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Comment by sjwright | Hacker News Reader

> The most significant shift is that vinyl records read from outside in

Equally significant (and equally interesting) is how they spin. Whereas vinyl has constant angular velocity—they spin at the same rate from beginning to end, resulting in lower information density as you get towards the inner grooves—audio CDs have constant linear velocity, meaning that the rotational speed gradually changes as the disc is played so that the information density is equal from beginning to end.

For the era, this is very cool tech. (I'm also awestruck about how they managed to get the laser head alignment to work perfectly back then, it's practically magical.)

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Techmoan did a video about a record that is recorded "backwards" to take advantage of the increasing fidelity in the outer grooves.

https://www.youtube.com/watch?v=5Afikv6k1-c

Techmoan's recent video on extended play (2.5 hour) CDs is also quite interesting.

https://youtu.be/5fG1crhGqI0

You're right for audio CDs but very fast CD-ROM readers use constant angular velocity (CAV), mainly because they start hitting mechanical constraints (including the integrity of the CD itself).

Can't remember but, wasn't 52x spin the max (I heard urban legends about the disc spinning apart back in the late 90's)

Depending on how you define 52x, CAV, and the mechanical limits of a CD.

Some drives max at an advertised 48x, because early CDs still exploded at 52.

> resulting in lower information density as you get towards the inner grooves

It's actually the opposite: highest information density (musical notes played per surface unit) is in the innermost grooves.

Since linear velocity is lower in the center than the border, the size dedicated to each piece of data is larger in the outermost places, which may increase the quality of songs placed there.

I don't really know if the machine making the disc is precise enough and if the human ear is capable of hearing the difference.

Hmm, I would arrive at the opposite definition of "information density" than you, and I'm guessing the GP does, too.

Your definition is "musical notes played per surface unit", but I think that's inverted. Taking a musical note here to mean a fixed note length, more surface per note allows for additional expressiveness. You can even "fit more notes in" if the notes are a shorter duration.

A time span on the outer part of the record has more groove length dedicated to the same amount of playing time, so it has a higher information density in my book. I guess my definition for density would be groove length divided by playing time.

With your definition the information density is constant as in the outer part of the disk the wavelength need to be longer to compensate the higher velocity

I see what you mean. It's "temporal information density" then. Also called bandwidth :D

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