Digital Audio Stationary Head
The term DASH ( Digital Audio Stationary Head ) format is understood as a format agreement for the digital storage of audio data on magnetic tape with fixed heads . The format is used on many different tape machines with open belts .
Digital sound storage on magnetic tape
When recording on magnetic tape , one speaks of a linear recording technique, since every point in time of a recording is linearly related to a certain point on the tape.
For the digital storage of audio data on magnetic tape, two different systems are available, which differ in their drive technology . On the one hand there are digital tape machines with fixed recording and playback heads, on the other hand there are systems in which the heads rotate. Both processes were largely adopted by analog machines. The drive with fixed heads comes from the analog tape recorder, while the drive with the rotating heads was taken from the analog video machine .
Digital data cannot be recorded directly on magnetic tape. They must be prepared beforehand in order to avoid recording errors. This process is known as channel and error coding. This is very important, since individual areas of the tape can be illegible due to soiling or faulty areas, both during recording and during playback. However, the demands on the tape are much higher than with purely analog technology, as the density of the data stream is very high and even the smallest errors in the tape that would not be noticed in an analog recording can result in fatal sound impairments. This also affects the archivability. Many studios were able to determine severe damage to the digital information on the tapes after less than 10 years of archiving, not only with DASH recordings, but also with recordings from devices that use the Heliscan process (e.g. ADAT), i.e. with rotating heads work.
In addition, with channel and error coding, the data is provided with synchronous information, which ensures that the signal can be processed precisely during playback .
Digital sound storage on magnetic tape using fixed heads only takes place in very specific applications.
Digital sound storage with fixed heads
Recording devices for operation with open tape reels (DASH and Pro-Digi) were developed to replace the analog two-channel and multi-track tape machines in the studio. In theory, a mechanical cut is possible with these tapes, similar to the analog technique. In addition to the open tape reels, there were also closed versions such as the eight-track format from Yamaha and the consumer format DCC, which, however, did not catch on.
Properties of the DASH format
The DASH format was developed by Sony , Studer and Matsushita. You can work with two sampling frequencies, 44.1 kHz or 48 kHz, each with 16 bits . Depending on the sampling frequency, three different belt speeds are possible (see figure). These are:
- DASH-F (F = Fast / Fast)
- DASH-M (M = medium / medium)
- DASH-S (S = slow / slow)
| Sampling frequency | Belt speed (cm per second) | ||
| slowly | medium | fast | |
| 44.1 kHz | 17.50 | 35.00 | 70.01 |
| 48 kHz | 19.05 | 38.10 | 76.20 |
| Bandwidth | 1/4 inch tape | 1/2 inch tape | ||
| Recording density | normal | double | normal | double |
| Tracks (digital) | 8th | 16 | 24 | 48 |
| Control tracks | 4th | 4th | 4th | 4th |
| Number of digital audio tracks at the different tape speeds |
||||
| fast | 8th | 16 | 24 | 48 |
| medium | - | 8th | - | 24 |
| slowly | 2 | 4th | - | - |
Only the DASH-F format achieves the maximum number of 48 tracks. Either ¼ "or ½" pure iron magnetic tape is used as the recording medium. The digital tracks are written and read by special thin-film heads. A maximum of 48 tracks are possible at the same time. As normal or double density can be recorded, there are several recording formats from which you can choose depending on the number of audio tracks required (see table). The data is recorded in individual blocks, the recording density always being 1.51 kBit / mm. The block addresses and synchronization information for the digital audio data are on a separate control track (see illustration).
literature
- Hubert Henle: The recording studio manual. 5th edition, GC Carstensen Verlag, Munich 2001, ISBN 3-910098-19-3
- Michael Dickreiter, Volker Dittel, Wolfgang Hoeg, Martin Wöhr (eds.), "Handbuch der Tonstudiotechnik", 8th, revised and expanded edition, 2 volumes, publisher: Walter de Gruyter, Berlin / Boston, 2014, ISBN 978-3- 11-028978-7 or e- ISBN 978-3-11-031650-6