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The Personal Computer Based Music Studio
Bare Essentials for Your Home Studio

Which is better: IDE, SCSI, USB or FireWire?

IDE

Integrated Drive Electronics (IDE) (also known as ATA) has become by far the most common way to connect hard drives, CD-ROM, CD-Recordable and DVD-ROM drives to today's personal microcomputers, both Mac and PC. All current production Macs and PCs come with at least an IDE hard drive and CD-ROM or DVD-ROM drive installed as standard equipment.

There are several types of IDE interfaces and devices commonly used today:
 

Enhanced IDE (E-IDE) - Many CD-ROM drives made in the last couple of years use this older design. Hard drives made from about late 1994 to early 1997 are of the E-IDE design.

UltraDMA (a.k.a. DMA-33, Ultra33, ATA-33) - Most IDE drives made from 1997 to early 1999 are of the UltraDMA design. The latest CD-ROM and CD-RW drives are also UltraDMA.

ATA-66 (a.k.a. Ultra66, DMA-66) - Most IDE drives made in 1999-2000 are of the ATA-66 design.

ATA-100, ATA-133 - These IDE flavors allow data transfers as fast as 133MB per second (in theory, at least).

Serial ATA - The very latest IDE flavor is patterned after FireWire, which allows "hot swapping," connection of both internal and external hard drives, and very fast throughput.

The latest ATA-133 hard drives are just about as fast as the latest Ultra160 SCSI drives. The capabilities of the drives themselves, not the interfaces they attach to, define the upper performance limits. So if your goal is to obtain the fastest raw performance for the least amount of money, today's IDE drives are "better" than the latest SCSI drives. But—this cheap speed comes at a price. How's that? Read on...

IDE devices are connected using 40-pin ribbon cables. To facilitate their faster speeds, ATA-100 and ATA-133 controllers and devices need to use special shielded ribbon cables, which are basically the same as the normal 40-conductor IDE ribbon cables but with each conductor shielded (for a total of 80 wires on 40 pins).

All flavors of IDE share the same basic design and limitations. Each IDE interface 'channel' allows attachment of a maximum of two devices, one configured as the 'Master' device and the other as the 'Slave'.

Older versions of IDE used a protocol known as "PIO" or (Programmable Input/Output) to control data transfers. PIO uses the host computer's CPU to control disk I/O, and so requires a lot of your computer's attention when reading from or writing to disk.

The term "Bus Mastering" describes a protocol where the device itself performs the basic computations necessary to perform input/output, thus freeing the CPU for other tasks. UltraDMA (ATA-33) was the first version of IDE to fully use the Bus Mastering protocol. Bus Mastering interfaces and devices are usually faster than PIO.

So far, all versions of IDE have been backwards-compatible with previous versions.

In a Power Macintosh G4 computer there are two ATA-100 IDE channels built into the logic board, used for the hard drive and the combo DVD/CD-R drive. The two drives are connected as Master on each of the two ATA-100 channels. Consequently, you can install two additional IDE drives as Slaves in a G4 system, for a maximum total of four drives. You must use the USB or FireWire ports to connect external devices.

In a PC, there are two interface channels built into the motherboard—one is called the Primary IDE interface and the other the Secondary IDE interface. This means there can be no more than four IDE devices connected at a time in a typical PC (Primary Master, Primary Slave, Secondary Master and Secondary Slave).

Pentium III, Pentium 4 and Athlon PCs come with the latest ATA-133 controllers. This is required for full-speed operation of the new ATA-133 hard drives.

Windows 95 and Windows NT 4.0 support the E-IDE and UltraDMA (ATA-33) interfaces, while Windows 98 Second Edition, Windows Me, Windows 2000 and Windows XP add support for ATA-66, ATA-100 and ATA-133 interfaces.

PCs have limited resources with which to add devices. One limiting factor is that each device installed in the system must have its own Interrupt Request Line, or IRQ. There are 16 IRQs in total (numbered 0 to 15) but many of these will already be occupied by basic system devices like the floppy drive controller, the serial ports, parallel port, PS/2 mouse, etc.

If you use IDE hard drives, each IDE controller channel installed will need to use an IRQ (remember, each channel can hold only two IDE devices maximum). This means, for instance, that your two hard drives will need to take IRQ14, while your CD-ROM and CD-Recordable drives will take IRQ15 (IRQs 14 and 15 are reserved for the Primary and Secondary onboard IDE controllers on PC motherboards). You will now have no room left to add more IDE devices, like an internal DVD-RAM drive. This leaves only a handful of IRQs available for add-ons (like your expensive 'pro' soundcard) in the typical PC. If your PC has a built in RAID controller you will be able to add up to four more IDE hard drives, but this will use another IRQ (usually IRQ 11).

Another liability of IDE is that there is no provision for connecting external devices to IDE interfaces. In today's PCs, external devices are usually connected to the onboard USB controller, which takes up another IRQ (if you have a FireWire port, that uses yet another IRQ). Older printers connect to the parallel port, using yet another IRQ (IRQ 7). This is why PC users find themselves with so many mysterious "hardware conflicts" right out of the box.

SCSI

Now let's take a look at this same situation, but this time using SCSI (Small Computer System Interface, pronounced "scuzzy")... First, the SCSI controller will take up only a single IRQ. An Ultra2Wide (also known as Low Voltage Differential or LVD) or Ultra160 SCSI controller card will allow up to fifteen SCSI devices to be attached to the computer. Each SCSI device gets its own unique SCSI ID number (0 through 15, with ID 7 taken by the controller). This means that as long as you use all SCSI devices, you can have two hard drives, a DVD-ROM, a DVD-RAM and a CD-Recordable all hooked up, and still have room for a scanner, a Jaz drive, and up to seven more devices without needing to use more than a single IRQ!

If you configure your PC system to boot from a SCSI hard drive and you have no IDE devices installed, you can turn off the built in IDE controllers on your motherboard and get the use of IRQ14 and IRQ15 back for installing PCI cards. If you have only a SCSI scanner and external drives and no USB devices, you can even turn off the USB controller. Now you have all of your internal and external drives connected to the SCSI controller, which uses only one IRQ. No more IRQ conflicts!

Note that you can add a PCI SCSI controller to your present IDE system, no problem. It's very common for a PC to boot from an IDE hard drive with an IDE CD-ROM or DVD-ROM installed, while using a PCI SCSI controller to connect additional hard drives, a removable drive, a CD-Recordable drive, and a scanner. This also holds true for owners of the full-size Apple PowerMac G4 towers, but not for the iMac or G4 Cube (these have no PCI slots).

Another important consideration is that reading and writing operations to and from E-IDE or UltraDMA hard drives take up more of your CPU's resources than when performed with SCSI drives. These reading and writing operations to and from the hard drive are referred to as "disk I/O". The reason for IDE's higher processor usage is that the IDE interface needs the CPU to process the commands that control its basic disk I/O functions. The control circuitry on a good SCSI adapter (such as the Adaptec AHA-2940U2W or AHA-19160, Tekram DC-390F or AdvanSys ABP-940UW) will perform these disk I/O commands by itself, without the help of the host computer's CPU - by a process called "Bus Mastering". This allows the host CPU to concentrate on other functions, like processing that Waves TruVerb DirectX plugin's reverb algorithm in real time. The result is that reads and/or writes to a Bus Mastering SCSI hard drive won't interrupt your work (i.e. you will get better 'multi-tasking' performance).
 

On my old Pentium 233MMX based system, writing a 75 MB file to my UltraDMA hard drive used up as much as 75% of my CPU's processing time, while writing the same file to my UltraSCSI (Narrow, not Wide) drive used only 35% at the most. I had the latest Bus-Mastering IDE drivers installed (with DMA enabled), as well as the latest drivers and ASPI layer for my Adaptec 2940AU SCSI card. I measured my processor usage in the WinNT Resource Monitor applet. Both the UltraDMA (Fujitsu 4.3GB) and the UltraSCSI (Micropolis 4.0GB) drives had a continuous throughput of over 7MB/sec, as measured in DskBench. Both drives have a rotational speed of 5400 rpm.

I definitely noticed a decrease in the performance of Sound Forge, WaveLab, and DirectX audio plugins when I used my UltraDMA hard drive to store my audio files instead of my UltraSCSI drives. Curiously, my UltraDMA drive measured faster than my SCSI drives in both WinTune 98 and DskBench.

Up until a couple of years ago, all Apple Macintosh computers used the SCSI interface to attach external hard drives, scanners, CD-ROM and CD-R drives to the system. The latest PowerMac G4 computers running MacOS 9.x no longer sport a SCSI interface, but are equipped with ATA-66 hard drives and DVD-ROM or DVD-RAM drives, with room for two additional IDE drives. G4s rely on USB and FireWire for adding external devices to the system (more on this below). A SCSI controller card can be installed in an available PCI slot in a G4 tower, but remember, the iMac and G4 Cube have no internal expansion options available, and no external SCSI interface — so no SCSI for the iMac or Cube.

NEWS FLASH: It appears that the writing is on the wall — SCSI is fading away. While IDE (ATA) is not as flexible and reliable, and FireWire and USB are not as fast, these technologies are all much less expensive than SCSI. The result is inevitable: SCSI will be abandoned except for the most demanding server applications. Those of you who are buying a new computer for music production and aren't already using a lot of SCSI devices may want to start looking into FireWire (see below). Is FireWire the future? It sure looks like it... But USB 2.0 may also become a "standard." All I can say is that FireWire is faster at this point in time.

USB

The Universal Serial Bus (or USB for short) allows multiple devices to be daisy-chained together, all attached to a single USB port (using a single IRQ in a PC). USB is supported on the Apple iMac, iBook and G4, and on new PCs. There are PCI expansion cards with USB ports available for older Power Macs and Windows PCs. Unfortunately, USB 1.0/1.1 is only good for comparatively low bandwidth devices like scanners, printers, monitors, keyboards... and MIDI interfaces (USB MIDI interfaces have become quite popular).

There have been reports that using a USB MIDI interface on a PC equipped with a first-generation USB controller (the type with only two USB ports instead of four) can cause clicks, pops and dropouts in audio. If you plan to use a USB MIDI interface in a PC, make sure you are using a system built on a recent motherboard and chipset (such as Intel i845E, i845G, i850, VIA KT133A, KT266A or KT333, or nForce2). Apparently, using a USB MIDI interface on a G4 Macintosh is not a problem.

There are USB hard drives available, but these have a maximum throughput of no more than 2 MB/second, which is slower than today's CD-ROM drives—and for a hard drive, that's slow! However... The new version of USB, USB 2.0 will soon be commonplace, and will be much faster. How much faster? Speeds approaching those of FireWire are promised, but we'll have to wait and see what we really get...

FIREWIRE (IEEE-1394)

FireWire (also known as IEEE-1394 or Sony i-Link, developed by Apple Computer) is the hottest new high speed interface for attaching peripherals to computers. FireWire allows multiple devices, including hard drives, to be connected to a single adapter... and — it's FAST. It is worth noting however that FireWire is a serial (first in, first out) bus technology, as opposed to a parallel tasking technology like SCSI, so data transfers over FireWire may take more CPU cycles to accomplish than when using SCSI. This is not a problem now that two gigahertz-speed processors are so affordable — but I wouldn't try recording eight tracks 'live' to a FireWire hard drive on your old overclocked Celeron 300A! (I should point out that Digidesign still recommends that an LVD or Ultra160 SCSI controller and matching "scratch disk" hard drive be installed in a ProTools workstation meant for recording multiple tracks simultaneously.)

At this time, FireWire is most commonly used for connecting digital video (DV) recorders to computers for video editing. Apple Computer equips its new computers with FireWire interfaces. On most PCs, a FireWire to PCI interface card will need to be installed if you wish to hook up FireWire devices to the computer (although Sony VAIO and some other new Pentium 4 and Athlon XP PCs come with FireWire ports already installed). Windows 98 Second Edition, Me, 2000 and XP all support FireWire, while Windows 95 and NT 4.0 require special software in order to use it.

FireWire hard drives are now readily available. Although they are not quite as fast as the latest Ultra2Wide and Ultra160 SCSI hard drives, FireWire drives are "hot-swappable," which means that if you fill up one drive during a recording session you can pull the first drive out and replace it with a blank, formatted drive— without rebooting the computer. You can even find FireWire RAID cabinets. (For more info on the latest hard drive technologies, check out Granite Digital. There are also quite a few FireWire CD-Recordable drives, while Mark of the Unicorn has released a multichannel FireWire audio interface called the 828. I would expect to see more peripherals like these showing up in the very near future...