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DASH

A description of the technology and function of DASH (digital audio stationary head) multitrack recorders.

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DASH stands for Digital Audio Stationery Head. The DASH specifications include matters such as the size of the tape, the tape speed and the layout of the tracks on the tape; also the modulation method and error correction strategy, among other things. The format is based on two tape widths: 1/4” (6.3 mm) and 1/2” (12.55 mm). For each tape width there are two track geometries, Normal Density and Double Density and there are also three tape speeds, nominally Slow, Medium and Fast (a further variation is caused by each of the three speeds being slightly different according to whether 44.1 kHz or 48 kHz sampling is used). According to the above, there must be twelve combinations all of which conform to the DASH format. This could make life confusing, but just because a particular combination of parameters is possible, it doesn't necessarily mean that a machine will be built to accommodate it.

The original Sony 3324, and recent 24-track machines, use the normal density geometry on 1/2” tape which allows twenty-four digital audio tracks, two analog cue tracks, a control track and a timecode track. (The cue tracks are there so that audio can be made available in other than normal play speed +/- normal varispeed). The tape speed at 44.1 kHz is 70.01cm/s. The 3324 is totally two-way compatible with the larger 3348 which can record forty-eight digital tracks on the same tape. To give an example, you may start a project on a 3324, of any vintage, and then the producer decides as the tracks fill up that he or she really needs more elbow room for overdubs. So you hire a 3348, put the twenty-four track tape on this and record another twenty-four tracks in the guard bands left by the other machine. Continuing my (hypothetical) example, when it is decided that the project is costing too much and going nowhere, the producer is sacked and another one brought in who decides that the extra twenty-four tracks are unnecessary embellishments and the original tracks, with a little touching up, are all that are required. Off goes the 3348 back to the hire company, the tape - now recorded with forty-eight tracks - is placed back on the 3324 and the original twenty-four tracks are successfully sweetened and mixed with not a murmur from the tracks that are now not wanted. We are now accustomed to new products and systems which offer new features yet are compatible with material produced on earlier versions. This must be audio history's only example of forward as well as reverse compatibility. It shows what thinking ahead can achieve.

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By David Mellor Tuesday February 1, 2000
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