All About DVD Part 4: DVD-Audio and SACD

When acim audio emerged, most music producers and music lovers were absolutely sure that Audio CD provides the best quality sound possible and nothing else will ever be required. However, some audiophiles instantly refused to accept new Audio CD and reverted back to old vinyl disks. At first no-one took them seriously, however, as the time passed an increasing number of listeners started to notice, especially when comparing CD albums with the same records on vinyl, that CDs are not capable to transmit full range of sound. Eventually it becomes clear that there are scientific reasons behind this, namely dynamic range and sampling rate.

Dynamic range is the difference between the loudest and softest sounds in audio record. Sound data is recorded on CD in 16-bit PCM format and therefore its dynamical range does not exceed 96 decibels. And the upper range for human ear is 120Db. The difference is obvious. While the lack of dynamic range might be hard to notice for the pop music, but for something like symphony orchestra sound engineers often have a problem with too soft or too loud instruments which are beyond dynamical range and therefore lost in record.

Apart from dynamic range it also became clear that Audio CDs have insufficient sampling rate. Digital record consists of small pieces called samples, each of them contains information about sound in particular point in time. The more samples are per unit of time, the higher is the quality of record. Basically, sample is a digital analogue of sound oscillation and for that reason they are measured by using the same unit, hertz (Hz). The maximum frequency of sound that humans can hear is around 20 thousands hertz (20 kHz). To reproduce sound accurately, the sound information have to be digitized with sampling rate higher than a human can hear.

During Audio-CD development it was believed that doubling this rate – up to 44.1 kHz would be sufficient. But for some records part of sound was lost and turned into noise. Especially often that happened for classical music and jazz, with high-frequency instruments such as violin and flute being the most unlucky ones. It is scientifically proven now that for the most accurate sound the sampling rate has to be not lower than 64 kHz. This, of course, is far beyond Audio CD capabilities.

To address these issues the DVD-Audio format was developed. It took more time to approve than DVD-Video, as DVD-Forum had to justify between two alternative technologies: one from Toshiba and the other from Sony-Philips (they also developed Audio CD) alliance. The problem was to decide, which format was better. Both of them successfully solved the drawbacks of Audio CD.

Toshiba simply improved existing PCM standard and turned it into 24-bit one. The dynamic range grew to 144db and sampling rate soared up to 192 kHz. These values exceed human ear capabilities by a good margin. This format also had support for multi-channel sound.

Sony and Philips took another route and invented new single-bit audio format called DSD (Direct Stream Digital). The idea was to read data from disk in significantly smaller quantities (one bit at a time, compared to 16 bits for Audio CD and 20-24 bits for DVD-Audio) but with insane sampling rate – 2.8224 MHz. Similar principle is actually employed in the vinyl audio pickup. Due to this the dynamical range for new format grew to 120db. Apart from that multi-channel support was added along with compatibility with Audio CD. The latter was achieved by adding second layer with sound in Audio CD format. If such disk is inserted in generic Audio-CD player, it will be played just like any Audio CD, with all limitations of that format.

At the end, in 1999 Toshiba finally prevailed. This was due to several reasons. First of all, DSD soundtrack is hard to make even in studio environment. Second, when this technology first appeared, there was no special equipment that could support it. All existing devices were able to work with PCM sound only, and DSD required conversion. This eventually reduced the benefits of DSD to zero. Toshiba, however, selected DVD as a medium for the new format, which made it possible to create audio disks in this format even at home. The situation with Sony-Philips creation was completely different. It had to be recorded on special disks, which were similar in appearance and sizes to normal DVD, but totally different otherwise. These disks also required special equipment for printing and recording. All of these were the reasons why Toshiba received rights for the DVD-Audio brand-name.

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