Learn How Bits Are Actually Stored: NRZI, GCR, MFM, and RLL Explained

Ever wondered how data is actually stored on tapes, floppy disks, and hard drives? We explore the evolution of data encoding formats from NRZI (Non-Return to Zero Inverted) to PE (Phase Encoding), GCR (Group Coded Recording), MFM (Modified Frequency Modulation), and RLL (Run Length Limited). By the end of this video, that alphabet soup of acronyms will make perfect sense!

Join me as we unravel the challenges of storing digital data on magnetic media and how ingenious encoding schemes have overcome these hurdles. Whether you're a tech enthusiast, a vintage computing fan, or just curious about how your data is stored, this video is for you.

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Twitter: @davepl1968 davepl1968

[ilovecatsandsynths]

Dave didn’t go into this specifically, , but for those who remember “single density” vs. “double density” floppy drives, single density used FM encoding while double density used MFM encoding.

[lotusdj]

I understood about 40% of this but loved every minute of it.

[Jed__Blitzen]

You have great pacing and cadence when you explain concepts. Very clear.

[nst]

I lived through the transition from MFM to RLL, but never had a good understanding of the differences between the two formats until your lucid explanation. Thank you for that. Also your new book helped me understand my son, who was diagnosed as ADHD but seems to also have some aspects of the spectrum.

[martyb]

Great video. Many moons ago I used to change the interleave on my mfm hdd to get better performance on my 20Mb hard disk. Your wealth of knowledge regarding PCs is astounding. Your articulate presentations are very informative and easy to understand. Well done.

[michaeltyniec]

Dave you are a godsend! I share your videos with as many people as I can. You've helped me better connect with my autistic son and realize that I'm on the spectrum too.

[nezbrun]

Although you didn’t mention it, this is all to do with clock recovery: the receive side clock must synchronise with the incoming data so uses these transitions to do so.

Furthermore, for some signalling, sometimes there’s a physics need to maintain DC balance, so over time the encoding leads to equal time spent in each state. Coding such as FM, MFM and Manchester achieve this.

Manchester encoding is still used extensively, for example in USB C power delivery signalling over the CC pins of a USB C connector on the physical layer, and 4B5B (like GCR) on top of that at the packet layer.

NRZI is used on USB D+/D-, plus a zero bit is stuffed after six ones to avoid too long a period without a transition, so is a semi-variable length coding scheme.

So this is all still very relevant.

[Nightspyz]

Love how you broke down the features and made it easy to understand. Super helpful

[JamieStuff]

You are absolutely correct that you could use an RLL controller on an MFM drive. In fact, Perstor sold "aftemarket" RLL controllers specifically to use with MFM drives. I used one on my 71MB Maxtor XT-1085 to get over 100MB of storage. I also swapped out one of my floppies to add in my old ST-223 for a 30 MB boot drive. Good times...

[andrewsydney]

That was awesome!
These techniques/concepts also apply to communications cables (Ethernet, HDMI, USB, SATA) and to optical media (CDs, DVDs)

[Michael_]

Having dedicated the better part of two decades to study and invention in data compression, I found this to be a really enjoyable watch. Thank Dave!

[wadz]

Thank you SOO much for this video. I just completed a program for restoring old C64 disk images encoded with GCR, but I've always wondered what the difference was between GCR and MFM. Now i know!! I started with zero knowledge of GCR or the 1541 and ended up with a program that could successfully reproduce even the most difficult protection schemes, thus preserving these images for eternity in their original glory

[eric_d]

Been a subscriber for years, but this is probably my favorite video on this channel so far. I do need to watch it again to absorb all the knowledge, but I surely know more now than I did 20 minutes ago. I have a rule to never click like on any videos where the creator asks for a like, but I gave a like on this one anyway because it deserves it.

[gtc]

There were a lot of conversations on BBSs of the time about putting your MFM disk on an RLL controller and getting more space available. The common thread was YMMV, sometimes it worked and you got more space/performance, sometimes it failed. I recall it being highly dependent on the drive and controller combination, but being so long ago, hard to remember any specifics. What I do remember was, at least for performance, it was better to get a 1:1 MFM controller (WD 1006 card) and stick with the same encoding rather than chance doing an RLL conversion and possibly losing files. If you did a low-level format on an MFM drive with RLL, the drive could end up very iffy...

[TinkerbatTech]

Definitely worth tring a MFM drive on a RLL controller. Especially when they started using plated media. Very reliable and another bonus: 50% more storage per track also meant faster read/write data transfers as the drive was rotating the same speed but the data was transferring that much faster. Made for noticeably faster operation.. Win-win! Ran dozens of different drives that were MFM originally with RLL controllers for years. My old multi-line BBS ran with 3 old Priam(?) drives on an RRL controller. Never lost any data, rand for years... Great video! Love these deep dives into stuff like this. Thanks! Blessings, Stu

[XGP]

I wrote mainframe tape analysis software in the 70s. Thank you for the walk down memory lane!

[stupot]

When I worked in the HSBC data centre in London (formerly Midland Bank) back in the early 80's, I remember using PE coded tape reels. A couple a couple of years later transitioning to GCR tapes. Also used 80 column punch cards.
Thanks Dave, you brought back some old memories.

[msromike]

Waiting for the electrical interface vid! SCSI, ESDI, SATA, IDE for hard drives and maybe the various floppy drive interfaces.

[BlackHoleForge]

I often find myself being interested in some of these niche topics. I also watched usagi's video.
After I watch a video I like to do research on the topic to get a better understanding. But most of the time the research pulls me out of the happy mood I was in because the topic or idea is hard to wrap my head around.
Thank you Dave for making supplemental material that is both interesting and easy to understand.
Thank you Dave.

[nakfan]

Your data output and format is magically synced to my brains input circuitry...