Notes about audio

MiniDisc (MD) was developed by Sony, and was introduced in 1992 as a format set to replace Philips Compact Cassette (CC) as a durable, reliable, convenient, portable and, most importantly, recordable audio medium. A digital medium similar to Philips Compact Disc (CD), MD is not yet a popular format in 2000, although it is beginning to be more widely adopted, and may yet replace the cassette.

This document assumes a basic understanding of the material in the Audiotek document Notes about Compact Disc, as the MiniDisc format will be explained using Compact Disc as a point of comparison.

What are the different types of MD?

All MDs (with the exception of those used for data applications) have the same external shape and size. However, there are two broad types of MD: prerecorded read-only discs and recordable magneto-optical discs.

Pre-recorded discs are essentially a physically smaller incarnation of Philips Compact Disc (CD) which are housed in MD's protective cartridge. They consist of a pitted aluminum sheet embedded within a plastic substrate which is scanned by a laser. The beam is reflected by the surface, and the varying intensity decoded to retrieve a digital signal.

The recordable magneto-optical variety of MD have a different structure, but are essentially read in the same way. Instead of a pitted aluminium sheet, a grooved layer containing a compound of terbium, iron, and cobalt is used. The magneto-optical properties of this compound are exploited to allow digital signals to be recorded. Similar to the original rewritable specification for CD specified in the Orange Book, Sony uses a contact magnetic head on the reverse side of the disc to modulate a north-south pattern into the disc^[1]. This pattern is written while the read laser, driven at a higher power than normal, heats the revelvant point in the groove to its Curie temperature. Once the laser passes by, the polarity of the head over time is preserved in the written groove. The varying reflectivity of north and south pits is then exploited when reading the disc.

How much are MD and CD the same?

MD and CD are both digital recording formats with similar end-user specifications—potentially flat response up to 22.05 kHz with greater than 100 dB dynamic range. The quality of both is therefore essentially the same. Both are a major improvement over CC, even when top-of-the-line Dolby S encodings and high precision decks and transport mechanisms are used. There was debate early over MD's advantage over Dolby S CC, but those who made such comparisons weren't listening to the current state-of-the-art in MD.

The key advantages of digital media are the profoundly reduced susceptibility to speed variations—the so-called "wow" and "flutter" associated with tape—and the reliability in accurately reproducing the original data. In addition, optical media and the recordings thereon are not degraded by playback, which means that both formats can live as long as the disc remains intact and reflective (potentially several decades, at least.) Both MD and CD are winners here.

Physically, size issues aside, the MD disc itself is very similar to CD, and at the level of bits, the encoding is almost identical. It is only when it comes to framing bits into more complex structures and encoding the audio signals that a clear distinction between MD and CD can be drawn.

But once we rise to the level of percieved audio quality and audio capacity, we find that CD and MD are once again highly comparable. Both use a 44.1 kHz audio sampling rate, which can produce sounds with frequencies up to 22.05 kHz. Both can store 74 odd minutes of stereo audio programming. CD uses a fixed linear sample width of 16 bits, but MD is more adaptable, and employs scaling to achieve a wider signal range. However, typically MD uses 16 bit linear samples as a benchmark, so there is not normally a great gain from this flexibility.

How are MD and CD different?

MD and CD use the same pitches of pitting and similar low-level encoding and error correction schemes, yet both obtain a recording capacity of 74 minutes despite a vast difference in physical size. It is obvious that a bit of trickery is involved to allow this, and you will be unsurprised to learn that this trickery has a name: ATRAC.

ATRAC stands for Adaptive Transform Acoustic Coding, and it is an audio compression scheme not dissimilar to Dolby's AC scheme (better known as Dolby Digital) or MPEG's audio compression schemes. ATRAC compresses digital audio data so that it occupies 20% of the space it would have under CD's PCM encoding. This is achieved primarily, without noticable degradation to human ears, by exploiting weaknesses in our ability to distinguish soft sounds behind loud sounds (called the "masking effect,") recognising the frequencies we are less able to hear (determined by "equi-loudness curves,") and making use of knowledge about the way our ears analyse sounds (in so-called "critical bands.")

While ATRAC is a lossy compression scheme, in that some data is discarded, and therefore original waveform is never perfectly reproduced, it cannot be emphasized enough how little it affects the listening experience. Early ATRAC encoders were less successful in this respect, but today it is possible to record sounds which are almost impossible to distinguish from the same sounds recorded on CDs entirely uncompressed. Indeed, when ATRAC is used to record from sources other than CD, it is possible for MD to exceed CD quality in terms of dynamic range.

Why use MD and not CD?

MDs are smaller than CDs, which makes them particularly well suited to portable applications. Another desirable attribute which makes MDs more robust than CDs is that they are enclosed in a cartridge. This means that they are less prone to being scratched or otherwise damaged. Possibilities for safely loading such cartridges are also increased from a top-loading Discman arrangement.

But beyond this, MDs offer quick and easy recordability, editing and rerecordability. Rewriting can occur millions of times, so there should be no problem with recording wear. By using a fragmented approach to storage, taking full advantage of the random-access possibilities available on a disc based medium, MDs are easily mutable. Don't like a song? Erase it, and put in something new. Don't like the order you've recorded things? Change it. And it's not much more difficult than that. Cut and paste and splice until your heart is content. Way beyond what Philips write-once CD Recordable (CD-R) can provide, and arguably easier than CD-RW. Certainly, we are yet to see a portable CD recorder.

The final advantage of MD over CD is the integral and widely implemented titling system. All tracks on an MD disc can have a title, as can the disc itself as a whole. All equipment, from the lowliest portable to the top-of-the-line deck, will allow this information to be displayed. Gone forever are the mystical track numbers we find on CD.

Notes about Audiotek MDs and numbering schemes

The ATKE catalogue and its subcatalogues contain all variants of original density audio MiniDiscs. That is 60, 74 and 80 minute MiniDiscs, irrespective of whether they are monaural or stereo or mixed, or prerecorded or magneto optical. It does not however include MD Data or any future high density MD format which would be unusable in current generation audio players.

The ATKE catalogue lists whole second timings for tracks and MDs, and for each track the title and primary artists are given. The title of each MD is derived by the primary artists and the longest form of the title, and the publisher is listed. For soundtrack MDs, the designation "Soundtrack" is included in the title. Detailed data on the mastering of tracks is listed as far as it is known, including the primary data format (stereo/mono,) the mastering bitrate (where applicable,) the hardware and software used, any matrixing and NR involved, as well as the number of passes through a range of digital and analogue quality gateways (including D/A conversion, digital and analogue compression and modulation.)

The title information on the MD is not guaranteed to be identical to that in the ATKE catalogue. Abbreviations may be used in the MD titles, and full artist and track title designation may be omitted. Also, additional punctuation may be included in the MD title information, as canonical forms are less important. It is typical for the title information on Audiotek MDs to follow the catalogue conventions:

Disc titles are usually given if full. Track titles are sometimes abbreviated for convenience. Artist details are omitted where these are unimportant or can be inferred from the disc title. Punctuation and capitalization conventions generally follow the model used in all Audiotek catalogues. Broadly speaking, punctuation is minimalized and capitalization is used only for the initial letters of all words in titles and names. "The," "A" and "An" and full names are retained in proper order in both the Audiotek catalogue and in the MD title information.

Audiotek mastered MDs are numbered from 001–799 within the ATKE catalogue. Each disc is allocated a consecutive number as it is finalized. This mirrors the use of these numbers in the ATKC Compact Cassette catalogue, but contrasts with the use of these numbers in the ATKV VHS Videocassette catalogue, which allocates prerecorded items from other publishers in the same range. Discs may be deleted from this range, and the number reallocated (or not) as required.

Prerecorded MDs are allocated a number in the same way as the ATKC catalogue. That is, they are assigned to one of the subcatalogues, ATKEA MD Albums or ATKES MD Singles, with numbers from 01–99 in both cases. Deleted MDs are not reallocated in these subcatalogues.

Mirroring the ATKC and ATKV catalogues, the range 900-999 of the ATKE catalogue is reserved for MDs which are produced by Audiotek for external agents or those which have been created for internal backup purposes. Numbers are allocated to discs in consecutive order starting at 900. Allocations in this range are permanent, and there is no requirement that any disc catalogued actually physically exist in the Audiotek library.

MDs ability to be rerecorded conveniently yields the necessity for an additional partition of the ATKE catalogue from 800–899. These content of these discs is rotated between various readily accessible digital audio sources in the Audiotek library. To provide some order within this chaos, each disc in this range is assigned a genre, the ranges being as follows:

Within each of these genre ranges, numbers are allocated to discs in consecutive order. New subranges of ten consecutive numbers may be established within the 840–899 range as the need arises. MDs in the 800–899 range typically do not have their tracks titled. The "tracks" listed in the ATKE catalogue for these discs are actually records depicting the source albums which have been used in their preparation.

^[1]  The specification given in Orange Book actually varies the laser, not the polarity, which is not quite as reliable.

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Last updated: Sunday 28th May 2000