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U. Glaeser

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FIGURE 34.10 A typical read channel architecture.<br />

the controller. A typical read channel architecture is shown in Fig. 34.10. During a read operation, the<br />

head generates a pulse in response to magnetic transitions on the media. Pulses are then amplified by<br />

the preamplifier that resides in the arm electronics module, and fed to the read channel. In the read<br />

channel, the readback signal is additionally amplified and filtered to remove noise and to shape the<br />

waveform, and then the data sequence is detected (Fig. 34.10). The data to be written on a disk are sent<br />

from a read channel to a write driver that converts them into a bipolar current that is passed through<br />

the electromagnet coils. Prior to sending to read channel, user data coming from computer (or from a<br />

network in the network attached storage devices) are encoded by an error control system. Redundant<br />

bits are added in such a way to enable a recovery from random errors that may occur during reading<br />

data from a disk. The errors occur due to number of reasons including: demagnetization effects, magnetic<br />

field fluctuations, noise in electronic components, dust and other contaminants, thermal effects, etc.<br />

Traditionally, the read channel and drive controller have been separate chips. The latest architectures<br />

have integrated them into so called “super-chips.”<br />

Analog Front End<br />

As a first step, the read signal is normalized with respect to gain and offset so that it falls into an expected<br />

signal range. Variation of gain and offset is a result of variations in the head media spacing, variations<br />

in magnetic and mechanical and electronic components in the drive, preamplifier and read channel. The<br />

front end also contains a thermal asperity (TA) circuit compensation. Thermal asperity occurs when<br />

head hits a dust particle or some other imperfection on a disk surface. At the moment of impact, the<br />

temperature of the head rises, and a large signal at the head’s output is generated. During TA a useful<br />

readback signal appears as riding on the back of a low frequency signal of much higher energy. The<br />

beginning of this “background” signal can be easily predicted and the TA signal itself suppressed by a<br />

relatively simple filter.<br />

© 2002 by CRC Press LLC<br />

Preamplifier<br />

Read/Write<br />

Head Head<br />

Write<br />

Precompensation<br />

Variable<br />

Gain Gain<br />

Amplifier<br />

Read Read Channel<br />

Low-Pass<br />

Filter<br />

Thermal<br />

Asperity<br />

Compensation<br />

+<br />

Gain Gain<br />

Control<br />

Offset<br />

Control<br />

LMS LMS<br />

Coefficient<br />

Adaptation<br />

Analog<br />

FIR FIR<br />

Filter<br />

Adaptive<br />

Equalizer<br />

Servo<br />

Address<br />

Mark<br />

&<br />

Burst<br />

Detector<br />

Quantizer<br />

Timing<br />

Recovery<br />

Recovered Clock<br />

to the Controller<br />

to the Controller<br />

Qulity<br />

Monitor<br />

SYNC<br />

Mark<br />

Detector<br />

Viterbi<br />

Detector<br />

Post- Post-<br />

Processor<br />

EE EE Control<br />

Encoding/<br />

Decoding<br />

Modulation<br />

Encoding/<br />

Decoding

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