Cedar AZX+, Руководство пользователя

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PAGE 24
AZIMUTH CORRECTION
CEDAR
’s first version of the Azimuth Corrector was called the Phase/Time
Corrector, and ran on the
CEDAR
-2 Production System, an early
PC
-based suite of
processes that ran under
DOS
. This was developed primarily for the broadcast
and video industries, for whom mono compatibility is of great concern. The name
was, however, misleading because the algorithm did not correct ‘phase’ errors
within the signal. It corrected timing errors.
The next incarnation of the process was implemented in the
AZ
-1 Azimuth
Corrector, a
CEDAR
Series 2 rackmount unit. The reason for the change of name –
despite the functionality remaining broadly unchanged – was purely a marketing
decision. Audio engineers knew what azimuth errors were, and the unwanted side
effects that they created.
Unfortunately, the use of the term ‘azimuth’ implies that these errors only occur
when analogue tape recorders are involved in the signal chain. This is not the case.
Indeed, timing errors can occur whenever two signal paths are inconsistent with
each other. They can even occur – although to a lesser extent – if the pick-up
cartridge in a turntable is misaligned. Nevertheless, this is the name that makes
most sense to the world at large, and that is why the
AZX
+ is called an Azimuth
Corrector rather than a stereo timing corrector.
To understand what these timing errors do to a signal, let us first consider some
simple cases involving just sinewave oscillators. A sinewave oscillator can be
described in its entirety by just three parameters: its frequency, amplitude, and
phase. The frequency and amplitude describe, to a good approximation, the
perceived pitch and volume of the signal. The phase, however, is of no audible
significance until we combine more than one such sinewave.
For the moment, let us consider two sinewaves of the same frequency and
amplitude, but different phases. Figure 1 shows a simple sinewave climbing away
from ‘zero’ at T=0. The figure also shows another, identical waveform with
identical phase (i.e. starting at the same time). As you would imagine, the two
waveforms add together to produce the same sound, but louder.
But now consider figure 2. This also shows a simple sinewave climbing away
from ‘zero’ at T=0, with another, identical waveform offset by half its cycle.
If we add these two waves together, they cancel each other out, and we hear
nothing. Although, in isolation, both oscillations sound identical, combining
them results in silence.
This is a very simple result, and demonstrates perfect addition and perfect
cancellation. These can also be represented by the Lissajous diagrams shown
in figure 3 (in phase), and figure 4 (out of phase). If you combine the waveforms
with other delays (i.e. at different phases) you get other results that lie between
the maximum volume and silence, and the Lissajous figure becomes an elipse.
What’s more, if, instead of combining these waves into a single signal, you
output them through different speakers, you hear a different result. When the
signals are in phase you hear the original tone, perfectly reproduced in mono.
But as the phase shifts, you hear the tone shift across the sound field. (Figure 5.)