电脑硬盘结构及基本知识.ppt

Read-back Waveforms at Different User Densities UD = 0.75 UD= 1.5 UD = 2.0 Asynchronous Detection Peak Detection PLL Detector has NO knowledge of the bit timing PLL knows the bit timing No communication to Detector Synchronous Detection Peak Detection PLL Detector has knowledge of when a pulse may occur (bit timing) Can make a here / not-here decision Makes better decisions Signal to Noise Ratio (SNR) can be lower Sampled Detector allows for post compensation Model and remove ISI as an error source Synchronous Channel: Sampled Peak Detection 0.0 -0.2 -0.9 0.0 0.9 0.1 -0.1 -0.8 0.7 -0.8 -0.1 0.1 0.9 0 -0.9 -0.2 0.0 0 0 -1 0 1 0 0 -1 1 -1 0 0 1 0 -1 0 0 0 0 1 0 1 0 0 1 1 1 0 0 1 0 1 0 0 50% -50% Detection Threshold Received Samples Target values Detected Data Synchronous Channel: Sampled Peak Detection 50% -50% Detection Threshold -0.2 -0.4 -0.8 0.0 0.7 0.3 -0.2 -0.7 0.3 -0.7 -0.2 0.3 0.8 0 -0.8 -0.4 - 0.2 0 0 -1 0 1 0 0 -1 0 -1 0 0 1 0 -1 0 0 0 0 1 0 1 0 0 1 0 1 0 0 1 0 1 0 0 Received Samples Target values Detected Data Missing 1 transition Or have one too many transitions Sequence Detection We know certain sequences shouldn’t exist. Make use of the fact ! Step 1: Determine the rule for which sequences exist For sampled peak detection = polarity of pulses must alternate Step 2: Compare the observed samples with the expected samples from all possible sequences. Choose the closest sequence. Closest = sequence with minimum squared error Closest = most likely sequence → Maximum Likelihood St