Notes about audio

Compact Cassette (CC) was developed by Philips, and became the standard cassette format for home audio recorders from the 1960s. The standard is used around the world and the only consideration is the quality of the medium in use as, unlike video, the format has no local variation. This owes to the simple linear nature of encoding on CC.

How is music recorded on a cassette?

A CC has two sides, usually denoted A and B, but sometimes 1 and 2 are used. The term "sides" does not refer to different physical tape surfaces, such as the "inner" and "outer" surface. It instead refers to the edge of the tape which the audio signals are recorded against. This is the case because only one magnetic tape surface is ever exposed—the other side is unmagnetizable and always faces inward towards the hubs.

Music is recorded in four tracks on a stereo cassette (two per side,) and two tracks on a monaural cassette (one per side.) It is possible to have monaural on one side, stereo on another, ore a mix of both on either side with no restriction. This is because monaural tracks are recorded in a stereo compatible manner, using the space allocated to a stereo track. There is essentially no difference between a tape produced with a monaural recorder or a tape produced on a stereo recorder with a monaural input. Both will play in the same way.

Tapes are labelled so that the signals are recorded on the half of the tape nearest to the panel labelling the side. The tab in the top-left corner of a labelled side is the write protect tab for that side. When removed, leaving a hole, recording on that side is not possible. Covering the hole with a new tab (perhaps some adhesive tape) will allow recording once more.

The signals on each track are recorded linearly as the tape passes a fixed head at a fixed speed. The level of magnetization of the tape gives the amplitude of the waveform at that point in the sound in a simple manner. Sound quality is dependent on the wow and flutter present in maintaining the speed of the transport mechanism, the age of the tape, the type of surface, the alignment and age of the heads, and the alignment of and the amount of residue on all the components of the mechanism. Also, because magnetic tapes require physical contact between the head and the tape, usage will wear down the tape, and cause dropouts on the magnetic surface as well as a gradual degradation of the information encoded there.

Blank cassettes exist in a number of predefined lengths, ranging from the C-10 which runs ten minutes (5 minutes per side) to the C-120 (1 hour per side.) Prerecorded cassettes are usually clipped to the length of the longest side. Cassettes also have different medium grades which affect the bias used to record and playback. These are now discussed.

What are the different medium grades?

There are three IEC grades for the magnetic surface used in a cassette, numbered I, II and IV.

• Type I, or normal position, is suitable for most purposes, and has a rated bias of 120 µs. The tape surface is normally made from an iron oxide, which is overlayed on plastic a backing.
• Type II, or high position, is suitable for home mastering purposes and has a rated bias of 70 µs, which means it has a frequency response almost twice as good as type I. The tape surface is normally made of a chromium oxide, which is overlayed on a plastic backing.
• Type IV, or metal position, is more durable than type II but has the same rated bias of 70 µs. Metal is more common in high-end applications.

All CC recorders and players are suitable to use when dealing with type I. However, older equipment cannot take proper advantage of the higher densities of types II and IV, and even some modern equipment requires a tape type selection switch to be set appropriately.

Notes about Audiotek CCs and numbering schemes

Audiotek CCs are included in the ATKC catalogue. Audiotek mastered cassettes are numbered from 001–899 within the ATKE catalogue. Each cassette is allocated a consecutive number as it is finalized, and cassettes may be deleted from this range, with the number being reallocated (or not) as required.

The range 900–999 of the ATKC catalogue is reserved for cassettes which are produced by Audiotek for external agents or those which have been created for internal backup purposes. Numbers are allocated to these cassettes in consecutive order starting at 900. Allocations in this range are permanent, and there is no requirement that any cassette catalogued actually physically exist in the Audiotek library.

Prerecorded CCs are treated as a special case, and are allocated a number in one of the subcatalogues, ATKCA Cassette Albums or ATKCS Cassette Singles, with numbers from 01–99 in both cases. Deleted cassettes are not reallocated in these subcatalogues. Selected Audiotek produced albums and singles may also be included in these ranges instead of the main catalogue.

Typically, Audiotek CCs are type I (normal position) and recorded on good quality stereo equipment. High speed dubbing^[1] is avoided unless time constraints are imposed or the volume of requests is large. C-90 is the typical tape length. C-100s and C-60s are sometimes used, but C-120s are avoided.

Audiotek has a set of subjective quality measures for the audio recorded onto a CC. These measures refer to the quality of the source originally recorded, and do not take into account tape degradation.

• Mono (single channel) signals are rated as being either of AM or FM quality, where these designations approximate the quality one can expect from AM and FM radio respectively. AM therefore means poor quality with poor bass response and extremely poor high-frequency response. Conversely FM means good quality, with quite good bass response and good high frequency response.
• Stereo (two channel) signals are rated on a scale of I to V, with I being the best and V being the poorest. The type of source corresponding to these designations is given in the table below.

  Level	Source examples
  I	CD; Dolby Digital; MPEG Layer 3 Audio
  II	High-grade cassette (Dolby B); cassette recorded from Level I; VHS hi-fi video
  III	High grade FM; cassette recorded from cassette
  IV	Low-grade FM; beyond second generation cassette
  V	Low-grade cassette; non-hi-fi gramophone

• Dolby and Dolby HX Pro^[2] signals employ Dolby B noise reduction^[3] and are not rated for subjective quality—only high quality sources or prerecorded materials will have these characteristics. All Dolby recordings are considered to be stereo, or possibly may use the Dolby MP surround matrix^[4],[5], although this is unlikely.

Timings, where given in the Audiotek CC catalogue, should be taken as approximations only as the accuracy cannot be guaranteed. CC recorders and players have a lot of variation in speed, both between each other and within themselves, and this variation can be as much as 5%. Also, it is sometimes unclear what a timing includes, as it may or may not consider introductory commentary, the full length of the fades, or the gap between songs.

^[1]  High speed dubbing refers to tape copying performed at double the normal playback speed on standard non-optimized equipment. Such dubbing reduces the quality of the output, particularly in the high-frequency spectrum.

^[2]  Dolby HX Pro (for "headroom extension professional") is a filter applied on top of noise reduction³ which enhances the "depth" of recorded music, improving the clarity of some percussive sounds.

^[3]  Dolby B is a widely used noise reduction filter developed by Dolby Laboratories. Noise reduction filters reduce tape hiss, rumble and other unwanted background artifacts not present in the original source but introduced by the recording mechanism.

^[4]  The Dolby MP Matrix provides quadraphonics using only two encoded channels. i.e. The four source channels are reduced to a two channel quasi-stereo signal. The four channels are the normal left and right sources, and additionally an in-phase centre source (reduced by 3 dB) and a Dolby B encoded surround source (reduced by 3 dB and filtered to 0.1 to 7 kHz) which is phase shifted by +/-90 degrees into the left and right outputs. The separation of adjacent channels is then 3 dB⁵, producing a balanced quadraphonic sound field.

^[5]  A Dolby Pro-Logic decoder will allow 30 dB channel separation, as it employs an active adaptive matrix decode step.

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Last updated: Sunday 28th May 2000