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Trace A is a portion of a captured disk drive sector. Trace C is a histogram of the averaged Viterbi input samples. There are three distributions due to the peak locations, zero crossings, and trough locations. Trace D is a histogram of the actual Viterbi input samples. Measurements performed on Trace C are Media Signal-to-Noise (msnr), Residual Signal-to-Noise (rsnr), and the ratio of Media Noise to Residual Noise (m_ to_r).
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Media Noise
Media signal-to-noise (msnr) automatically measures repetitive noise that has been recorded on the media. Residual signal-to-noise
(rsnr) automatically measures the non-repetitive noise, which is often referred to as electronics noise. Media noise to residual noise
(m_to_r) automatically measures the ratio of media noise to residual noise. If this ratio is greater than 1.00, the signal is media noise–dominated. If not, the signal is electronics noise–dominated. In the example (the screen shot above), MSNR = 24.811 dB, while rsnr = 23.796 dB. There is less media noise (better signal-to-noise ratio) than electronics noise. The noise is dominated by the residual noise and the m_to_r ratio is less than one (.896).
Granularity in magnetic media produces zigzag transitions. The exact location of the zigzags changes from write to write, causing media noise (also known as transition noise or zigzag noise). During a read, the read head effectively averages across the track, the location of all of
the zigzags. If the recorded track width is wide, there are many zigzags and very little variability in the averaged transition location. If the track width is narrow, there are fewer zigzags, and the variability in the averaged transition location increases.
This figure represents a
single recorded magnetic transition. An ideal transition is a straight
line. However, granularity in the media produces zigzag transitions. The
zigzag locations change from write to write, producing variability in the
read-back signal or media noise.
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