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 (10
10
) 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.
