Types of Hard Disk Drives

Disk drive technologies have advanced quickly over recent years, making terabytes of storage available at reasonable cost. When researching the type of hard disk storage system appropriate for your needs, keep in mind the format and data rate of the video you’re capturing. Depending on whether you work as an independent video editor or collaborate with others, the amount of storage you require and the bit rate of data transfer will be important factors to match up with your storage needs. Outlining all of the hard disk storage technologies is beyond the scope of this documentation, but four common choices include:

ATA Disk Drives

There are two kinds of ATA disks:

  • Parallel (Ultra) ATA disks: These are found in Power Mac G4 computers.
  • Serial ATA disks: These come with Power Mac G5 computers.

ATA disks do not offer as high a level of performance as LVD or Ultra160 SCSI disks. If you plan to use Ultra ATA disks, make sure that:

  • The sustained transfer speed is 8 MB/sec. or faster

  • The average seek time is below 9 ms

  • The spindle speed is at least 5400 rpm, although 7200 rpm is better

Parallel (Ultra) ATA Disks

Many editors use parallel ATA (PATA) disks (also called Ultra DMA, Ultra EIDE, and ATA-33/66/100/133) with DV equipment. Parallel ATA disks are disks that you install internally. Because imported DV material has a fixed data rate of approximately 3.6 MB/sec., high-performance parallel ATA disks typically can capture and output these streams without difficulty. The numbers following the ATA designation indicate the maximum data transfer rate possible for the ATA interface, not the disk drive itself. For example, an ATA-100 interface can theoretically handle 100 MB/sec., but most disk drives do not spin fast enough to reach this limit.

Parallel ATA disks use 40- or 80-pin-wide ribbon cables to transfer multiple bits of data simultaneously (in parallel), they have a cable length limit of 18 inches, and they require five volts of power. Depending on your computer, there may be one or more parallel ATA (or IDE) controller chips on the motherboard. Each parallel ATA channel on a computer motherboard supports two channels, so you can connect two disk drives. However, when both disk drives are connected, they must share the data bandwidth of the connection, so the data rate can potentially be reduced.

Serial ATA Disks

Serial ATA (SATA) disks are newer than parallel ATA disk drives. The disk drive mechanisms may be similar, but the interface is significantly different. The serial ATA interface has the following characteristics:

  • Serial data transfer (one bit at a time)

  • 150 MB/sec. theoretical data throughput limit

  • 7-pin data connection, with cable limit of 1 meter

  • Operates with 250 mV

  • Only one disk drive allowed per serial ATA controller chip on a computer motherboard, so disk drives do not have to share data bandwidth

FireWire Disk Drives

Although not recommended for all systems, FireWire disk drives can be effectively used to capture and edit projects using low data rate video clips, such as those captured using the DV codec. However, most FireWire disk drives lack the performance of internal Ultra ATA disk drives or of internal or external SCSI disk drives. For example, a FireWire disk drive may not be able to support real-time playback with as many simultaneous audio and video tracks as an internal Ultra ATA disk drive can. This can also affect the number of simultaneous real-time effects that can be played back.

Keep the following points about FireWire drives in mind:

  • FireWire disk drives are not recommended for capturing high data rate material such as uncompressed SD or HD video.

  • Certain DV camcorders cannot be connected to a computer while a FireWire disk drive is connected simultaneously. In many cases, you can improve performance by installing a separate FireWire PCI Express card to connect your FireWire drive.

  • You may be able to improve performance by reducing the real-time video playback data rate and the number of real-time audio tracks in the General tab of the User Preferences window.

  • You should never disconnect a FireWire disk drive prior to unmounting it from the desktop.

SCSI Disk Drives

Small Computer System Interface (SCSI) disk drives used to be among the fastest drives available, although newer computers may no longer provide SCSI ports. Although no longer highly popular, SCSI technology has been implemented in various ways over the years, with each successive generation achieving better performance. Two fast SCSI standards for video capture and playback are:

  • Ultra2 LVD (Low Voltage Differential) SCSI: Ultra2 LVD SCSI disk drives offer fast enough performance to capture and output video at high data rates when a single disk is formatted as a single volume (as opposed to formatting several disks together as a disk array).
  • Ultra320 and Ultra160 SCSI: These are faster than Ultra2 LVD SCSI disks.

SCSI disks can be installed internally or connected externally. Many users prefer external SCSI disk drives because they’re easier to move and they stay cooler. If your computer didn’t come with a preinstalled Ultra2 LVD, Ultra160, or Ultra320 SCSI disk drive, you need to install a SCSI card in a PCI Express slot so you can connect a SCSI disk drive externally.

A SCSI card allows you to connect up to 15 SCSI disk drives in a daisy chain, with each disk drive connected to the one before it and the last terminated. (Some SCSI cards support more than one channel; multiple-channel cards support 15 SCSI disks per channel.) Use high-quality, shielded cables to prevent data errors. These cables should be as short as possible (3 feet or less); longer cables can cause problems. You must use an active terminator on the last disk for reliable performance.

Note: Active terminators have an indicator light that goes on when the SCSI chain is powered.

Figure. Diagram showing a computer connected to a daisy chain of SCSI hard disks.

All devices on a SCSI chain run at the speed of the slowest device. To achieve a high level of performance, connect only Ultra2 or faster SCSI disk drives to your SCSI interface card. Otherwise, you may impede performance and get dropped frames during capture or playback.

Note: Many kinds of SCSI devices are slower than Ultra2 devices, including scanners and removable storage media. You should not connect such devices to your high-performance SCSI interface.

Using a RAID or Disk Array

You can improve the transfer speed of individual disks by configuring multiple disk drives in a disk array. In a Redundant Array of Independent Disks (RAID), multiple SCSI, ATA, or FireWire disk drives are grouped together via hardware or software and treated as a single data storage unit. This allows you to record data to multiple drives in parallel, increasing access time significantly. You can also partition the array into multiple volumes.

Creating a disk array is necessary only if high performance is required to capture and play back your video at the required data rate without dropping frames.

If you require rock-solid data integrity, consider purchasing a RAID. Many RAIDs record the same data on more than one disk, so that if a drive fails, the same data can still be retrieved from another disk. There are many RAID variations available, but one that offers high performance for both digital video capture and data redundancy is RAID level 3. Because they use specialized hardware, RAID level 3 systems can be more expensive, but they should be considered whenever the safety of your media is more important than the cost of your disks.

When you create or purchase a disk array, there are two important considerations:

  • Compatibility: Make sure the software you use to create the array is compatible with Final Cut Pro. For more information, go to the Final Cut Pro website at http://www.apple.com/finalcutstudio/finalcutpro.
  • Ventilation: If you’re creating an array yourself with an off-the-shelf drive enclosure, make sure to allow for good ventilation. Disk arrays store information on several disks simultaneously. If one of your disk drives fails, information on all the disks is lost. One of the most common reasons a disk drive breaks down is overheating, so make sure that your disks stay cool.

Important: Check the manufacturer’s specifications before buying disks to make sure the disks offer the level of performance you need.

Fibre Channel Drive Arrays and RAIDs

Fibre Channel is a hard disk drive interface technology designed primarily for high-speed data throughput for high-capacity storage systems, usually set up as a disk array or RAID. Fibre Channel disk drive systems typically have performance rivaling or exceeding that of high-performance SCSI disk arrays.

One of the most common ways of connecting a computer to a Fibre Channel disk drive system for video capture and output is called a point-to-point connection. A single computer, equipped with a Fibre Channel PCI Express card, is connected to a single Fibre Channel disk drive array. Unlike SCSI systems, Fibre Channel cables can be run extremely long distances, up to 30 meters using copper cables and 6 miles (10 kilometers) using optical cables.

Figure. Diagram showing a computer with a Fibre Channel card connected to a disk array.

For all its advantages, a Fibre Channel disk array requires more setup than other storage options, making it unsuitable for portable use. Fibre Channel disk arrays usually have extremely high capacity (potentially several terabytes of disk storage). Although this can make them more expensive relative to other storage solutions, the cost per megabyte is often considerably lower.

Storage Area Networks

A storage area network (SAN) such as an Apple Xsan system consists of one or more disk arrays that are made available to multiple computer systems simultaneously. Broadcast and post-production facilities can use an Xsan system to share a single set of media files among multiple editing systems.

Xsan software allows an administrator to control SAN access privileges for each editing system. For example, a capturing edit station may have read-and-write access to the SAN, while an assistant editor station may only have read access to media files for a particular project. An administrator may also control permissions to make sure editors capture material only to specific folders.

Advantages of Xsan include:

  • Media files are instantly accessible from multiple editing systems.

  • Storage capacity and bandwidth can be scaled as needed.

  • Editors can move between editing suites and continue working on the same project without moving media files.

  • Assistant editors can load, output, or archive media without disturbing an ongoing edit session.

  • Producers can view dailies or finished sequences for approval without being in an editing suite.

For more information, go to http://www.apple.com/xsan.