Why Philips Put the CD's Data on the Label Side
The Objects We Held
341 views
120 mm across, 1.2 mm thick, about 16 grams. The side everyone protects is
the clear one. The data is not on that side.
The reflective aluminium layer sits directly under the printed label,
separated from the outside world by a lacquer coat a few tens of microns
thick. Under the clear side there is 1.2 mm of solid polycarbonate. The side
that looks like it matters is armoured. The side with the artwork on it is
the fragile one, and a ballpoint pen pressed hard enough to mark a label can
reach the layer the music is stored in.
That arrangement is deliberate, and the 1.2 mm underneath is not there for
strength. It is an optical device. The laser enters the disc as a cone about
0.7 mm wide and converges to a spot roughly 1.7 microns across at the data
layer, which means the surface of the disc is never in focus. A scratch or a
fingerprint blocks part of a cone nearly a millimetre wide and the light that
gets past still reaches the data. Move that same scratch to the data layer
and it sits exactly where the beam is smallest.
This also covers:
Why radial scratches are survivable and circular ones are not
Why a DVD is measurably less scratch-tolerant than a CD, and why Blu-ray
needed a hard coating before it could be sold without a caddy
Why the disc slows from about 500 rpm at the centre to 200 at the edge
How the audio is scattered along the spiral so a continuous scratch arrives
as thousands of tiny recoverable errors instead of one unrecoverable block
Why slightly more than half of what is physically written to a CD is
neither music nor error correction
Why the sampling rate is 44.1 kHz, a number inherited from video recorders
Why 115 mm became 120 mm during the Philips and Sony negotiation
Why a recordable CD fails in a completely different way from a pressed one
Why a disc that plays perfectly today may already be failing
Accounts of the 74-minute playing time story differ between tellings. What is
not disputed is that the disc grew from 115 mm to 120 mm during the
negotiation and that playing time was the reason.
#CompactDisc #VintageTechnology #EngineeringHistory
Why Philips Put the CD's Data on the Label Side
The Objects We Held
341 views
120 mm across, 1.2 mm thick, about 16 grams. The side everyone protects is
the clear one. The data is not on that side.
The reflective aluminium layer sits directly under the printed label,
separated from the outside world by a lacquer coat a few tens of microns
thick. Under the clear side there is 1.2 mm of solid polycarbonate. The side
that looks like it matters is armoured. The side with the artwork on it is
the fragile one, and a ballpoint pen pressed hard enough to mark a label can
reach the layer the music is stored in.
That arrangement is deliberate, and the 1.2 mm underneath is not there for
strength. It is an optical device. The laser enters the disc as a cone about
0.7 mm wide and converges to a spot roughly 1.7 microns across at the data
layer, which means the surface of the disc is never in focus. A scratch or a
fingerprint blocks part of a cone nearly a millimetre wide and the light that
gets past still reaches the data. Move that same scratch to the data layer
and it sits exactly where the beam is smallest.
This also covers:
Why radial scratches are survivable and circular ones are not
Why a DVD is measurably less scratch-tolerant than a CD, and why Blu-ray
needed a hard coating before it could be sold without a caddy
Why the disc slows from about 500 rpm at the centre to 200 at the edge
How the audio is scattered along the spiral so a continuous scratch arrives
as thousands of tiny recoverable errors instead of one unrecoverable block
Why slightly more than half of what is physically written to a CD is
neither music nor error correction
Why the sampling rate is 44.1 kHz, a number inherited from video recorders
Why 115 mm became 120 mm during the Philips and Sony negotiation
Why a recordable CD fails in a completely different way from a pressed one
Why a disc that plays perfectly today may already be failing
Accounts of the 74-minute playing time story differ between tellings. What is
not disputed is that the disc grew from 115 mm to 120 mm during the
negotiation and that playing time was the reason.
#CompactDisc #VintageTechnology #EngineeringHistory