A dozen years ago there was a strong push to replace the compact disc. As Stephen Dawson explains, it hasn’t worked out the way it was planned.

When the compact disc fi rst hit the market in 1982 it was technology right at the edge of the commercially possible. (Indeed, one of the fi rst two players didn’t even attempt to fully deliver the possible 16 bits of resolution.)

And it was immediately dealt a blow by some sections of the high fi delity community which declared its sound quality inferior, usually laying the blame at the limits to its resolution.

I recall that, oddly, the critics tended to prefer the sound from the lower resolution (14 bit) Philips player to the higher resolution (16 bit) Sony player.

Nonetheless, it was plausible that the resolution may not have been suffi cient. The sampling rate, in particular, of 44,100Hz meant that a harsh anti-aliasing fi lter was required. This had to somehow leave a 20,000Hz signal unharmed, while ensuring that nothing above 22,000Hz was reproduced. That could result at least in severe phase shifts in the top octave or so of the sound.

So there was always some interest in providing a higher resolution format for audio. And by the end of the century another defi ciency in the CD had become apparent, this one of commercial importance.

COPYING

Sony and Philips, the developers of the CD, hadn’t put a great deal of work into copy protection of its contents. So with the march of technology it was soon easy to rip a CD and burn a copy. Some efforts were made on the legislative front to have the computer makers hobble their systems to prevent copying, but fortunately this failed.

So a promising solution was something that would be a win-win: a new format which offered much higher resolution than the CD, overcoming perceived defi ciencies in the sound, as well as strong copy protection to preserve copyright holders’ control over their music.

But this effort in the end turned out to be very close to a total fail.

The CD goes on as the dominant physical sales medium for music. But it is being hit hard not by high resolution alternatives, but by internet downloads of the same music in arguably lower quality digital formats such as MP3 and AAC.

The principle reason was that the high resolution offerings were never anything of interest to the mass market. A subsidiary reason was that, quite unhelpfully, two competing and incompatible formats were released at roughly the same time.

DVD AUDIO AND SUPER AUDIO CD

At first glance there seems little difference between these two formats: DVD Audio and Super Audio CD (SACD). Both offer stereo and multichannel sound. Both offer far more technical precision than the 44,100Hz and 16 bits of the CD. Both are on 12cm discs, so both could fit physically into the same player.

But they are oh so different in other ways. DVD Audio is closely related to the standard DVD. Physically it is identical, and indeed they typically have a regular DVD version of the content – but in a compressed audio format on the disc.

The high resolution audio is kept in a different part of the disc, not accessed by a regular DVD player.

The main audio is normally in a format called Meridian Lossless Packing (MLP). This is an advanced compression format that allows the original digital sound to be perfectly reconstructed without any of the losses inherent in things like MP3, Dolby Digital, DTS and so on. MLP typically allows a space saving of roughly 2:1, but that depends upon the audio being compressed.

The underlying digital data, though, is the same Pulse Code Modulation (PCM) format as the original CD, just with a lot more detail. Rather than 44,100 samples per second, the norms are 48,000, 88,200 and 96,000, with 176,400 and 192,000 also appearing from time to time. Those frequencies allow recordable bandwidths of, conservatively and respectively, 20,000Hz, 40,000Hz, 40,000Hz, 80,000Hz and 80,000Hz.

The number of bits used to record each sample is typically 24, rather than the 16 of CD. That is, rather than counting on a scale of about 66,000, DVD Audio counts on a scale of about 16.7 million.

The theoretical noise floor of a 16 bit system such as CD is around -96dB. For a 24 bit system is it around -144dB. Up to 5.1 channels are supported on DVD Audio for the sampling frequencies up to 96kHz and two channels for the higher ones.

DVD Audio discs come with basic video menus to allow you to navigate between different audio streams and so on, and some also provide photos, lyrics and video extras.

DIRECT STREAM DIGITAL

The SACD is very different. It uses a vaguely DVD-like physical structure, but with some differences. The most obvious one, and the biggest advantage it has, is that it supports a second physical layer of data specifically compatible with regular CD. Many SACDs are ‘hybrid’ discs, with a standard CD version of the music on one layer, and a high resolution version on the other.

That high resolution version is often in both stereo and multichannel forms. But in both cases they don’t use a PCM format at all. Indeed, the audio is not compressed. Instead a digital audio format called ‘Direct Stream Digital’, or DSD, is used.

This was marketed as somehow being more ‘analogue’ sounding than the PCM format used on CDs (and, effectively, on DVD Audio). But this is of course nothing more than marketing. It is a digital format (a form of Pulse Density Modulation) which shares the same virtues as other digital formats: primarily, robustness against degradation during copying.

Effectively, each channel of the sound is represented by a stream of bits running at 2,822,400 per second. Instead of each sample of the sound being recorded as a number, the level of the sound at any instant is represented by the number 1 bits as opposed to 0 bits. The more 1s, the higher the wave form is at that moment.

The capacity of an SACD is close to 8GB, not far short of the 8.5GB for DVD Audio. But because there is no data compression, there is less space for high resolution audio than with DVD Audio. It was often claimed that the DSD format gave a frequency bandwidth of up to 100,000Hz and a signal to noise ratio of 120dB, which is impressive indeed.

The reality isn’t quite that good. In order to get a noise floor as impressive as -120dB, a fairly aggressive ‘noise shaping’ technology was employed. Rather than the natural digital quantisation noise being spread evenly over the entire reproduced spectrum, it is pushed up into the ultrasonic region. That is, noise is low below 20,000Hz, but the noise floor increases at higher frequencies. Indeed, examining real-world recordings, the noise actually rises to a higher level than the signal at somewhere between 25,000Hz and 35,000Hz.

Nonetheless, like DVD Audio, this is a massive improvement on a technical level to CD.

In practice the SACD – despite an allowance for such in its specification – does not have provision for video, so there is no on-screen menu navigation. SACD also had implementation problems in its multichannel incarnations, in that while there are many well developed tools for manipulating PCM audio, PDM is less well handled. So in order to do things like adjust the channel timing which is necessary for the bulk of realworld surround sound systems it had to be converted to PCM.

BUT…

CD was a clear and obvious improvement to the LP and the audio cassette, in a way which was obvious to the great majority of listeners. Not only was the sound relatively noise and scratch free, the discs were somewhat more robust than LPs (although not as robust as the early claims made for them), and far more practical in use with reliable cueing and such features as automatic repeats, random playback and so on.

Within a decade CD had achieved sales dominance, and now the LP has been relegated to an important, but niche, corner of the market.

The advantages of SACD and DVD Audio were nowhere as clear to the mainstream music consumer. They were, if anything, somewhat less practical in use and the case that their clear technical quality actually improves perceived audio quality has yet to be made.

So they never really took off. But nor did they die. You can still purchase players for both, and indeed discs are still being produced for both.

Just last month, for example, I purchased newly prepared DVD Audio multichannel remasters of Emerson Lake and Palmers’ first two studio albums.

But eventually, it seems likely, both formats will be supplanted by the obvious replacement: Blu-ray. Indeed, lately EMI has been releasing ‘Immersion’ box sets celebrating some of Pink Floyd’s mid-70s greats. The high resolution version of Dark Side of the Moon wasn’t DVD Audio, nor SACD. It was on Blu-ray… physically. The actual content was a port of an earlier SACD version.