Ever wondered why a CD, and thus all the CD-ROM and DVD derivatives, has a 12cm diameter? OK, you probably haven’t, but I have.

I had assumed that this fundamental dimension came about through an engineering compromise based on data densities, linear velocity at the laser reading head, and other immutable physical constraints.

It turns out the truth is a little more prosaic.

Dutch engineer Kees Immink, one of the engineers working for Philips in the team that collaborated with Sony to develop the Compact Disc format, states that Philips’ top brass wanted the new format to be comparable in size to the compact cassette - presumably a marketing requirement. The CD team took this edict to the max by borrowing the very biggest cassette dimension - the 11.5cm diagonal across its face. As Immink explains, with the data densities then feasible for mass-produced players, this gave a suitably round playing time of one hour.

Sony, no doubt with portable players in mind, initially wanted a smaller 10cm disc. So how did we end up with a compromise that was bigger than either starting point?

The 12cm CD has a playing time of 74 minutes, which according to Philips’ official history, was determined by a particular piece of music.

“Sony vice-president Norio Ohga, who was responsible for the project, did not agree [with the 11.5cm specification]. ‘Let us take the music as the basis,’ he said. He hadn’t studied at the Conservatory in Berlin for nothing. Ohga had fond memories of Beethoven’s Ninth Symphony [and] that had to fit on the CD ... a check was made with Philips’ subsidiary, PolyGram [which said] the longest known performance lasted 74 minutes ... This therefore became the playing time of a CD. A diameter of 12 centimetres was required for this playing time.”

Only that’s a little too neat.

Back to Immink again, talking to Irish newspaper the Sunday Tribune in October 2005:

“Philips owned Polygram, one of the world's largest distributors of music. Polygram had set up a large experimental CD disc plant in Hanover, Germany, which could produce huge amounts of CDs having, of course, a diameter of 11.5cm. Sony did not yet have such a facility.
If Sony had agreed on the 11.5cm disc, Philips would have had a significant competitive edge in the market. Sony was aware of that, did not like it, and something had to be done.
The long-playing time of Beethoven's Ninth imposed by Ohga was used to push Philips to accept 12cm, so that Philips' Polygram would lose its edge on disc fabrication.
It was all about the money and competition in the market, and not about Ohga's great passion for music.”

So there you have it. The 12cm size was a compromise based on arbitrary marketing concerns, honed through a process of infighting and back-stabbing among consortium members.

If I’d really thought about it, I guess I should have known that from the very start.

Right, it’s lunchtime and I’m off to buy some draft 802.11n gear...