Digital Audio Media - Magnetic digital media

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These media systems have similar characteristics to those used in analogue recording. Indeed the same principle of the aligning of magnetic particles depending on the signal-rate is used, with the difference that in this case it is a signal that represents the sample-sequence taken from the analogue signal. This kind of support is particularly sensitive to different kinds of interferences which may alter its content. Especially those generated by televisions, mobile phones (never keep a magnetic digital support in the same bag as your mobile phone!) and computer monitors. However, these kind of supports are widely employed, seeing that they transfer data quickly and at relatively reasonable costs. Let's divide this type of support into two categories: rotating magnetic head and fixed magnetic head supports.

19.2.1. Rotating head media

The fundamental principle is the same as that which underlies common video-recorders where an elicoidal tape-scan takes place. This means that a part of the tape is taken out from its case and wrapped around a cylinder-shaped magnetic rotating head. The head is inclined at a certain degree-angle in relation to the tape's rolling-direction. This results in the magnetization of the tape in oblique "parts", as illustrated in the following diagram:

Digital Audio Media  - Oblique magnetization of a magnetic tape

Oblique magnetization of a magnetic tape

The rotating head contains both the recording head and the reproduction head. In actual fact there are systems which have many recording and reproducing heads inside the rotating head, which increases quality because there is always at least one head making contact with the tape (the quality of paused images in video-recorders depends on this factor). The most widespread supports that use this system are:

  • R-DAT: Recording Digital Audio Tape - one of the most widespread systems for the production of master stereos. There are two different versions:

    30 mm tape, 4 heads, 90o winding angle:

    Digital Audio Media  - 90o Winding

    90o Winding

    15 mm tape, 2 heads, 180o winding angle:

    Digital Audio Media  - 180o Winding

    180o Winding

    The head's rotation speed is 2000 rpm [40 ] whereas the tape's speed is 8.15 mm/sec. It allows two channels to be recorded (Left and Right) with a sampling frequency of 44.1 KHz or 48 KHz. The quantization bits can either be 16, 20 or 24.

    The information is stored onto the tape as illustrated in the following diagram:

    Digital Audio Media  - Stored data format

    Stored data format

    The audio data is located in the central area indicated by the initials PCM (which stand for: Pulse Code Modulation, the type of data encoding). The ATF section (Automatic Track Follower) stores the codes that are used by a correction system which takes care of the right alignment of the head in relation to the tape. The sub-code section allows extra information to be stored in addition to audio data. Furthermore, on the DATs it is possible to record the synchronization time-reference in a separate area, thus allowing it to be changed at will even after the audio signal has been stored. Various different error control systems are employed, such as the parity check system and the CIRC code [CIRC (Cross Interleaving Reed-Solomon Code) ] as well as the 8-10 modulation system which converts 8 bit sequences into 10 bit sequences, which, as we will explain further on, reduces the number of transitions to be stored per time unit. Furthermore, a system called SCMS (Serial Copy Management System) is used, which protects against pirate copies. Once the DAT has been passed on to another DAT (which remember takes place digitally and therefore without any data loss), the SCMS codes are altered, ensuring that no other copies of the DAT can be made.

  • ADAT: Alesis Digital Audio Tape - Every ADAT machine can record up to 8 tracks and up to 16 of these devices can be synchronized together, resulting in 128 available recording tracks. The internal timing can be synchronized with an SMPTE signal [SMPTE time code ] or MTC [MTC - MIDI Time Code ] issuing from external equipment in order to not waste an audio track for the recording of the time-code. The type of data-encoding is linear at 16 bit with a 48 KHz sampling frequency (oversampled x 64). It uses an improved version of normal VHS video cassettes which need to be formatted in advance.

  • Tascam DA-88: Uses a Hi-8 video format. It has a 4-head rotating head which allows 8 tracks to be recorded. Allows 100 minutes of audio to be recorded onto a 90 minute videotape.

  • A-DAM: Employs an 8 mm videotape and a 40 mm-diameter rotating head with three heads. Each video cassette allows 17 minutes' recording. It includes an auxiliary track for the time-code.

19.2.2. Fixed head supports

This kind of support generally consists in simple reels on which the digital signal is stored. The signal is saved along the the tape's rolling direction by static heads. Note that they are not oblique, as was the case with rotating heads. The most widespread models are:

  • DASH: Digital Audio Stationary Head, a system devised by Sony. These machines can record from 2 to 48 tracks. Their sampling frequencies are: 44.1 KHz and 48 KHz, whereas the tapes' width can either be 1/4" or 1/2".

  • DCC: Digital Compact Cassette - Allows 9 tracks to be recorded in every direction onto a 1/8" width tape, with a speed of 17/8 ips.

[40 ] rpm stands for "rounds per minute" and it is the unit commonly employed for measuring the rotation speed of a disc support (i.e the old "vinyl records" aka Long Playing- LP's had a speed of 33,3 rpm).