Business card CDs are so 15 years ago. USB business cards are the new ‘old’ tech now
CD’s were first released about 1983, when Bill Gates was pondering why anyone would need more than 640K of RAM. A device that held 600MB of data was yuge back then (and even yuger at pushing the envelope when under development for the previous few years). It pushed the limits of technology, particularly for content that could be mass-produced.
And IIRC the math, it had 44,000 samples/sec stereo, 12 bits(?) per sample, so it needed to read with one laser hundreds of thousands of bits per second - one bit at a time. Any device reading that fast meant the track had to travel under the laser pretty fast. Unlike a vinyl record, IIRC it speeds up as it reads closer to the center to ensure a constant bits per inch rate.
There’s no reason an irregularly-shaped CD has to vibrate or cause bearing wear. It just needs to be balanced to the center of mass. Every solid object has a distinct center of mass, and for a thin sheet like this it’s not hard to find.
One problem, probably fairly minor, is that at very high speeds the disk will stretch slightly, and for an irregularly-shaped disk it will stretch asymmetrically. That shouldn’t really cause any reading errors, since all the data is contained within a circular region near the center, but it could cause some small vibrations. I suspect it’s less than other natural variations, though, like with the thickness of the ink printed on the top. The drives can handle some vibration.
Nitpick: Starts at the center fast, then slows down as it moves to the outside. For Constant Linear Velocity.
(As opposed to Constant Angular Velocity)
This reminded me of my copy of “Dunninger’s Illustrated Encyclopedia of Magic” that I had as a child. It was full of all kinds of illustrations of 1920’s-era illusions and mechanisms.
Somewhere between the various sword-through-box devices and coin conjuring gadgets there was an explanation of how to spin a random chunk of cardboard on a pencil. The illustration showed poking two holes through it near the edges, then using a pencil as a pivot and a weighted string as a plumb-bob to strike a line through the cardboard as it hangs first from one hole, then from the second hole: where they cross is the center of balance.
Not a stunning revelation, but still pretty cool for a kid to read.
Sounds like a pretty good way to get some nasty malware installed on your machine!
I think it is pretty surprising, personally. It’s not all that obvious that it works in all situations. In fact, just now I had to think about it for a minute to convince myself that it doesn’t matter where you hang it from (though the closer to the edge you hang it from, the better the precision will be).
If you know how torque works, it’s not too difficult a problem, but even torque is not an immediately obvious thing. You can get to a result by various arguments by contradiction, like that it would spin forever if the plumb line didn’t go through the center of mass, but those are pretty sophisticated arguments for an adult, let alone a child.
I loved that book!
Doh! Thanks. I knew that… Of course, or mini-CD’s (which I have several) wouldn’t be a thing.
One complication is that, for an irregularly-shaped CD, the usable area is the largest circle centered on the hole and within the shape. So you might want to put the hole where it maximizes that area, rather than at the center of mass. For sufficiently-symmetric shapes, they’ll be the same, but not for all shapes.
Nitpick:
Basically, the CD is covered with microscopic bumps and valleys. Each bump stands for a 1, and each valley stands for a 0. Each 1 or 0, or bit, is read by the laser, based upon how the light reflects off of the bump or valley.
I have a bunch of Babylon 5 soundtrack CDs with the silvered area forming various starry shapes (the discs themselves are circular as usual, with clear plastic in the areas outside the design). That works because the discs only have about 30-40 minutes of music, so the outer points beyond the inner diameter where the data is are just decoration.
Not as much as you might think. In addition to the NRZI encoding you mentioned, CDs use what’s called “eight to fourteen modulation” (ETF), which guarantees that any two ones have between 2 and 10 zeroes between them. Among other benefits, this means that the timing system only has to be good enough to distinguish between those nine cases; i.e., with about 10% accuracy. The reader will also have a retiming system that recovers an accurate clock from the disk; as long as there are enough bit transitions (and with ETF, there are), this is a trivial problem.
If you had square CDs, why not add some curves along the sides for extra burning space?
There used to be some protocol or program that let you do that when recording a CD-R. I don’t remember what it was called now (and can’t google it up). I only used the a couple of times, as I usually filled the discs with data.
Lightscribe:
LightScribe uses specially made blank CDs that let you etch a customised design into a layer of a photosensitive dye. That layer is only on the label side of the CD; the data side is normal. @Darren_Garrison, on the other hand, seems to mean a CD that is made of clear plastic in certain parts.
I had a LightScribe burner in the 2000s. Very nice for those cases when you compile and create a custom-made CD as a gift for someone; the graphics design in lieu of a label added a beautiful touch to that. I always had the suspicion, though, that the designs would fade after a couple of years. I’ll have to see if I still have LightScribe CDs from that time to see how well they’ve aged.
No, Lightscribe is what I was thinking of. I remembered it doing images on the data side, though. Maybe the reason I only used it a couple of times is that I didn’t want to buy special, more expensive blanks?
There were other technologies that could do that, called DiscT@2 and its successor, Labelflash. The former used the unused parts of the data side to engrave images; the latter used a layer of dye in between the label and data sides. Seems that, like LightScribe, they’ve all been abandoned by their original manufacturers.
Yeah, that’s it.
I know this is a zombie thread from 2005, but I can’t help wondering if the original question was inspired by the fact that computer floppy discs, of the sort that used to be ubiquitous but are now long obsolete, are square.
Of course, it isn’t really the floppy discs themselves that are square. Those square plastic cases hold discs that are round and do spin. But I suspect that there were people who didn’t know that.