What happens in A/V Optimization?

What is a real-time operation?
An A/V application is a real-time operation. This means that data must be “recorded” at full speed as it is presented to the computer, and data must be “played” out of the computer with no pauses or gaps. Once data has started flowing into or out of the computer, nothing must be allowed to interrupt that data stream.

What can interrupt the data stream?

Drive speed –
The speed of the disk drive can cause data interruption. If the disk is too slow it can’t record data as fast as your digitizer, camera, or VCR is sending it. This will cause dropped or missing frames. Data played from a slow disk will have jerks and pauses. A fast drive is essential, and setting the drive to record and play as fast as it can is a big part of drive optimization. Making the drive record and play data as fast as possible is done through proper setting of the drives’ internal cache memory.

Errors –
If the drive has errors and tries to correct them your data stream will be interrupted. Disk drives are designed to deal with imperfect media and components by building in several layers of error correction. Error correction is a good thing if you are concerned about all of your data being perfectly recoverable from the drive all of the time. But error correction takes time – while a drive is correcting errors is can’t be transferring data. For example – if you are recording a data stream into your computer and your drive encounters a data error, it will correct the error. Your data will be correct, but while the drive did the error correction is stopped recording and you dropped several frames of video. This is why for A/V operations you do not ever want to correct data errors.

Drive Logging –
Most newer, high-speed drives collect statistical information about their performance as they run. This data is stored every so often within the drive. When the drive is storing this information it cannot record or play data, so all internal drive logging must be turned off or else data stream interruption can occur.

Optimizing for A/V –
To summarize, when a drive is optimized for A/V use its’ cache is set up for the highest data throughput, error correction is minimized, and internal logging is turned off.

What about “data” use? –
When you use a drive to store “regular” (non A/V) data you want the opposite of all of the above. Data integrity is more important than speed, so the drive must be set up to insure that the data is always recoverable.

Written by: Dr. SCSI