Unlocking the Secrets of Magnetic Storage: NRZI, GCR, MFM, and RLL Explained 

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.

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

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.

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

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...

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.

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.

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.

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

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

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!

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

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

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...

Yes! I remember my 20mb RLL drive on an Atari 800XL using an ICD MIO. Bonus, with the MIO you could connect a DSSD 3.5in drive for a whopping 320kb of storage. I remember putting every single scrap of data onto the hard drive. Games, programs, I mean everything. Then wondering, "what am I going to do with the 16mb leftover on the hard drive?"

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

DAT, DCC, DVI, HDMI and USB 3.0 (among others) use 8b/10b encoding which sounds like a variation on GCR; for every 8 bits of data, there's a 10 bit pattern which is used to avoid long strings of 0 / 1 at a time. This helps to ensure that the receiving end can extract clock / timing data from the data stream, without needing a separate clock channel (parallel printer ports usually had 8 data lines and a clock line, at a bare minimum). There are also 64b/66b and 128b/130b variations used on other, newer systems. Same basic idea but pushing a greater percentage of the total signal as payload.

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

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.

Ah ok, the last bit about MFM explains a lot. I remember back in about 1988 being told I could buy a 30MB Seagate harddrive and format it at 40MB under DOS 3.3, or some numbers along those lines. The question back then was "how is that possible?". Nobody knew. Even the people at the computer store didn't know, they just said we could do it even if they didn't know why it worked. Well, it did work, and very well. 10MB was a major win back then.

Loved this. I spent time back in the day using my Amiga 500 and writing a 68000 MFM decoder to read files from floppies without using the Amiga ROM routines. Thanks.

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.

Great from the ground up explanation of encoding for magnetic media that extends to almost all other storage, RF and optical applications.

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.

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

I am old enough that I have worked on tapes and disk drives that used all of the mentioned recording techniques. The greatest improvement was when the CRC code was written at then of each block so that the data read could be verified against the CRC code and if it was wrong then the hardware would try 3 times to retry reading the data and getting the correct CRC code but if the CRC still indicated an error in the data then the software would use the CRC code to do a mathematical calculation to produce the correct data. I have changed many read/ write heads on head per track disk drives and the old removable disk pack drives requiring special alignment packs and oscilloscope. A few years before I retired all the physical magnetic tapes had been replaced by virtual tapes stored on arrays of hard drives. All the hard drives for fast storage had been replaced with arrays of SSD drives. Original head per track drives that I worked on held 10 MB of data driven by a 1 HP synchronized AC motor, 5 drives per cabinet and 50 MB capacity.

When I looked at the title, I just though " I Know all of them but GCR". but when you start explaining how GCR is working, it came back to me. When I worked on mainframe we had to know all the details.

Great presentation Dave, thanks for that :-) I remember getting more capacity on the same hard drive when upgrading OS. I can't remember the details, but it certainly happened, and the explanation given at the time was "different encoding" - which you confirm here. The first CP/M computers I worked on (1 MHz Z80, later 2 & 4 MHz) had two single sided, single density 8" floppies with 256 KB on each. Later came double sided, single density (512 KB) and in the end double sided, double density (1 MB). Imagine - TWO MEGABYTES total disk space! A HUGE step up from the Apple II, which was my very first computer, with its two 72 KB 5.25" drives for a total of 144 K! That limitation was what launched me on a career in computing, but that's a VERY long story that I'll spare you ;-)

Yes the MFM on an "RLL" drive did actually work that way, at least on certain SCSI controllers.

You are absolutely correct that hard drives using the old "ST506" interface could get an instant 50% capacity upgrade by replacing the MFM controller card with an RLL card and then performing a low-level format. Case in point, the venerable ST-225 (20 MB) drive and the ST-238R (30 MB) were essentially the same drive, but the 238R had an RLL controller, while the 225 had an MFM controller. Drive manufacturers warned you to not to use an unsupported controller like this. They said that the surface and timing circuitry of an MFM drive could not reliably handle RLL data, and if you did this, the result would be unreliable and you would void the warranty. Maybe this was technically true, but lots of people did it anyway and it almost always worked. Of course, once RLL drives started shipping, nobody went back to MFM. I think all drives today use some kind of RLL encoding. Also of interest is the GCR encoding used by the Apple II floppy drives. Apple actually used two different kinds of GCR over the years. The original one (DOS 3.2 and older) created disks with 13 sectors (of 256 bytes) per track. With 35 tracks and one head, that produced a capacity of 113.75 KB per disk. But when Apple released DOS 3.3 for the Apple II, they changed to a different GCR encoding that was able to pack more bits on a track. DOS 3.3 disks had 16 sectors (of 256 bytes) per track, for a total capacity of 140 KB per disk. Interestingly, had Apple chosen to format and use all 40 tracks that the drive could (I assume) have formatted, the DOS 3.3 encoding would have produced 160 KB per side - very close to the single-sided capacity of an IBM PC's original floppy drive (which was 9 sectors of 512 bytes per track, for 180 KB on single-sided media or 360 KB on double-sided media). But Apple never went there. I assume there was a technical reason why they limited their 5.25" floppies to only 35 tracks. Also interestingly, Apple stuck with GCR encoding for their original double-density 3.5" drives. But they use a variable-speed spindle motor in order to slow down the rotation and pack more bits (and therefore more sectors) on the outer tracks, where there was more linear space to write them. With an 80 track drive, this produced a single-sided capacity of 400 KB and a double-sided capacity of 800 KB (vs the PC-standard MFM 3.5" disks that were 360 KB single-sided and 720 KB double-sided, using a fixed-speed motor and the same number of sectors per track for all tracks). Apple eventually relented and used industry-standard MFM for their high-density 3.5" floppies, producing the same 1.44 MB that PCs have. And this is why it is nearly impossible for anyone without an old Mac to read an Apple 400K or 800K disk. The GCR encoding and variable number of sectors per tracks makes it impossible for a standard PC floppy drive (with a fixed count of sectors and MFM encoding) to read anything meaningful.

Great video, Dave; it was a good review of the early years of data storage. Next, you have to cover the different RAID levels on disk storage and what you get with each level.

I've been using some of these acronyms all my life and never knew how they worked. Thanks! And, oh yeah, you could plug an MFM PC drive into an RLL controller and get more space. But some drives would work and some would be damaged by this.

I was a computer operator in the 70's. I heard the letters NZRI. Never knew what it meant at the time. As I was listening to this video, about errors and tape stretching and all the things that could have gone wrong, i must say that aside from the odd data check, the working correctly percentage would have to have been 99.9%. I think the vacuum tubes on the tape drives were there to stop the stretching. Can't remember the tape hardware numbers. Then we had tapes that did not have the vacuum tubes, those desk like ones. I am always amazed at the sheer mental grunt that has gone into computing over the decades. As I was doing my job, there were genuises beavering away all over the world. Like Dave. He can sit there and just rattle off the evolution of magnetic tape storage. I feel so small when I think of the effort and brilliance of human beings in all manner of endeavours, that have made this world what it is today.

When we were involved in PCs in the 80s and early 90s, we encountered literally thousands of RLL controllers running MFM drivers without issue. It only phased out as IDE drives became the norm. Thanks for the lookback!

As someone who was growing up just as MFM/RLL drives were being phased out for IDE I found this quite interesting.... I did format drives MFM/RLL with their controllers but never really understood the technology. Thanks Dave! And yes you could MFM/RLL swaps with the right controllers and drives.

Extremely cool delivey of the computer science & application. I absolutely love your channel! My favorite out of all content creators. Thank you Dave!

Great video! It brought back memories of working with carrier transmission equipment in the early 90's that used HDB3 and CMI line codes to transmit signals up to 140mb/s. The codes were not only used to embed clocking info, they also had to invert in order to stop capacitance in the coaxial cables from building up and flattening the AC signal.

Yey! The outro is back! Its what charmed me into staying between the technical talk. I just found Usagi Electric a few weeks before this video.

I remember having two 20MB seagate drives on my BBS and I bought a brand new on the market RLL controller. I copied all the BBS software and scripts to floppies as well as the second hard disk, reformatted the first Hard drive to RLL and copied it all back - Viola ! 30MB. Re-Loaded (copied) all the software back to that drive after re-installing DOS and did the same thing to the second drive and now I had two 30MB drives. It so seemed like magic at the time. Some time after that, I got an ESDI drive, then a SCSI drive and then an IDE drive. It was an incredible journey.

you explained this so elegantly that i was surprised when you said you had struggled to understand GCR encoding, as I had understood it as the words left your mouth

Yes, you could format an MFM drive with RLL - but only if the drive was good for it. Early MFM drives could not do RLL as they were not precise enough for RLL, but as the technology grew more mature they could handle the more precise timings of RLL just fine. Modern day hard drives are all RLL encoded internally.

I really like these computer technology history explanations!

The controller created/decoded the analog signal of each track. The small cable carried these signals to and from the head selected over the large digital cable. The disk was a blank magnetic media until the low level format ( remember debug G=C800:5 ) laid down the framework of tracks and sectors. This is how the magic 50% could be created. I forget the details now but there must have been an increase in sectors per track. I never trusted RLL for longer then 6 months even on drives sold as RLL compatible. If I got the drive back before it really corrupted Spinrite would fix it for another 6 months by rewriting the entire track, LL format and data of all tracks. A wonderful explanation of the long forgotten. Huge thanks big Dave.

Mesmerising, I don't understand any of this but it's still mesmerising. Thanks Dave.

Ooohhhh, Thank you for answering a many decade old question from the back of my mind. I was in grade school at the time but I definitely remember a day where my dad brought the family 286 back from the shop and our previously fairly full 40Mb HDD was now reading as 60Mb and seemed much roomier and even a bit faster. My dad had told me that it wasn't a replacement drive but a replacement drive controller (I thought it was MFM to RLL but that might just be confirmation bias after seeing the video.) I always wondered how a controller change could change the capacity of a drive by such a large amount and now I know!

PRML (Partial Response/Maximal Likelihood) was a key advancement in the read-channel that significantly boosted bit density. Disk drives can get rather nasty at the inner cylinders where the bit-density is higher and bits interfere with eachother. Funny fact: When I graduated in 1985, at one of my interviews the sys admin was very proud to show-off their brand new 1GB disk drive. It was the size (and noise-level) of a residential dishwasher. Today, my dashcam has a 256GB microSD that's smaller than my fingernail.....what will we see 40 years from now ????

Like a lot of folks, I used to double the capacity of 5-1/4" floppies for my Commodore 64 using a notch cutter so that I could flip the disk over and write on the other side. Gee, that was a long time ago. I had no idea how the data was written. Thanks, Dave..

As well as regular improvements in data recording devices, there was a corresponding improvement in the recording media. Greater data densities required denser magnetic particles which needed stronger magnetic forces to record on. A similar situation occurred with audio cassettes. Just as a recorder designed to work with basic ferric oxide tapes were unable to successfully record on higher quality Chrome tapes, 40 track single density drives couldn’t write to media designed for use on 80 track double density drives even though they could usually read the higher quality media if the data on it had been written to it at the lower specification by a drive capable of writing at the higher densities.

Great video Dave! I remember taking my 20MB HDD and attaching it to an RLL controller and it formatted out to 30MB. I wasn't sure it would work, but to my surprise it worked great! Was cool to brag about it back in the day. 🤠

I loved your reference to disk drives as "spinning rust"! Hadn't heard that one before. 😀

You're right. In fact I always heard that the Seagate st-225 (20mb mfm) and st-235 (30mb rll) were the same drive. I loved those old systems I had a 60mb mfm drive formatted via rll to 90mb. One of the platters had a bad track 0. I was able to tell the low level format utility to ignore that platter. Lost some space there. My grandfather worked for IBM back in the day, often bragged about being the inventor of RLL. Wish I had some confirmation of it.

About the MFM vs RLL encoding scheme.. I do remember finally getting enough money together to get a 20 MB hard drive for my IBM PC clone. It was MFM encoded. A year later or so later a friend of mine said he got a 30 MB hard drive for his IBM clone. He said the drive mechanism was identical to mine except it used RLL encoding. So I do remember those discussions about RLL being able to store 50% more then MFM. I wouldn’t have had the confidence to try swapping just the controller on my drive to see if it worked. Especially typically I built my PC’s at the time by just buying all new technology anyway. My next hard drive was in a whole new computer, everything was much faster, stored more, faster ram ,etc.

I definitely remember getting an RLL controller to replace my MFM controller and low-level reformatting my hard disk to get something like 50% more space. This was in the early 1990s, when everyone was switching to IDE, so old MFM and RLL controllers and drives were easy to find second-hand, and cheap too. Especially Western Digital controllers. And here's why: There had been an open source hardware project in a German magazine that allowed you to connect an ISA MFM or RLL controller and hard disks to an Amiga, and they said it didn't work with Western Digital controllers and didn't know why. So it was almost impossible to find other controllers because everyone was buying those, but Western Digital controllers were a dime a dozen. I actually built one of those projects with a Western Digital controller, and reverse-engineered the software (with a little help of the hard disk BIOS listings in the IBM PC/XT Technical Reference manual). Turns out the only reason why it didn't work with Western Digital controllers was that the software didn't hold the Reset line long enough. That was easy to fix, and I used a 70MB hard disk on my Amiga as a 105MB hard disk for years. And that was a lot of space back then!

I remember formatting a 20MB MFM hdd with RLL , and getting 35MB from it. There were more chances of reports of bad sectors, but even then the 5.25” hard disk I was using was somewhat antiquated!

Your’re correct, I used MFM drives on RLL controllers all the time to double my hard drive space. Worked great on most MFM drives.

I worked for Hard Drives International back in the MFM/RLL/ESDI/SCSI days (and early IDE/xt-ide). Yes, you could take a Seagate st225 MFM drive, put it on an RLL controller and if you were lucky you'd get the same capacity of a ST-238r. Basically, a 50% increase in storage. If you weren't lucky, it would work but you would end up with data corruption.

I repaired 8" to 5 1/4 hard drives 1980's. It took a mfm disk controller to rll controller to get 50% more! Two megabyte became 3 megabyte! Most the time on a drive, it was the lubrication that was wrong!

My first hard drive was an MFM 20MB ST-225, and the controller that I got with it was a single port controller. When I got my second drive, it was a 30MB ST-238R, and came with a dual port controller card. At some point, I ended up reformatting the ST-225 to RLL on the dual port card, giving me 60MB of storage in one XT machine. Those drives were with me until I went to upgrade Windows 3.0 to 3.1, and 3.1 no longer supported the 8 bit controller. At that point, I picked up a 50MB Quantum Fireball IDE, and the old drives got used to make a gaming computer for the wife and kids ( think 286 CPU and monochrome graphics, great for hangman and board games. )

GO:C800;1 do you remember the embedded HDD formatting on the ISA controller

I remember some of that. It's been a while! Thanks for the memories!

Your site is very informative and interesting to follow😊 Some 60 years ago I had the opportunity to play on a recorder using a steel/iron wire on reels😬 I think the decoding on that play machine missed decoding other that a mechanical counter. What a development over the years. I serviced some test/stress chambers at Tandberg in Norway way back when they was a pretty huge producer of tape storage systems on some big tape cassettes. The development of other storage devices made them to go bankrupt. Still the company I worked at until recently use small tape cassettes for daily back-up. In early days they used a huge magnetic disk storage for salespeople. The disc was may be twice the size of an LP music player. Belt driven with a high speed where the flat belt flew off every other day. Took some time and effort to get that belt back.

These are very similar problems to those encountered in the telecoms industry when trying to get streams of 1s and 0s to travel along non fibre-optic, copper cables. Transmission lines rounded the square wave transmissions making transitions between low and high states difficult and unreliable to detect. Another issue was the need to have the net voltage on the line around the midpoint between high and low over a period of time regardless of the number of consecutive 1s or 0s transmitted in a sequence. Also, as the telecoms networks became digital, synchronisation and timing information needed to be recovered reliably from the messy signal arriving to ensure network stability and propagation down the synchronisation hierarchy was well managed. All good fun stuff made so much easier by fibre optic cables.

Yes, you could use a MFM drive with a RLL controller with the caveat that the drive might not be of high enough quality to support it well. It's a matter of manufacturing tolerances and qualities of the magnetic media. I'm sure some drive manufacturers sold their RLL drives that didn't pass quality checks as MFM drives. Quite often the the difference between an MFM and RLL drive was just the label having a different suffix letter on the model number.

Great video Dave ! MFM is the one I heard of .

I don’t remember how i did it, but back in the day on the Amiga i think i used DMA to read the MFM data in the middle of the disk. Decoded the MFM with the blitter (also DMA) and analyzed the BAM to proceed to read the meaningfull data of the disk. This could read and decoded the data without using cpu power. The goal was to use the cpu to send the data over the parallel cable so we could copy disks from Amiga to Amiga at incredible speed at copy parties 😇 I tested, but never finished the “networking” part, a bit of a shame because the idea was great.

Maintaining sync is often cited as being the reason why encoding schemes are required for storing long groups of zeros. This is only a minor reason, the major reason that is rarely ever mentioned is due to magnetic fields only being able to induce a current if they are changing. To read a changing magnetic field the signal is differentiated using an op-amp. This is a mathematical, calculus differentiation where the output voltage is directly proportional to the input voltage’s rate-of-change with respect to time. A second op-amp configured as a comparator can then be used to detect the zero crossings, with its output sent to a one shot multivibrator used in the next stage to create the digital pulses. The flux transitions in the signal are like a continuous, 3D gaussian curve where it can go up and down. Ideally, the head attempting to read the signal needs to be precisely positioned over the peeks and troughs of the gaussian curve. Due to tolerances between different drives, or even hysteresis in the same drive, this can be hard to do and the head can end up on the shoulder of the gaussian curve. To help with this the signal is first amplified by an auto-gain amplifier. A flux transition is used to represent a 1 and the absence of a transition is used to represent a 0. But now there is a problem if you need to store many zeros between the flux transitions. For example, when moving from one transition going up and the other going down the distance between the peeks dictates how many zeros are stored between the two ones. But the more zeros inserted, the more the slope between the gaussian peeks becomes shallower and shallower. The more shallow it becomes the higher the auto gain amplifier ramps up. If there are too many zeros then the slope can become so shallow it almost becomes a constant straight line. Induction stops in the presence of a constant field so this is bad. Mathematically, differentiating a constant is zero. The derivative of a shallow sloping, almost constant signal can be close to zero. So with the first amp with its auto gain maxed and the second differentiating close to zero, any noise can easily cause a random incorrect zero crossing to be sent through to the one shot and be incorrectly detected as a one. Some floppy disk copy protection schemes deliberately did this on purpose. When reading a section of the disk with a slowly changing magnetic field or a section of the disk that was manufactured with no magnetic material multiple times, the code expects the data to be random. If the data is always the same each time this section of the disk is read then the code knows it is a copy.

Adrian's Digital Basement channel had an episode of recovering data on some old MFM and RLL drives. He also went through explaining the encodings used. It was very interesting. My first HDD was an MFM 40MB drive. I think I paid $500 for it at the time. Had to low-level format it before you could then partition it then high-level formatting the drive so the OS can store any files on it. Ah, those were the days.

Yes. I do remember taking an MFM drive, putting it onto an RLL controller and having it work. Granted, the MFM drive required low-level formatting to work on the RLL controller.

I remember the MFM controller on the Amiga 500, which slapped into the left side of it and stuck up about 6 inches. I had a 300mb HD hooked to it. That was my first hard drive experience. Noone these days has to deal with low level formatting a hard drive.

Hi Dave, Dave here. I'm glad to see again that cute old sign-off segment with the tiny furniture and welcoming narration. I've been lucky enough to live through all of the technologies you've talked about being used. I *think* I've seen a drum drive, has the chance to make the heads of an IBM 1620 disc drive move using assembly language. I still have a CPM machine with a Tarbell Tape Drive board in it. Now I took an the proud owner of boxes of my old hard drives from XTs to the current computers. Most haven't been spun since their host computers were recycled. And there is the rub, what am I going to use on my 40 plus year old Seagate? I'm sure the technology is out there, but I'm not that motivated to relearn the old ways of forgotten hardware and software. Sigh.

Very interesting. Some of the signal processing methods were invented by my FIL in a backyard hammock in Raleigh NC (for IBM). I still find it interesting to compare some of the patents of his methods to the implementations used for these devices.

On the subject of using the same drives with both MFM and RLL controllers, I had the opposite experience. I had a 30mb RLL drive/controller combination that was proving very un-reliable. I was constantly having issues and re-formatting the drive. Eventually I conceded defeat and connected the drive to an MFM controller, re-formatted and ended up with a very reliable 20mb drive. Lost 50% capacity but at the time it was worth it for rock solid reliability. I guess the head alignment was slightly off and struggled with RLL but was perfectly happy with MFM. Good times!

Great video Dave and your videos are always educational.

When you mentioned frequent modulation I thought you were going to go into how some early consumer computers stored data on audio cassettes. The second computer I built (from a kit in Australia) used this method. I replaced the original 8080 CPU with a Z80, doubled the clock speed, and then used a few spare gates on the board to software select the original clock speed when reading or writing tape as the system ROM routines relied on the system clock for generating and reading the audio signals.

yes, Dave. I had an Amiga 2000, w/ 8088 bridgeboard. & video toaster. I had a RLL controller card instead of the usual SCSI commadore drive due to costs. we just hooked it to a MFM drive & it worked. I had a 32 meg HDD with it partitioned 50/50 for Amiga & IBM. Had to boot the amiga ( workbench)from 3.5" floppy drive, wait 60 for the IBM side to initialize, then run Janus software so that i could use both sides of the pc. my biggest limitation was having 1 meg of memory. i swear that unit could run circles around almost any pc up to including win 95 machines in most respects. I do miss those days.

Yes, you could take an RLL controller onto a MFM drive and get more space. My buddy and I did that as well, AND.... if you you really got crazy, you could change the clock speed on the RLL controller, and you could cram even more on the same disk. There was a limit though because too fast and as you'd expect data errors really started to creep in. What a great time to be playing and learning 'cutting edge' computers and their hardware. I do miss setting interrupts with jumper pins though!

Yep it was my first job working for a importer distributor in new zealand selling hard drives, controllers to pc dealers over the whole country. we had Miniscribe 8425 5 1/4 " 20mb drives that used MFM ISA controller where as a 8435 5 /14" 30mb drive used a 27RLL ISA controler which was the same as the 20mb drive, but was rated as being more reliable to handle the extra 50% increase in data density onto the platters. we also had and also 3425's 3.5" 20mb drives later on. And yes there was the case where miniscribe shipped bricks on ships... they were failing at producing enough products that were capable of keeping up with performace improved western digital drives....

On PE, part of why it was popular was you could do faster encoding without worrying about syncing up to your clock signal which I believe often meant you could do faster encoding those putting more data per inch of tape.

Reed Solomon is everywhere. They are used in QR codes and take up around a 3rd of the bits in any given QR, which is why they can scan with damage or a logo in the center.

That brings memory when I worked on mainframe from late 7i's to early 90's

PE is used for the data tracks on credit cards. You can record it as simply as pulsing in two polarities. Reading it back it shows up as pulses either up or down on a scope and is EZ to decode using a comparater + AND gate.

Hi Dave! This was a good one.

My first PC (386SX) in 1991 had a full height 5.25" 40MB hard drive. I remember looking through the manual for the hard drive controller and there was a mention of the default encoding being MFM but after changing a jumper it could be RLL. I didn't really know what this was... but I changed it, did a low level format and my 40MB drive turned into a 63MB drive! It was magical 🙂

I remember when I low-level formatted my hard drive and changed the interleaving factor from 3 to 2 and greatly increased performance. At that time, the disk was spinning 3 or 2 revolutions per track, so that the electronics could catch up. Now we are in deep water!

Such good stuff, Dave! Thank you!

In the 80z my 48k 8 bit sinclair spectrum had a tape drive but was using sound to store the data on a tape recorder. It sounded like a modem when it was loading or saving a program and was fun mostly.

What's funny is that back in the old days I thought I was taught that there were different kinds of hard disk drives: RLL, SCSI, IDE, etc. But then I couldn't figure out why "run-length limited" would be compared as a connectivity type such as integrated device electronics, even though it didn't sound like one. So it turns out that it's not one and some people just mixed up connection types with encoding types. Because obviously now in storage connection types we have M.2, SATA, USB, IEEE1394, PATA/"IDE", SCSI, whatever type name those MFM hard disk drives used, 34-pin floppy, and then whatever came before those, including DIN serial, IEEE 488, and buscard, etc. What are the official names for 34-pin floppy and the dual-ribbon type for the MFM-era hard disk drives?

I go waaaaay back to using tape drives on Commodore and Atari computers. Reliability was a regular issue back then, that is until the better encoding algorithyms were implemented. The early floppy drives had fewer but occasional errors. Glad to see the Friendly Giant Programmer back on a regular basis. 😆

I definately remember doping the the MFM/RLL Conversion, which made my 20-MB to a 30MB hard drive on the early PC's. I DO remember being warned that this could loose data, as the flux density of a 20MB drive was really pushing it to get 30MB. AKA, more flux changes etc... I did run it and never had a problem though.

First Tape Drive on a computer that I used was in 2nd grade in 1984. It was a new Apple 2e. I played a math game on it. I remember it took a long time to load the game, probably 4 or 5 mins. I got mad because I was enjoying the game, but we had to share the computer and the load time seemed longer than my play time. I could be wrong, but that was my 2nd grade memory of it, and I'm sticking with it! 🤪

“Closet full of spinning rust”…. Made me lol.

That was absolutely fascinating. I want to explore and flash and how it stores data in multi layer nand.

SMR drives, or SLC vs TLC vs QLC flash, bring these sorts of tradeoffs right up to date.

I had a 20MB Seagate MFM drive that became a 30MB RLL drive by swapping the controller in the late 80s. Doing this controller change required a low level format. Adding another 10MB was a big deal back then.

I closely followed the research on the GMR effect in Switzerland. The GMR effect was discovered by Albert Fert and Peter Grünberg, for which they were awarded the Nobel Prize later in 2007. It had a significant impact on hard drive technology, as the read heads utilized this effect. The first hard drives featuring this technology were released by IBM with the Deskstar 16GP in 1997 and were not even that expensive. I bought two 14GP models from IBM at the time.

Thanks a lot for this one - I recognized some of these, but not all - fun. One question you didn't touch is how modern drives store data. I can only guess (or hope) that something has changed over those 40 years since first RLL drives :)

this really dusted off my memory. in the late 80s and early 90s - I was installing hdd upgrades for customers, and lived thru the MFM to RLL inflection in the market. I do remember the RLL drives though to be slower,. Back then, I had no RU-vid or the internet to know the science behind MFM and RLL, as Seagate and others at the time didn't care to communicate the tech to us poor field installers.

My first hard drive was a full height 10MB. Added an Adaptec RLL controller to get 15 MB. "More storage than you will ever need!" they used to say! If I remember it correctly, there were only a few drives that could accept RLL. Mine was a Seagete.

It's amazing how so many people don't know what an acronym is. I suspect they're being taught at school/college by other people who also don't know. An acronym is a word. One that's made up of the initial letters of some other words. Like RAM, ROM, LASER, RADAR, GIF, etc. Things that are read letter by letter, like NRZI, PE, GCR, MFM, RLL (none of which are pronounced as words) are initialisms.

Yes, can remember swapping out MFM controllers and installing RLL controllers. Worked a treat. You could improve a 10MB drive to about 15MB. A big deal in the day…