24. Why is Manchester coding not a good choice for writing data to a magnetic disk?
Manchester coding is not a good choice for writing data to a magnetic disk due to the following reasons:
Bandwidth Requirements: Manchester coding doubles the data rate compared to the non-return-to-zero (NRZ) encoding, as it transitions between high and low states in each bit period. This increased data rate translates into higher bandwidth requirements for the storage medium. Magnetic disks typically have limited bandwidth, and using Manchester coding would require a significant increase in the disk's bandwidth capabilities, which may not be feasible or cost-effective.
Clock Recovery: Manchester coding relies on a clock signal that is embedded within the data itself. This clock signal helps in synchronizing the receiver with the transmitter. However, magnetic disks have their own timing mechanisms, such as rotational speed and sector synchronization, which are separate from the data being written. Manchester coding's dependency on the embedded clock signal conflicts with the disk's internal timing mechanisms, making it challenging to recover the correct clock timing from the encoded data.
Storage Density: Magnetic disks are designed to achieve high storage densities, packing a large amount of data within a limited physical space. Manchester coding, with its requirement for additional transitions within each bit period, results in a less efficient utilization of the available storage space. It would require more physical area on the disk to store the same amount of information compared to other encoding schemes that offer higher data density.
Magnetic Interference: Manchester coding's frequent transitions between high and low states produce rapid changes in the magnetic field, resulting in increased electromagnetic interference (EMI). This EMI can introduce errors in the stored data and affect the overall reliability and performance of the magnetic disk. Other encoding schemes, such as run-length-limited (RLL) coding, are specifically designed to minimize EMI and improve the robustness of data storage on magnetic disks.
Overall, Manchester coding is better suited for applications where clock recovery from the data stream is crucial, such as in certain types of digital communication over physical channels. However, for magnetic disk storage, alternative encoding schemes that are optimized for storage density, timing synchronization, and electromagnetic interference are typically preferred.