Sound Equipment - Playback Sources

Reel-to-reel (open reel)

Reel-to-reel or open reel tape recording refers to the form of magnetic tape recording in which the recording medium is held on a reel, rather than being securely contained within a cassette. Reel-to-reel was the main recording format used by audiophiles and professionals through the 1980s, when digital audio recording techniques began to allow the use of other types of media (such as DAT cassettes and hard disks). Even today, many artists of all genres swear on the analogue tape's musical, natural and especially warm sound.

Reel-to-reel tape also allowed relatively easy editing to be performed, using a chinagraph pencil, splice tape and cutting block. For this reason, reel-to-reel machines became popular in the theatre world, where they were used for sound effects and audio playback. Some models, notably the B77 were extremely widely used, and some models also featured an 'auto-stop' feature via the use of a small pilot lamp and the use of translucent tape sections.

Typical open reel format was ¼ inch (6.35 mm) wide, each stereo track being 1?16 inch (1.5875 mm) wide, and running generally at either 3¾ or 7½ inches per second (95.25 or 190.5 mm/s). Professional machines were capable of running at 15 inches per second (381 mm/s). Track layout in stereo mode was 1+3 in one direction, 2+4 in the other. In general, the faster the speed the better the sound quality.

Compact cassette: CC

The compact audio cassette medium for audio storage was introduced by Philips in 1963 under the name Compact Cassette. In essence, the audio cassette is simply a cassette-based (and smaller) version of the older reel-to-reel tape format. Four single audio tracks are available on the tape. These are paired up to produce two stereophonic tracks - one played when the cassette is inserted with its 'A' side facing up, and the other when it is turned over (with the 'B' side up). As opposed to reel-to-reel machines, the stereo tracks are laid next to each other.

The tape runs at 1 7/8 inches per second (ips). Tape length is usually measured in minutes total playing time, and the most popular varieties are C46 (23 minutes per side), C60 (30 minutes per side), C90, and C120 (usually thinner tape, more likely to be destroyed in use). In a 90-minute cassette, the tape is 443 feet (135 meters) long.

The compact cassette was originally intended for use in dictation machines. It soon became a popular medium for distributing pre-recorded music, and popularity grew from 1980 with the launch of the Sony Walkman series of portable cassette players.

Technical development of the cassette effectively ceased when digital recordable media such as DAT and MiniDisc were introduced in 1992. Philips introduced the Digital Compact Cassette - a DAT-like tape in the same form factor as the compact audio cassette - but this attempt failed in the market. Since the rise of cheap CD-R discs, the phenomenon of 'home taping' has effectively switched to compact disc.

Compact Disc: CD

The compact disc offers many advantages over the LP record. Noise and distortion produced in recording and playback are eliminated. Dynamic range and channel separation is greater, and the disc is less vulnerable to damage because the information is not on the surface but is sealed in. Consequently, it does not deteriorate with repeated playings. It can store extra data in the form of programme details, expired time, and even two extra channels to give quadrasonic reproduction. Its 12cm single side plays for 1 hour, which is longer than both sides of an LP.

Mini-Disc: MD

The Mini-Disc is a 7 cm x 6.75 cm x 0.5 cm case containing a disc of 64 mm in diameter. MD stores audio using a data reduction technique and thus has a capacity of about 160MB for a "74 minute disc" in standard audio mode (140MB when used in Data Mode). This compares to an audio CD of 740MB (650MB for a CD-ROM). The maximum actual playing time of a 74 minute MD is 74 minutes and 59 seconds.

Since MD stores audio using a data reduction technique (see ATRAC, below) there are considerable differences between the audio signal from a CD and that of an MD digitally recorded from it. Whether or not these differences are audible however depends to a large degree upon who is doing the listening, most MD users consider the sound quality of modern MD equipment to be essentially the same as CD. But even those who can hear differences usually find them to be undisturbing and inconsequential, frequently being unable to say which is which.

Mini-Disc Long Play: MDLP

Minidisc Long Play was introduced in September 2000 on the Sony MDS-JB940 deck; it is a new encoding method for audio on Minidisc that offers two modes: one gives 160 minutes stereo (LP2), the second gives 320 minutes stereo (LP4). LP2 is stereo encoded (each channel is independent), LP4 is joint-stereo (both channels are coded simultaneously, using an encoding scheme called ATRAC3).

Hi-MD

Introduced by Sony in January 2004, Hi-MD is a nearly complete revamping of the original Minidisc system. The most significant change is the introduction of Hi-MD media, which includes a new 1GB blank in the existing MD form-factor and a reformatting of existing MD media that doubles its capacity to 305MB. In addition, several important and long wished-for Minidisc capabilities have been incorporated into Hi-MD equipment.

• Hi-MD equipment can record audio to Hi-MD media in uncompressed PCM (16bits/44.1khz, i.e. CD format) as well as in Sony's ATRAC3plus encoding at 256kbps ("Hi-SP") and 64kbps ("Hi-LP").
• Hi-MD recorders can be used directly as USB connected data drives on any PC supporting USB storage devices (no additional drivers needed).
• Microphone and line input (i.e. analogue source) recordings made on Hi-MD equipment can be uploaded at high-speed to Windows PCs via SonicStage (Sony's copyright-protecting audio transfer software).

There are two Hi-MD media formats: a new 1GB magneto-optical medium based upon DWDD ("Domain Wall Displacement Detection") technology, and the original Minidisc magneto-optical medium (60', 74' and 80') reformatted to double its capacity. The Sony 1GB Hi-MD disk is the first commercial use of DWDD technology (as of January, 2004).

Digital Audio Tape: DAT

Digital Audio Tape is a small audio cassette on which the sound signal is stored digitally. This gives a higher quality signal on replay than ordinary analogue musicassettes.

DAT cassettes can carry up to two hours of sound on each side and are about half the size of standard cassettes. DAT players/recorders were developed in 1987. Pre-recorded cassettes are copy-protected. The first DAT for computer data was introduced in 1988.

DAT machines are constructed like video cassette recorders (though they use metal audio tape), with a movable playback head, the tape winding in a spiral around a rotating drum. The tape can also carry additional information; for example, a time code for instant location of any point on the track.

The music industry delayed releasing pre-recorded DAT cassettes because of fears of bootlegging, but a system has now been internationally agreed whereby it is not possible to make more than one copy of any pre-recorded compact disc or DAT. DAT is mainly used in recording studios for making master tapes. The system was developed by Sony.

Digital Compact Cassette: DCC

A late contender in the digital tape stakes, DCC from Philips is claimed not to rival DAT because it is a mass-appeal format, being to DAT what the compact casette was to open-reel systems. It was envisaged to become the popular digital tape format, leaving DAT the choice of professionals and high-end users.

A remarkable feature of the DCC system is its compatibility with ordinary analogue compact cassettes. The DCC cassettes are the same size and shape, and the tape is the same width. Tape speed is also the same in each case. An analogue cassette can thus be played on a DCC machine with no extra hardware requirements, providing the read-head is designed to accomodate DCC and stereo analogue.

DCC uses PASC to encode audio data - a lower-loss compression algorithm than MD (4:1 versus 5:1), and thus could sound better. For comparison, DCC has a capacity of 175MB (for a 60 minute tape).

ATRAC versus PASC

ATRAC (Adaptive TRansform Acoustic Coding) divides the 16 bit 44.1 KHz digital signal into 52 sub-bands in the frequency domain (after a Fast Fourier Transform). The sub-bands in the low frequencies are finer than the ones in the high frequency range. A psycho-acoustic transfer function that takes advantage of the masking effect and the absolute hearing threshold then removes enough information to reduce the data stream to 1/5th of the original size. Each channel receives that treatment separately.

PASC (Precision Adaptive Sub-band Coding), as used in Philips DCC, divides the digital signal into equally spaced sub-bands and removes less information (to only 1/4th of the original size). PASC is essentially the MPEG Layer 1 audio standard.

Both are data compression algorithms, used to store the information content from a stream of 16-bit samples in fewer bits. The purpose of compression is to reduce the rate of data bits, and to reduce the total number of bits stored. There are many compression algorithms. The ones used for computer data (for example in archiving programs) are lossless; the result of decompression is identical to the input.

PASC and ATRAC are both "lossy" algorithms. In order to get greater compression, they do not attempt to preserve every bit of the original data, but rather only the acoustically 'important' bits. Considerable cleverness goes into finding the sounds masked by properties of the human auditory system, ones that you would not hear even if they were reproduced. By all accounts the two schemes do amazingly well, considering they operate in real time.

See also: