Frequently Asked Questions

What are the characteristics of an A/V application?

Since disc drives only store data digitally, an analog signal such as audio or video must be converted to digital for storage. Upon retrieval, this data must be converted back to analog for its intended usage.  This conversion is the primary function of an A/V application.

What would define a drive as A/V capable?

An A/V capable drive is a drive that can store or retrieve data at a sustained rate sufficient to make the input or output data appear to be seamless after the digital/analog conversion. An inability to sustain a transfer rate sufficient for the application being used will result in noticeable "dropouts" in the analog output.

The single most important factor in A/V capability is the “internal” transfer rate of the drive, which is inherent in the design. The movement of digital files associated with A/V activities would typically be defined, from a storage point of view, as large block transfers of sequentially stored data. Typically, SCSI drives are more suitable for these types of operations.

What is a “sustained” transfer rate, and how is it related to “external” and “internal" transfer rates?

Sustained transfer rate is an average rate at which the drive can move many blocks of data over a relatively long period of time.

External transfer rate is the rate at which the drive and controller can exchange data in a buffer-to-buffer transfer in a “burst” across the cable. This external rate is defined by the level of SCSI interface that both the controller and the drive support (buffer to buffer transfers over the SCSI cable).

Internal transfer rate is the rate at which a drive can move sequentially stored data (A/V data is typically stored sequentially) between the physical disc platters and the drive's on-board buffer. This rate is determined by the rotational speed and recording track density inherent in the drive design.

How they relate: In order to "sustain" a rate, the buffer on the drive must be refilled (during a read) or emptied (during a write) by moving data (internal transfer) to or from the media (platters) before the “burst” (external transfer) can occur again.

Example: A read operation would typically consist of reading an amount of data from the media sufficient to fill the buffer (at the internal transfer rate), transferring the data to the host (at the external transfer rate) and reloading the buffer with the next sequentially stored data (again, at the internal transfer rate). A typical A/V application would do this thousands of times for a particular piece of audio or video. The average rate over these thousands of transfers would be the “sustained” transfer rate.

Can a drive be modified for A/V applications?

Current generation Seagate SCSI drives have an embedded servo, so there is no thermal recalibration to interrupt the data flow. The high internal transfer rates, along with the default settings on data prefetch may make the drives sufficient for many A/V applications with no further "tuning".

If desired, the performance can be further enhanced by:

• Increasing the cache buffer segment size, which will maximize the data prefetch feature of the drive
• Turning off error logging (S.M.A.R.T.)
• Turning off or limiting retries in case of data errors