Helicoidal (Video Derived) Tape Formats (March 1997)

Some fundamental differences exist in how data is written to magnetic media, PDM attempts to explain the pros and cons of linear and helicoidal techniques.

The technology used for the storage of digital data on tape falls into two camps, linear and helicoidal. The lineage of the former is well known to the geophysical industry, being a major consumer of linear tapes since the early days on 21 track, 1 inch tapes, through the evolving 9 track tape with densities increasing from 800 bits per inch to 6250 bpi. In the 80's the round reel tapes began to be superseded by the still linear cartridge tapes of the 3480 format, and today from the same stable, we have the 3590, which shares the 3480's external physical characteristics.

The demands for bandwidth in the last decade or so have pushed the scientific world to look for higher capacity drives than current linear tape technology could provide, and there has been considerable development of non linear, helicoidal devices from a variety of manufacturers. These devices all come from the world of video.

VCR technology

The helicoidal technique is used in home video recorders and for those who have peeked inside their VCR, involves dragging the tape out of its normal path, over a skewed, spinning head, which reads and writes the data diagonally across the tape. More recently the broadcast world has gone digital, and it is actually sampled (digitised) data that is written to tape. This data is written in frames, or in individual pictures and the technology is designed around the writing of one frame per scan of the head. The helicoidal technique effectively multiplies the useable length of the tape, while allowing the linear speed of the tape to be kept within reasonable limits. The idea of recording data to these devices was first spotted by the US Navy, whose sonar recordings from their submarine detection arrays were pushing the linear tape technology beyond its limits. The seismic industry, whose activity involves a different kind of underwater recording followed close behind.

To blip or not..

The passage from analogue to digital video potentially offered the data storage world a free lunch, with the advent of a new cheap (because of the high volumes) digital technology. Things were not quite so simple. The video industry, even when recording high quality for broadcast can put up with a certain amount of noise on the image, partly because no on is going to be killed by, or lose money from a blip on a video, and partly because such blips are rather amenable to on the fly "error hiding" processing. Digital Video, which has been developed and standardised by various ANSI classifications appears in a variety of manifestations; D1 (Sony), Metal Oxide tape, high quality for broadcast, D2 (Ampex/Sony), Metal Particle tape, lower spec, but higher capacity and D3 (Panasonic), although the latter has not gained widespread acceptance in the broadcast industry and Panasonic is now rolling out a D5 format.

Confusion labelling

It should be noted that contrary to ideas received, the D1, 2 3 etc. nomenclature does not necessarily represent an evolution towards greater sophistication and capacity. In fact the highest specification drives in the video industry are based on the D1 technology. In the following we refer to the initial video specs with a DV prefix. The move from digital video to data has involved considerable redesign of the hardware to reduce the error rate to acceptable levels for the computing business. This has generally been done by third parties. Emass and Ampex developed their "DD2" format (also known as DST) from DV2. StorageTek acquired Panasonic's DV3 helical scan technology and produced their "D3" Redwood drives for a rumoured $100 million investment. Meanwhile Sony produced a digital tape format ID-1 from the DV1 spec, and is now shipping a new format DTF based on the Digital Betacam format. This format is used in Sony's PetaSite Robotics with a mind boggling 2.5 Petabyte maximum capacity.

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