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By Conrad Persson
The technical information on which this article was based was derived from a Sony technical paper en titled Digital Audio Tape (DAT).
The hot word in audio lately has been "digital," just as it has been the hot word in almost everything having to do with electronics for the past decade. We now have compact digital audio discs and digital tuning. Poised for a splash into the U.S. market is digital audiotape (DAT).
DAT promises considerable advantages over conventional audiotape. The most important of these advantages is sound quality.
As with the digital audio disc, digital audiotape will provide greatly improved sound quality while eliminating noise sources such as, in the case of tabs, tape hiss.
Other significant advantages of the new DAT format are long recording times on a tape cassette, compact size and the ability to search for the start of each re corded track.
Digital audio on tape can be achieved in one of two methods.
One of these uses a fixed head as in current audiotape recorders and is called stationary DAT or S-DAT.
The other format uses a rotary head, in a manner similar to the method employed in VHS video cassette recorders. This approach is called rotary DAT, or R-DAT.
At the moment, because of a number of advantages, R-DAT is favored. At the present time, S-DAT is regarded as a possible future technical innovation.
The signal recorded on a tape in the R-DAT system is recorded using a technology called pulse code modulation (PCM), the same method that is used in recording digital audio compact discs. This technology allows high quality recordings (similar to that of a master tape) or a tape cassette that's about half the size of analog compact cassettes, but will play for two to three hours. The long re cording time and small size are possible because the R-DAT system records and plays back at a tape speed of 1/4 IPS, compared to the tape speed of a conventional cassette of 1 7/8 IPS.
Format Refer to Figure 1. Notice that the audio track is recorded on the tape in a series of diagonal strips, similar to the way the video signal is recorded on a VCR tape. The width of the track is 13.591µm; about one-tenth the thickness of a human hair. The length of the track is 23.501µm. Each bit of data within the track is 0.67µm in length. The recording density for an R-DAT tape is thus 114Mbits per square inch, the highest tape recording density ever achieved.
The signals being digitized for recording on the tape include the entire audio range of 20Hz to 20kHz. According to information theory, when using a sampling process, in order to accurately reproduce a range of frequencies, the signals must be sampled at a rate of at least twice the highest frequency that will be en countered. That would mean that a sampling frequency of at least 40,000 samples per second would be necessary. The R-DAT format uses 48kHz. Because each sample consists of 16 digital bits of information, and in a stereo system there are two channels, the system must process 1.5Mbits per second.
In order to compensate for errors in the signal, extra information must be added. This increases the signal data rate to about 2.45 M-bits/s. The addition of sub codes to enable various functions to be controlled pushes the data rate to 2.77Mbits/s.
Heads make contact only part of the time DAT is a digital process and uses a buffer to store some of the signal information between the time it is read off of the tape by the play head and the time it is processed into analog music and fed to the amplifier. Because of this, it is possible to have the play heads in contact with the tape only part of the time. Specifically, (see Figure 2) the R-DAT head drum is 30mm in diameter and is in contact with the head drum only through an angle of 90° or one-quarter of the drum's circumference. Because the heads are on diametrically opposite sides of the drum, 180° apart, the signal is recorded or played back only 50% of the time.
During the other 50% of the time, the signal is interrupted because the heads are not in contact with the tape.
This discontinuous approach to recording and playing back the signal confers certain advantages.
Three advantages accrue be cause the tape wraps only 90° around the head drum:
1. Only a short length of tape is in contact with the drum. This reduces damage to the tape, and allows the system to operate at high-speed for transport and search while the tape is in contact with the drum.
2. Tape tension is low, which contributes to long head life.
3. If four heads are used instead of two (Figure 3), the heads can be separated by 90° so that you will be able to play back at the same time as you're recording, thus pro viding a record monitor function.
Two more advantages accrue because the record/playback signal can be converted to a high range signal of 7.5Mbits/s. This enables the system to overcome any inadequacy in low-range characteristics.
1. The small head drum size and small size of the rotary transformer is a direct consequence.
2. The signal-to-noise ratio is improved.
The helical recording method of recording many narrow diagonal strips with a rapidly rotating head on a slowly moving tape allows recording and playback of higher frequencies in videotape and R-DAT. That approach is obviously not possible with S-DAT.
This limitation is overcome with S-DAT by distributing data across the width of the tape (see Figure 4). That's the theory. In the real world there are limits imposed by tape noise, dropouts, compatibility, the fact that heads must be mass produced and the complexity of the circuits. With current technology, the tape speed is in the same range as that of an analog compact cassette, and the tape needed for S-DAT is expensive.
Right now, S-DAT is being looked at as a future technology.
Unfortunately, at the present time DAT seems to be the victim of its own success. Because the quality of sound on this system is so good, and because a recorded tape can be dubbed onto a blank tape of the recording artists, record companies and anyone else who has a vested interest in recorded music have expressed concern that there will be wholesale illegal copying of tape recordings, thus siphoning off funds that should be going to pay for legitimately produced tapes.
Legislation is pending that might require anti-recording chips in all DAT recorders shipped into the United States. Hearings concerning this sales restriction were begun in May before appropriate House and Senate sub committees. Previously, this product-specific amendment was part of the giant Omnibus trade bill (H.R.3), but was deleted to make the DAT issue a freestanding bill. Testimony opposing the bill will be presented by representatives of the National Association of Retail Dealers in America (NARDA).
NARDA's position is that such legislation would deprive consumers of the opportunity to enjoy the latest state-of-the-art technology, and it would deprive retailers of the opportunity to sell the product.
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