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Analog Enhancement Layer (AEL) - the next step for CD?

The CD format has been with us for over twenty years, yet attempts to supersede CD with new domestic formats such as DVD-Audio and SACD are now agreed to have failed. Is CD the limit? Has audio quality in the home gone as far as it possibly can go?


Although the sound of CD is acceptable to many people who like music, but have no real interest in recording, sound engineering or hi-fi, there are very many people who profoundly wish that audio had some kind of a future - a future where continuous improvement is possible, an ever-forward striding towards the ultimate sound.

The invention of CD was certainly very welcome in its day. At last it was possible to listen to excellent quality sound, free from background noise and clicks. Many people are indeed still very content with the quality of CD and do not wish for anything more.

However, it is simple human nature never to be satisfied; always to yearn for something better. But the problem is that CD is fundamentally limited by its 16-bit, 44.1 kHz resolution. [Which means that the audio signal is divided up into 65,536 possible levels, 44,100 times per second.]

This is a hard and fast limit. It is not possible to record any finer detail than this, nor is it possible for any CD player, no matter how good, to extract any more information from the disc.

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People who love hi-fi, and the quest for great sound, dislike this intensely. They would prefer to see ever increasing standards. And their voice has not been ignored. Manufacturers have developed two rival CD-like, but better than CD, formats: SACD (Super Audio CD), developed by Sony and Philips, and DVD-Audio (not to be confused with DVD-Video), developed in response by virtually all the other hardware manufacturers.

The problem is that neither of these formats has won acceptance from the record companies. DVD-Audio requires a special player - most DVD players cannot play DVD-Audio discs. SACD discs can be played on a normal CD player, but an SACD player is required to extract the extra data that is available on the disc. Since both types of player are very few in number, record companies - and pressing plants - are reluctant to get involved.

Hence, as of now, it is fair to say that these systems are both dead.

Even if one or other were to become successful, there would still be a finite limit on how good the sound could be, so in a few years time yet another new format would be required.

Analog technology

It is important to remember that digital audio is not inherently superior to analog. It merely has had the benefit of more research and development effort. Also, 'analog' does not equate to 'vinyl', as some hi-fi enthusiasts seem to think. It simply means that the audio signal is stored as a continuous variation of some characteristic of the chosen storage medium. Analog formats include magnetic tape, optical sound on film (which is still in near universal use in cinema) and of course the excursion of the groove of a vinyl record. But it is not limited to these.

Imagine a disc carrying a pattern of indentations, or pits as we shall call them. The spacing of this pattern varies according to the waveform of the input signal. The disc is tracked by a laser which measures the variations in spacing and converts these variations back to an audio signal. Two points...

  • This is analog
  • The description of a disc carrying a pattern of pits almost exactly matches the description of a conventional CD.

The only difference between this and a conventional CD is that the CD's pits are regularly spaced, but there is no reason why this has to be so. It is well within the capability of even a budget player to cope with random variations in spacing caused by the manufacturing process. But who says they have to be random?

It is perfectly possible that the CD format could be adapted to carry an 'analog enhancement layer', where the full 16-bit, 44.1 kHz signal is recorded in digital form, but there is an additional analog signal that carries a higher-resolution component from which the CD-quality signal has been subtracted.

A conventional CD player could play the disc as a standard CD, but an 'AEL' player could combine both signals to get the best possible sound.

There are some great beauties in this:

  • An AEL CD is playable on a standard CD player, at conventional CD resolution
  • There is no limit to how much an analog system can be improved through incremental measures - the level of quality is not fixed in stone as soon as the format is specified
  • AEL discs could be pressed on standard equipment, although the mastering process would require modification
  • Discs can be pressed in both standard and 'audiophile' versions.

Significance for manufacturers

All of the above would be great for consumers, and for music producers who care about sound quality.

It is also great for manufacturers, record companies and anyone connected with the record or manufacturing industry. Industry always requires a continuous stream of new developments to excite the public. Hi-fi magazines need something to write about. For these purposes CD, DVD-Audio and SACD are all dead in the water. Any new wholly digital format is surely destined to fail too.

The addition of analog technology gives us everything we have now, plus more, plus the possibility of continuous incremental improvement.

In short, pure digital audio was great in its day, but that day is over. The time has come for the best of both the digital and analog worlds, in the form of AEL - Analog Enhancement Layer.

You read it here first.

By David Mellor Wednesday September 21, 2005