Replay processing is therefore required to present
a complimentary characteristic to restore the
original audio balance. The electronic amplifier
which corrects for the equalisation applied during
recording is today universally called an RIAA
preamplifier. The design of good, accurate RIAA
equalisation has taxed the ingenuity of audio
hardware engineers for sixty years.
Pspatial Audio believe that, after 60 years,
hardware RIAA equalisation has just about reached
the end of the developmental road and that
equalisation is nowadays better performed in
software for the following reasons:
REASON 1: Greatly increased digital
resolution
Stereo Lab has internal processing in software
which uses double-precision floating-point math.
CD audio represents the audio signal with a
dynamic range of 96dB, smaller then than the
capacity of the human hearing system, which has a
dynamic range of about 120dB. The very best
analogue circuits can just about match this dynamic
range. But, by contrast, double precision floating-
point math is a binary format which has a precision
of 53 bits or 320dB. That's a dynamic range which is
ten billion (1010) times greater than the hearing
system.
REASON 2: Filter accuracy and stability and
perfect left-right channel matching
Even the very best electronic components can only
be manufactured with a certain degree of accuracy
which is rarely better than 1%. Physical components
are also subject to ageing such that they go "off
value". Digital processing ensures perfect channel
balance and frequency response for ever, which
guarantees an uncoloured sound with superb
stereo sound staging.
REASON 3: Better warp and rumble handling -
with phase-linear filtering in software
In a perfect world, all gramophone records would
be perfectly flat, there would exist no inevitable
resonance of the arm-mass and the stylus mounting
compliance and the mechanical vibrations from the
driving motor would be eliminated. But LPs do not
exist in a perfect world so, when warp or rumble (as
these imperfections are named) are present, they
are better eliminated. The technique always
employed to remove these effects is high-pass
filtering and that is the approach taken in Stereo
Lab too: a slightly under-damped fourth-order filter
may be employed as a rumble filter so that the
response falls sharply below the audio passband.
However, very differently, the rumble filter is a
phase-linear, non-causal design which means there
is no phase distortion introduced. Phase distortion
is inevitable, real and audible in causal, filters and
can never be eliminated in analogue designs]. But it
is entirely eliminated in software processing. The
result is a bottom octave which will sound like
you've never heard it before.
REASON 4: Flexibility of equalisation
characteristics - to cope with records not
equalised with RIAA curve
Conventional wisdom has it that, by the mid-fifties
in a belle époque of international cooperation, most
American labels and most major European labels
had adopted the new RIAA standard and had
brought to an end a very chaotic situation which
had existed since the dawn of electric recording in
which the record companies all specified different
equalisation for their discs. However, the truth is
that many labels were much slower to adopt the
RIAA curve and disc recording characteristics were
not effectively standardised
until the late 1960s, or possibly even later. It is
therefore a great boon to the vinyl enthusiast to
have flexible equalisation characteristics. But such
flexibility, combined with perfect mathematical
precision is very complicated and expensive to
implement in hardware.
In providing digital equalisation as part of Stereo
Lab, Pspatial Audio wished to offer a
comprehensive range of these non-RIAA
equalisation options for gramophone disc collectors
whilst avoiding a complex and over technical user-
interface. So, Stereo Lab features equalisation
options to cover any disc (or indeed cylinder)
recording from the 1880s to the present day.
