|WikiProject Computing||(Rated Start-class)|
are bytes encoded individually or as a stream?????
The question is unclear. BITS are encoded as a stream.
This article is silly. All magnetic media recording codes are RLL codes. If it imposes a limit on the run length, it's RLL! For example, FM is a rate-1/2 (0,1) RLL code and MFM is a rate-1/2 (1,3) RLL code. Group Code Recording is, as the article says, a rate-4/5 (0,2) RLL code. Other popular ones are the rate-1/2 (2,7) RLL code (the original "RLL") and the rate-2/3 (1,7) RLL code.
Then folks developed PRML techniques that made (0,k) RLL codes more feasible, and that's what everyone uses on hard drives these days.
A lot of these techniques were developed at IBM, who were always pushing storage capacity limits in their mainframe heyday.
The other thing that needs to be mentioned is that all such codes assuming NRZI encoding afterwards, so a 1 bit is a transition, and a 0 bit is no transition. The minimum spacing between transitions d by (n+1 in an (n,k) code) is limited by the high-frequency response of the channel, while the laximum spacing (k+1 in an (n,k) RLL code) is limited by the clock-recovery jitter. (Even if the electronics have zero jitter, there is some in the source signal.)
First we have this statement: Run length limited codes were widely used in hard disk drives until the mid-1980's
Then this statement: Early disk drives used very simple encoding schemes, such as RLL (0,1) FM code, but higher density RLL (2,7) and RLL (1,7) codes became the de facto industry standard for hard disks by the early 1990s.
The first statement suggests that RLL was not widely used in hard drives after the mid 80s, whereas the second suggests they have been the defacto standard since the early 90s.
How can both of these statements be true?
- Because this is wikipedia! In all seriousness, this article is a lot of words with not much use. And no, I won't improve it, because I'm not an expert; if I were, I would not have come here to try to find out what RLL _IS_, which I walk away still not knowing. — Preceding unsigned comment added by 18.104.22.168 (talk) 23:59, 29 July 2011 (UTC)
a speed variation of even 0.01% - which is way better than what e.g. a floppy drive can possibly guarantee - could result in four bits being added to or removed from the 4,096 bit data stream. The math is wrong: 4 bits out of 4096 is 0.1%. I don't know how 0.1% compares to what a floppy drive can possibly guarantee, so I don't know how to fix this sentence.
In the specification for the Teac FD235HF floppy drive, the Long Term Speed Variation is given as +/-1.5%, and the Instantaneous Speed Variation as +/-2%, so 0.1% is far less than either. I've amended the percentage to 0.1% which is correct for 4 bytes in 4,096.