5.6.10 Minidisc The original Minidisc (MiniDisk, MD) appeared in two forms: as a mass replicated disc, which works according to the principles of optical discs, and as a recordable, actually rewritable, disc, which is a magneto -optical recording medium (cf Section 8.2 Magneto Optical discs). Both sub-formats may be read by the same players. The discs are of 2.5” (64mm) diameter and housed in a cartridge. Minidisc recordings employ Adaptive Transform Acoustic Coding (ATRAC),a data reduction algorithm based on perceptual coding. Data reduced formats, although highly developed (at least in the later versions of ATRAC), not only omit data irretrievably that would otherwise be captured by a non-data reduced format, but also create artefacts in the time domain as well as in the spectral domain.Such artefacts can lead to misinterpretations of spectral components as well as of time-related components, especially when analysing the signal by means of a spectral tool. The artefacts of data reduction codecs cannot be recalculated or compensated for at a post processing stage, as they are dependant on the level, dynamics and frequency spectrum of the original signal. ATRAC is a proprietary format, with many versions and variations, and for archival purposes it is advisable to re-encode the resultant files of compressed recording formats as .wav files. Many minidisc players have digital output which will allow the production of “pseudolinarised” data stream. The resultant file should comply with specifications laid out in chapter 2 Key digital principles and stored in accordance with that section. Metadata about the origin of such signals are imperative, as pseudolinearised signals cannot be distinguished from signals recorded without data reduction. This information would be recorded in the coding history of a BWF file, or be rendered as change history as per PREMIS recommendations (see Chapter 3 Metadata). In 2004 the Hi-MD was marketed, and it incorporated changes to hardware which, with the new media, would record up to 1 GB of audio data.With Hi-MD it was possible to record several hours of data reduced signals, but more importantly, it was also capable of recording linear PCM signals. For archival purposes these recording should be treated like CD signals and transferred as a data stream to a suitable file storage system. Extracting audio data directly from HD-MD at higher transfer rates requires specific proprietary software, some of which is available from manufacturers’ websites. It is advisable to purchase dedicated replay equipment and software immediately as prolonged manufacturer’s support cannot be guaranteed. The use of Minidisc as an original recording machine is not recommended (see section 5.7 Field Recording Technologies and Archival Approaches).