subjected to two mechanical processes. The laws of physics decree that at each remapping there is an increase in noise, a distortion of the wave form, and a blunting of transient sounds. At each mechanical process, wow and flutter are added. That the resulting sound is as good as it is is a tribute to much hard work, many years of engineering successes, some dramatic breakthroughs, some careful finessing of myriad tiny details.

The subsequent digitisation of the sound wave replica is accompanied by some noise and distortions of different kinds from the analogue processes (digital information theory is as spooky as quantum theory and cannot be understood by mere mortals), but once this is accomplished, there are no more significant distortions between this point and the digital electrical signal produced in the CD player. It has been said that CDs potentially give us a personal copy of the original master tape recording, a potential which has only been substantially realised by the SACD and DVD-Audio formats.

(There are people who believe that analogue recording processes, even in succession, result in a Divinely perfect living imprint of the original sound wave, whereas digital sound processing is a Satanic perversion which forever kills the life of the music. We have freedom of religion in Western nations, and this is not the place for a discussion of this opinion, or testimonials to Holy Martyrs to the Faith. From the scientific point of view, both digital and analogue sound processing can be meaningfully described in terms of noise level, resolution, and distortion level. Neither is perfect, both are flawed, and the result is as exactly as good as the care and money you invest in your project.)

In the case of recordings restored to CD from manufactured disk recordings, the journey is longer. The signal from the playback of the master tape recording first goes to a disc cutter head wherein a varying magnetic field is produced which causes an armature with an attached cutting stylus to move in, it is hoped, a replica of the original sound pressure wave. A mechanical transport device moves a vinyl surface by this vibrating cutting stylus which digs a meandering groove in the vinyl. Later this vinyl disk, or a contact replica of it, is moved under another stylus, this one a polished diamond, and this stylus moves in a replica of the sound pattern in the groove, generating a varying magnetic field which in turn generates a varying electrical signal. Now, we are ready to begin digital processing for the production of our CD, after six more remappings of the original signal as received from the master tape. Three additional mechanical processes have further increased the wow and flutter levels. Even if the processing is digital from hereon out, calling this disk an ADD is probably not accurate. It probably should be called an AADD disk.

During the high 78 rpm era, the nineteen teens through forties, records were routinely duplicated simply by playing the disk into disk cutting equipment. The quality of the result was horrendously bad, but there was no other way. In the analogue tape era, roughly the fifties through the eighties, master tapes were routinely duplicated onto other tapes for various reasons, such as: 1. Security copies in the event that the original deteriorated, was lost, or damaged in handling. 2. Early tape splicing tape was only designed to last long enough to duplicate the tape onto a new splice-free "master" after which the first tape, bearing the rapidly deteriorating splices, was discarded. 3. Unscrupulous employees would duplicate a master tape, keep the original and leave the duplicate in the vault. In a recent project to find "original master" tapes, it was found that due to careless storage and careless labelling it was not always possible to determine which of several tapes was actually the original master. Clearly, if they couldn’t tell, it shouldn’t matter, but the result should honestly be labelled "A[A?]DD."

One is always amazed at the accuracy of these remappings of the original signal, how low is the accumulating noise level, how bright remain the transients, considering what has been done to them. In one instance I have directly compared the wave form produced by an ADD CD transfer made from the original master tape against an "ADD" transfer made from manufactured LP disks using modern restoration methods and found no visible difference, and no easily audible difference. That it can be so is a marvel, a wonder, for which we should be deeply grateful.

 



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