higher frequencies. Upon playback, the
level of the treble is reduced; the
higher the frequency the lower the
output. This reduces the problem of
noise caused by surface dirt on the
disc, however it would also reduce the
high frequency content of the audio
signal itself. This is counteracted by
increasing the level of the
high-frequency portion of the audio
signal when it is mastered for the disc.
This general idea of lowering the level
of low-frequencies and/or boosting
highs when recording and doing the
opposite upon playback is a very old
idea in the audio industry and has been
used on many formats ranging from
film “talkies” to early compact discs.
Unfortunately, however, di
ff
erent
recording companies and studios used
di
ff
erent filters on phonographs for
many years.
Finally, in the mid-1950s,
the Recording Industry Association of
America (the RIAA) suggested a
standard filter description with the
intention that it would be used
world-wide for all PVC “vinyl” records.
Figures
and
show the responses
of the RIAA filters used in both the
mastering and the playback of long
playing vinyl records. Although there
are other standards with slightly
di
ff
erent responses, the RIAA filter is
by far the most commonly-used.
Figure 2.4: The “pre-emphasis” filter to
be used in the mastering to disc, as
described by the RIAA standard. The
black line shows the simplified descrip-
tion, and the red curve shows the real-
world implementation.
Figure 2.5: The “de-emphasis” filter to
be used for playback as described by
the RIAA standard. This standard filter
response is integral in what is now com-
monly called a “RIAA preamp”.
It may be of interest to note that
typical descriptions of the RIAA
equalisation filter define the transition
points as time constants instead of
frequencies. So, instead of 50 Hz, 500
Hz, and 2122 Hz (as shown in the
response plots), the points are listed as
3180
µ
s, 318
µ
s, and 75
µ
s instead. If
you wish to convert a time constant
(Tc) to the equivalent frequency (F),
you can use the equation below.
F = 1 / (2
π
Tc)
2.3 Mono to Stereo
In Edison’s first cylinder recordings, the
needle vibrated up and down instead
of left and right to record the audio
signal. This meant that the groove cut
into the surface of the tin foil was
varying in depth, and therefore in
width, as shown in Figure
Figure 2.6: Example of an audio sig-
nal encoded using a vertical cutting sys-
tem.
There are some disadvantages to this
system, such as the risk of the needle
slipping out of the groove when it is
too shallow, or su
ff
ering from
excessive wear if the groove is too
deep. In addition, any vertical variation
in the recording surface (such as a
cylinder that is not quite round, or
mechanical vibrations in the player
caused by footsteps in the room)
becomes translated into unwanted
noises upon playback.
Figure 2.7: An Edison cylinder player, on
display in the Struer Museum.
Figure 2.8: A closeup of the Edison
player.
Notice that the needle is
mounted to move vertically, modulating
a membrane located at the end of the
tonearm (the bent pipe).
Berliner’s Gramophone used a di
ff
erent
system, where the needle vibrated
sideways instead. This lateral cut
system produced a groove on the disc
with a constant depth, thus avoiding
some of the problems incurred by the
vertical cut recording system.
Figure 2.9: Example of an audio signal
encoded using a lateral cutting system.
2
see the Manual of Analogue Sound Restoration Techniques (2008), by Peter Copeland
3
Some 78 RPM discs use a vertical cutting system as well, including those made by Edison Disc Records and Pathé.
5
