Digital audio tape

The Digital Audio Tape, (from the English Digital Audio Tape and abbreviated DAT) is a means of recording and reproducing audio signals. digitized audio, which was developed and introduced by the Japanese company Sony in 1987. It was the first commercialized digital cassette format and, in appearance, is similar to a conventional audio cassette, using 4 mm wide magnetic tape encapsulated in a protective casing, but is about half the size at 73mm × 54mm × 10.5mm.

Recording principle

As the name implies, recording is done digitally instead of analog. Recording and conversion to DAT cassette has a higher, equal or lower sampling rate than for CD production (48, 44.1 or 32 kHz sampling rate and 16 bits of quantization). If you copy a digital source then the DAT cassette will produce an exact copy, unlike other digital media such as the Compact Digital Cassette or Hi-MD MiniDisc, which have lossy compression.

Like many video cassette formats, a DAT cassette can only be recorded on one side, unlike an analog audio cassette.

As more than 80 companies (mostly Japanese) were developing the DAT in parallel, there were many points where no agreement was reached on the standard to use. The research led to two technologies:

• S-DAT (Stationary Head DAT)
• R-DAT (Rotating Head DAT).

DAT was a format aimed at the professional sector that required at the time a digital recording system with which to make recording masters for CDs, since at the time of the invention of the compact disc all the masters or master tapes of 2 tracks, were made on 1/4 inch wide open reel tape.

The DAT completed a line of products, at that time, which included Sony's Multitrack PCM systems (in S-DAT or DASH "Digital Audio Tape Stationary Head" format), with the which could be recorded, mixed and subsequently transferred to CD digitally.

Currently it is surpassed by the power of PCs, which, equipped with powerful editing software, allow the mixing and subsequent creation of the CD, which will be "copied" at the CD manufacturing company.

Common Elements in the R-DAT and S-DAT

Dat Recorder and Player

The types of tape and their potential in the R-DAT and S-DAT are not the same. Although they have their nature, width and thickness in common.

DAT is a high coercivity (1400 oersted) metal particle tape, 3.81 mm (1/7 inch) wide and 13 microns thick and is about half the size of an analog cassette. R-DAT and S-DAT tapes differ in the specific dimensions of the cartridge in which they are inserted.

Bug fixes

Both R-DAT and S-DAT use the Reed-Solomon code for error detection, correction and cancellation. It is a very useful system if problems arise during playback (they don't have to be big problems, a simple speck of dust, scratch, or fingerprint can cause errors). To avoid errors, the data is not recorded in order, but the audio samples are interleaved. If a large number of samples are lost, they are not followed, so when the sound is reproduced, the loss will probably not be audible. To do this, during recording, a series of bits are allocated that control this data and also encode and record a certain amount of redundant information (the same data is recorded at once in a different location).

An error correction system can reconstruct the signal if the corrupted samples (either random errors or burst errors) do not exceed the system's capacity. When these minor errors occur, the playback system warns by flashing a green LED and the correction is carried out automatically.

When the number of errors exceeds the automatic capacity of the system, it performs the mathematical average between the adjacent values (previous and posterior). Even if the interpolated value is not correct, at least it will not produce an unpleasant effect. Interpolation is also called averaging or hiding.

Digital equipment indicates it using an orange LED when it interpolates. If there are many cases in which the value had to be averaged, it could be that the heads were dirty or the axis may have suffered a small deviation.

In cases where interpolation is not possible, because, despite the interleaving, some of the reference samples, the previous one or the subsequent one, are also damaged, the player retains the previous sample (hold). When the system makes a hold, it indicates it with a red light and the player system automatically silences the output if it detects several holds. Even if the equipment can reproduce the signal with errors, the resulting sound may be unpleasant, click, disappear or even break. Periodic checks of the equipment must also be carried out because imbalances that are difficult to detect may occur and may mean that a recorded sound is unreproducible. This can occur when recording a tape on equipment that has some wear on the heads or mechanisms. If the tape is played on that device, the sound will be correct, but if the playback is carried out on another device, the signal is completely distorted and is practically unrecoverable.

Multigeneration and SCMS system

Theoretically, the two DAT formats (R-DAT and S-DAT), like DCC, allow unlimited replication or multigeneration. However, Sony and Phillips introduced an anti-piracy (copy management) system: the SCMS (Serial Copy Management System) that allowed consumers to make a single copy on digital audio tape from another commercial medium (DAT, MD, CD or any other digital source). The copy could then be used personally but it prevented them from making unlimited copies to redistribute to third parties. To do this, among other things, the SCMS reads the original source codes, introducing a Boolean marker in the header of each audio object, and prevents DCC format players from making more than one copy of an original prerecorded source.

This system failed mainly because the hardware produced by some manufacturers did not support it.

R-DAT

The R-DAT (rotating head DAT) is based on the operation of home videos. During recording, both the cylindrical head (mounted on a rotating drum) and the tape move. The relative head-tape speed is very low (8.15, 4.075 and 12.255 mm/s) compared to the S-DAT (47.6, 43.7, 31.7 and 23.8 mm/s).

The R-DAT allows 4 recording-playback modes and two different playback modes.

The latest R-DAT models, still in use (2005), allow 20-bit recording with a sampling rate of 96 kHz.

R-DAT is a DAT tape (a high coercivity metal tape, with a tape width of 1/7 inch and a thickness of 13 microns); which differs from the S-DAT (which shares these characteristics) in its thickness (10.5 mm), its depth (54 mm) and its length (the 60 m long one provides 2 hours).

On the R-DAT, the tracks are diagonal (helical recording) with a width of 13.59 microns, without the need for guard tape. Tracks are recorded at an angle of 6° 23'. Each track follows a direction opposite to that of its adjacent track. Adjacent tracks that overlap can be recorded thanks to the information provided by AFT (Automatic Track Following).

As the tracks are recorded diagonally, they allow for a larger recording surface, which reduces tape consumption. The R-DAT contains 60 meters (120 minutes of playback) compared to 130 meters (90 minutes of playback) of the S-DAT.

The R-DAT, unlike the CC and S-DAT (and like video tapes), incorporates a cover that protects the tape when it is outside the equipment. The lid lifts automatically when the tape is inserted into it.

In addition, the R-DAT cassette has 4 holes that allow the equipment to immediately detect what type of cassette it is and whether or not it is prerecorded.

The R-DAT cassette does not have the traditional A and B sides, which is because its tracks are diagonal. (The S-DAT, since it has linear tracks, is organized as side A and B – the tracks on one side follow the opposite direction to those on the other.)

The R-DAT has 3 major drawbacks:

• With very high humidity levels, the tape can be glued to the drum.
• It does not have to be erased, which must be done by overwriting the tracks.
• It does not allow industrial duplicate of tapes with high-speed techniques. As if it were a CD, it needs a master disk and is made from a contact copy.

S-DAT

In the S-DAT (stationary head DAT), during recording, only the tape moves, the head remains fixed, therefore, the relative speed of the tape head is higher than that of the R-DAT which is They move both head and headband.

The playback or recording speeds of the S-DAT are: 47.6, 43.7, 31.7 and 23.8 mm/s, while those of the R-DAT are: 8.15, 4.075 and 12.255 mm /s.).

The S-DAT allows, like the R-DAT, also 4 recording-playback modes, but, unlike this one, it has only 1 prerecorded sound playback mode.

The size of the S-DAT cassette is slightly larger than that of the R-DAT, because it contains a longer length of tape. The S-DAT contains 130 meters (90 minutes of playback) compared to 60 meters (120 minutes of playback) of the R-DAT.

The S-DAT divides the tape into 20 parallel tracks of 65 microns, between which there is a guard separation (the R-DAT does not have it). Furthermore, the S-DAT, since it has linear tracks, is organized as side A and B, unlike R-DAT which does not have the traditional side A and B.

The S-DAT (like the analog cassette) does not have a cover. The R-DAT did have a cover that protected the tape and opened automatically when it was inserted into the equipment (as happens with VHS tapes).

The main drawback of the S-DAT, in relation to the R-DAT, is that it does not allow sub-code information to be re-recorded (recording start marks), etc., making it less operational than the R-DAT. DAT, on the other hand, overcomes all its drawbacks.