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extremely advanced method of RLL called Partial Response Maximum Likelihood (PRML) encoding. As hard drives pack more and more fluxes on the drive, the individual fluxes start to interact with each other, making it more and more difficult for the drive to verify where one flux stops and another starts. PRML uses powerful, intelligent circuitry to analyze each flux reversal and to make a “best guess” as to what type of flux reversal it just read. As a result, the maximum run length for PRML drives reaches up to 16 to 20 fluxes, far more than the 7 or so on RLL drives. Longer run lengths enable the hard drive to use more complicated run combinations so the hard drive can store a phenomenal amount of data. For example, a run of only 12 fluxes on a hard drive might equal a string of 30 or 40 ones and zeroes when handed to the system from the hard drive.

The size required by each magnetic flux on a hard drive has reduced considerably over the years, resulting in higher capacities. As fluxes become smaller, they begin to interfere with each other in weird ways. I have to say weird because to make sense of what’s going on at this subatomic level (I told you these fluxes were small!) would require you to take a semester of quantum mechanics. Let’s just say that laying fluxes flat against the platter has reached its limit. To get around this problem, hard drive makers recently began to make hard drives that store their fluxes vertically (up and down) rather than longitudinally (forward and backward), enabling them to make hard drives in the 1 terabyte (1024 gigabyte) range. Manufacturers call this vertical storage method perpendicular recording.

For all this discussion and detail on data encoding, the day-to-day PC technician never deals with encoding. Sometimes, however, knowing what you don’t need to know helps as much as knowing what you do need to know. Fortunately, data encoding is inherent to the hard drive and completely invisible to the system. You’re never going to have to deal with data encoding, but you’ll sure sound smart when talking to other PC techs if you know your RLL from your PRML!

Moving the Arms

The read/write heads move across the platter on the ends of actuator arms or head actuators. In the entire history of hard drives, manufacturers have used only two technologies to move the arms: stepper motor and voice coil. Hard drives first used stepper motor technology, but today they’ve all moved to voice coil.

Stepper motor technology moved the arm in fixed increments or steps, but the technology had several limitations that doomed it. Because the interface between motor and actuator arm required minimal slippage to ensure precise and reproducible movements, the positioning of the arms became less precise over time. This physical deterioration caused data transfer errors. Additionally, heat deformation wreaked havoc with stepper motor drives. Just as valve clearances in automobile engines change with operating temperature, the positioning accuracy changed as the PC operated and various hard drive components got warmer. Although very small, these changes caused problems. Accessing the data written on a cold hard drive, for example, became difficult after the disk warmed. In addition, the read/write heads could damage the disk surface if not parked (set in a non-data area) when not in use, requiring techs to use special parking programs before transporting a stepper motor drive.

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NOTE Floppy disk drives still use stepper motors.

All magnetic hard drives made today employ a linear motor to move the actuator arms. The linear motor, more popularly called a voice coil motor, uses a permanent magnet surrounding a coil on the actuator arm. When an electrical current passes, the coil generates a magnetic field that moves the actuator arm. The direction of the actuator arm’s movement depends on the polarity of the electrical current through the coil. Because the voice coil and the actuator arm never touch, no degradation in positional accuracy takes place over time. Voice coil drives automatically park