Read channels for hard drives

The read channel is a device situated between the drive’s controller and the recording head’s preamplifier (Figure 15.1). The read channel provides an interface between the controller and the analog recording head, so that digital data can be recorded and read back from the disk. Furthermore it reads back the head positioning information from a disk and presents it to the head positioning servo system that resides in the controller. A typical read channel architecture is shown in Figure 15.2. During a read operation, the head generates a pulse in response to magnetic transitions on the media. Pulses are then amplified by the preamplifier that resides in the arm electronics module, and fed to the read channel. In the read channel the readback signal is additionally amplified and filtered to remove noise and to shape the waveform, and then the data sequence is detected (Figure 15.2). The data to be written on a disk are sent from a read channel to a write driver that converts them into a bipolar current that is passed through the electromagnet coils. Prior to sending to read channel, user data coming from computer (or from a network in the network attached storage devices) are encoded by an error control system. Redundant bits are added in such a way to enable a recovery from random errors that may occur during reading data from a disk. The errors occur due to a number of reasons including: demagnetization effects, magnetic field fluctuations, noise in electronic components, dust and other contaminants, thermal effects etc. Traditionally, the read channel and drive controller have been separate chips. The latest architectures have integrated them into so called “super-chips.”