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A neat tidbit I found on the Last Word column in the NewScientist e-zine. In response to the question "Someone claimed they could identify a piece of music just by looking at the record. Is this true?". Peter Copland, who is the Technical Manager at the British Library National Sound Archive in London posted an excellent response:
It is certainly possible to identify a piece of music on an LP record just by looking at it and you can hone your skills by using scientific principles.

The first clue is the length of the track (or tracks). This can be estimated within a tolerance of 10% or so by a trained eye, which can also take into account different groove pitches. Errors are comparable to different performance times for the same piece of music, so they aren't significant.

The next clue comes from the dynamics of the music. This can be read more easily by shining a parallel beam of light onto the surface of the record. The light is diffused by an amount proportional to the root mean square velocity of the stylus which cut the sound waves in the groove. Thus the dynamic shape of music is easy to see.

The third clue is the spectral content of the music. By looking straight down onto the disc, the horizontal surface between the grooves (known as "land") is reflected back to the eye. Where low-pitched notes occur, the cutting stylus leaves comparatively long islands of land as it vibrates to and fro. By putting together these last two pieces of evidence, one can easily distinguish between, say, a solo piccolo and a solo double bass. So one can gain an idea of the instruments used and how it changes with time. In the days of 78 revolutions- per-minute records and early LPs when the range of recorded repertoire was much narrower, these three pieces of evidence could be sufficient to identify the music unambiguously.

More information can be found in a paper by G. Buchmann and E. Meyer: "Eine neue optische Messmethode für Grammophonplatten" (Electrische Nachrichten-Technik), 1930, vol 7, p 47. This is the original citation, but the mathematics of the principle were not described until E. Meyer did so in his book Electro-Acoustics (G. Bell and Sons Limited, 1939, p 7).

... and a side comment Peter later makes about reading CDs:

I can't say I've mastered the art, but it's easy to see the length of a recording (which starts at the centre), and this is a vital part of my work--because we copy analog sounds to CD discs, we often need to distinguish between blank and recorded CDs when we label them. I can often see when a CD has been assembled track-by-track instead of being "burnt" continuously, so I can find the track changes and check them. However, manufactured CDs are mastered continuously, so this technique will not help.

Every DJ knows that you can see the loud and quiet bits of a record, especially on 12" singles where the grooves are nicely spread out. The loud bits look white when viewed under a lamp at the right angle, and the quiet bits look dark. This means when mixing, you can pick up visual clues as to when the breakdown is coming, letting you know how long you've got to complete a mix even if you don't know the record too well.

You can even see the individual beats on house records with a strong kick drum, especially if those on adjaecent grooves line up with each other. This gives a nice spiral pattern towards the middle of the record.

In response to the comment above that it is impossible to identify a manufactured CD by looking at the data side:

Not strictly true. On CDs with "Secret Tracks", it is in fact possible to identify the CD by looking at the pits and grooves.

Try this: Take your copy of "Nevermind", by Nirvana. (You do have one, don't you? :-) Unless its one of the earliest pressings, it should contain the secret track "Endless, Nameless" after Something in the Way. Look closely at the back of the CD, and about 5mm in from the outer edge, the surface becomes slightly darker, returning to the normal shade after another 4mm or so. This dark patch is the ten minutes or so of silence between E,N and SitW.

If you know the exact times of secret tracks, and what those times correspond to in terms of radius on a disc, it is possible to make an educated guess as to what the CD is by looking for the "dark ring(s)".

I can only speculate as to the reason for silence being darker than sound, but my guess would be something along the lines of there being less variety of pits and grooves, thereby causing less interference/diffraction of light reflected off the CD.

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