Why is rll encoding more efficient

The term Run Length Limited is derived from the two primary specifications of these codes, which are the minimum number the run length and maximum number the run limit of transition cells allowed between two actual flux transitions. Several variations of the scheme are achieved by changing the length and limit parameters, but only two have achieved any real popularity: RLL 2,7 and RLL 1,7.

FM can be called RLL 0,1 because as few as zero and as many as one transition cells separate two flux transitions. MFM can be called RLL 1,3 because as few as one and as many as three transition cells separate two flux transitions.

Although these codes can be expressed as variations of RLL form, it is not common to do so. RLL 2,7 was initially the most popular RLL variation because it offers a high-density ratio with a transition detection window that is the same relative size as that in MFM.

This method provides high storage density and fairly good reliability. In very high-capacity drives, however, RLL 2,7 did not prove to be reliable enough. Most of today's highest capacity drives use RLL 1,7 encoding, which offers a density ratio 1. Because of the larger relative timing window or cell size within which a transition can be detected, RLL 1,7 is a more forgiving and more reliable code, which is important when media and head technology are being pushed to their limits.

Unfortunately, reliability suffered too greatly under the RLL 3,9 scheme; the method was used by only a few now-obsolete controllers and has all but disappeared. Understanding how RLL codes work is difficult without looking at an example. Within a given RLL variation, such as RLL 2,7 or 1,7, you can construct many flux transition encoding tables to demonstrate how particular groups of bits are encoded into flux transitions.

In the conversion table shown in Table 8. The selected transitions for a particular bit sequence are designed to ensure that flux transitions do not occur too closely together or too far apart. Limiting how close two flux transitions can be is necessary because of the fixed resolution capabilities of the head and storage medium. Limiting how far apart two flux transitions can be ensures that the clocks in the devices remain in sync. In studying Table 8. Encoding this type of byte is not a problem, however, because the controller does not transmit individual bytes; instead, the controller sends whole sectors, making encoding such a byte possible by including some of the bits in the following byte.

The only real problem occurs in the last byte of a sector if additional bits are necessary to complete the final group sequence. In these cases, the endec in the controller adds excess bits to the end of the last byte. These excess bits are then truncated during any reads so the controller always decodes the last byte correctly.

Previous page. Later on, many drive manufacturers introduced RLL certified models, and these were generally just as reliable as their cheaper, smaller MFM drives. The Miniscribe is an example. Technically, however, there was no such thing as an 'RLL drive'.

During the late s, the PC hard disk industry began using RLL encoding schemes to increase the storage capabilities of PC hard disks. Instead of encoding a single bit, RLL typically encodes a group of data bits at a time. The term Run Length Limited is derived from the two primary specifications of these codes, which are the minimum number the run length and maximum number the run limit of transition cells allowed between two actual flux transitions. Several variations of the scheme are achieved by changing the length and limit parameters, but only two have achieved real popularity: RLL 2,7 and RLL 1,7.

FM can be called RLL 0,1 because as few as zero and as many as one transition cells separate two flux transitions. MFM can be called RLL 1,3 because as few as one and as many as three transition cells separate two flux transitions. Although these codes can be expressed as variations of RLL form, it is not common to do so.

RLL 2,7 was initially the most popular RLL variation because it offers a high-density ratio with a transition detection window that is the same relative size as that in MFM. This method provides high storage density and fairly good reliability. In high-capacity drives, however, RLL 2,7 did not prove to be reliable enough.

Because of the larger relative timing window or cell size within which a transition can be detected, RLL 1,7 is a more forgiving and more reliable code, which is important when media and head technology are being pushed to their limits.

Understanding how RLL codes work is difficult without looking at an example. Within a given RLL variation such as RLL 2,7 or 1,7 , you can construct many flux transition encoding tables to demonstrate how particular groups of bits are encoded into flux transitions. In the conversion table shown below, specific groups of data that are 2, 3, and 4 bits long are translated into strings of flux transitions 4, 6, and 8 transition cells long, respectively.

The selected transitions for a particular bit sequence are designed to ensure that flux transitions do not occur too closely together or too far apart. Limiting how close two flux transitions can be is necessary because of the fixed resolution capabilities of the head and storage medium. Limiting how far apart two flux transitions can be ensures that the clocks in the devices remain in sync. In studying the table above, you might think that encoding a byte value such as b would be impossible because no combinations of data bit groups fit this byte.

So relative to FM, data can be packed into one third the space. Data recovery Salon welcomes your comments and share with us your ideas, suggestions and experience. Data recovery salon is dedicated in sharing the most useful data recovery information with our users and only if you are good at data recovery or related knowledge, please kindly drop us an email and we will publish your article here.

We need to make data recovery Salon to be the most professional and free data recovery E-book online. Find out more. Subscribe by RSS.