Hard Disk AV Capability

Audio-visual applications require different performance characteristics than are required of a hard disk drive used for regular, everyday computer use. Typical computer usage involves many requests for relatively small amounts of data. By contrast, AV applications – digital audio recording, video editing and streaming, CD writing, etc. – involve large block transfers of sequentially stored data. Their prime requirement is for a steady, uninterrupted stream of data, so that any dropout in the analogue output is avoided.

In the past this meant the need for specially designed, or at the very least suitably optimised, hard disk drives. However, with the progressive increase in the bandwidth of both the EIDE and SCSI interfaces over the years, the need for special AV rated drives has become less and less. Indeed, Micropolis – a company that specialised in AV drives – went out of business as long ago as 1997.

The principal characteristic of an AV drive centred on the way that it handled thermal recalibration. As a hard drive operates, the temperature inside the drive rises causing the disk platters to expand (as most materials do when they heat up). In order to compensate for this phenomenon, hard drives would periodically recalibrate themselves to ensure the read and write heads remain perfectly aligned over the data tracks. Thermal recalibration (also known as T-cal) is a method of re-aligning the read/write heads, and whilst it is happening, no data can be read from or written to the drive.

In the past, non-AV drives entered a calibration cycle on a regular schedule regardless of what the computer and the drive happened to be doing. Drives rated as AV have employed a number of different techniques to address the problem. Many handled T-cal by rescheduling or postponing it until such time that the drive is not actively capturing data. Some additionally used particularly large cache buffers or caching schemes that were optimised specifically and exclusively for AV applications, incurring a significant performance loss in non-AV applications.

By the start of the new millennium the universal adoption of embedded servo technology by hard disk manufacturers meant that thermal recalibration was no longer an issue. This effectively weaves head-positioning information amongst the data on discs, enabling drive heads to continuously monitor and adjust their position relative to the embedded reference points. The disruptive need for a drive to briefly pause data transfer to correctly position its heads during thermal recalibration routines is thereby completely eliminated.