Digital Audio Stationary Head

The term DASH (Digital Audio Stationary Head ) format refers to a format convention for digital storage of audio data on magnetic tape with fixed heads. The format is used in many different tape machines with open bands.

Digital sound storage on magnetic tape

When recording on the magnetic tape is referred to as a linear recording technology, as each time a recording in a linear relationship with a specific location is on the tape.

For the digital storage of audio data on magnetic tape, two different systems are available which differ in their drive technology. Firstly, there are digital tape whose recording and playback heads are fixed, on the other hand, there are systems which rotate their heads. Both methods have been widely adopted by analog machines. The drive with fixed heads comes from the analog tape machine, while the drive was taken with the rotary heads from the analog video machine.

Digital data can not be recorded directly on magnetic tape. You must be pre- treated to avoid recording errors. This process is called channel coding and error. This is very important, because both during recording and during the reproduction of certain areas of the tape by contamination or flaws may be unreadable. However, the demands on the band are much higher than with purely analog technology, as the density of the data stream is very high and even the smallest error in the band that would not stand out in an analog recording, can have fatal sound may result. This also has an effect on the archival quality. Many studios have seen archiving permanent severe damage of the digital information on the tapes after less than 10 years, not only in DASH recordings, but also from devices that use the Heliscan process ( eg ADAT), ie with rotating heads work.

In addition, in the channel and error coding, the data is provided with a synchronization information, which ensures that when an exact reproduction of the signal processing is possible.

The digital sound storage on magnetic tape using a fixed heads takes place only in very specific applications today.

Digital sound storage with fixed heads

Recording devices for operation with open reels of tape ( DASH and Pro - Digi ) were developed to replace the analog two-channel and multi-track tape machine in the studio. Theoretically, in these bands, a mechanical -section, similar to the analog domain is possible. In addition to the open reels of tape were also closed variants such as the eight-track format from Yamaha and the consumer format the DCC, however, not translated.

Properties of the DASH format

The DASH format was developed by the companies Sony, Studer and Matsushita. It is possible to work with two sampling frequencies, 44.1 kHz or 48 kHz with 16 bits each. Depending on the sampling frequency are three different belt speeds are possible (see Fig.) These are:

• DASH -F ( F = Fast / fast)
• DASH -M ( M = medium / medium)
• DASH -S ( S = Slow / slow)

Here, only the DASH -F format reached the maximum number of 48 tracks. The recording media is either ¼ " or ½" is used pure iron magnetic tape. Digital tracks can be written and read by special thin-film heads. There are more than 48 tracks simultaneously possible. As can be recorded with normal or double density, there are several recording formats from which, depending on the number of required audio tracks can be selected ( see table). The recording of the data takes place in each block, wherein the recording density is always 1.51 kbit / mm. The block addresses and sync information to the digital audio data is located on a separate control track ( see Fig.)