From Mar 95 issue

The technical bit

Silicon Audio uses a compression technique to enable it to fit a decent amount of music on to the memory card. The quantity of music held on the card is therefore defined by the compression technique used and the size of the memory card, measured in megabytes.

There are a variety of compression techniques used by other music formats, each using different algorithms. Sony's Mini Disc uses ATRAC (Adaptive Transform Acoustic Coding) and Philip's Digital Compact Cassette uses PASC (Precision Adaptive Subbed Coding).

Traditionally, the storage of the compressed data has been on optical or magnetic media, but recent developments in the capacities of silicon chips have meant that storing compressed data on a chip is now a realistic alternative.

And so we come to Silicon Audio, which uses a compression technique called MPEG/Audio Layer II. MPEG/Audio algorithms have three types of layered structures. The Layer I algorithm achieves subjective quality with a reasonable hardware size, while Layer III gives the best possible sound quality but is more expensive in terms of complexity. Layer II, the algorithm used, is a compromise between the two. The literature that comes with Silicon Audio claims that Layer II technique enables CD-quality music to be produced at a better compression ratio than that used by DCC or MD.

The maximum length of a recording is determined by the memory capacity of the card (as described above) as well as the bit rate employed in the algorithm and the number of channels used.

The prototype compresses a 20kHz, 770Kbits/s signal to one eighth of its original size (96Kbit/s), giving 24 minutes of recording on a 32Mb card. By the end of this century, NEC is hoping that memory-card technology will sustain over three hours of playing time on one card.