Grooves in close up
|These images were made with an Intel Play microscope at
200x magnification and the Mac OSX microscope software. The images were
quite saturated, bringing out the minute colour difference between old
an modern vinyl. Funny enough the old vinyl is browner, where as the new
vinyl is more blueish.
Note the square 'valleys' of an old mono EP and that the sides of the
grooves of a modern 45 rpm single are in a 90 degree angle, which allows
uncorrelated (stereo) soundswaves to be cut. In practice, this difference
hardly exceeds 18 dB, or a 1:8 factor. If the curves are too different,
the sides can get so close that the groove can actually dissapear. This
is why LP masterers check for 180 antiphase sounds in recordings. (especially
in low frequencies, as those waves tend to have higher amplitudes.)
The small details in the curves represent the high frequencies, the side to side whobble or movement of the curve ar bass frequencies. The white spots are dust and you can also see some fine scratches, probably caused by the sleeve. (You can't see the 90 degree angle to well here, as I lighted the single almost horizontally.)
Bass is recorded on a low volume on vinyl, high frequencies are boosted. This is then corrected in the phono preamp in the amplifier, according to the RIAA standard filtercurve. If vinyl records were recorded with a flat frequency response, the 'revolutions' are very likely to cross eachother.
I also made a little quicktime movie of a very modern (2002) pressing.
You might have to look twice to understand you're looking at a v shaped
groove. At certain places you can see some high frequencies that are in
anti-phase. [quicktime movie 7MB]