Manual

19
SHUFFLING
This is a fancy name for frequency-conscious width control and based - naturally - on
Blumlein's work. By inserting an equaliser in the S-signal, the gain may be changed
in a frequency-conscious manner which can lead to improvements in directionality in
certain types of stereo recordings. To increase the width of a stereo image at a
particular frequency or over a range of frequencies, the gain of the S-signal is
increased at those frequencies. Similarly, to reduce the width of a stereo image at
particular frequencies, the gain of the S-signal is reduced at those frequencies.
Later research has suggested that an equaliser should also be inserted in the M-
signal so that when the S-signal is increased at specific frequencies, the M-signal may
be reduced slightly at the same frequencies in order to maintain overall frequency
balance. Similarly, if the S-signal is reduced, then the M-signal is increased
slightly at the same frequencies.
Graphic equalisers would appear to be the obvious choice for shuffling, but they are
made up of a series of narrow-band (therefore 'ringy') filters and this can lead to a
'ripply' frequency response. Far better results will be obtained by using a constant-Q,
parametric equaliser with a very low Q-setting, eg 0.2, so that just one filter section
may be used to achieve the required result.
AD066-13 FLEX-EQ is such a filter set. This equaliser also satisfies a further
requirement - the transition to the 'equalised' part of the frequency spectrum should
be as gentle and smooth as possible. The skirts of the FLEX-EQ low-Q filters are of
the order of 6dB per octave.
Shuffling is too powerful and complex a technique to cover adequately in these
application notes, but it is worthwhile listing some of the situations in which the
technique will be advantageous.
Black records become easier to cut and reproduce when the width is reduced at low
frequencies. FM stereo reception noise can be reduced by reducing the width around
7kHz. Reduction of low frequencies in the S-channel helps to eliminate structure
borne vibrations. Increasing the width at low frequencies will bring into coincidence
high- and low-frequency components of an image which have become separated
because of imperfections in microphone polar patterns, eg where cardioid
microphones become more omnidirectional at low frequencies.
Dryness and lack of spaciousness are inherent in recordings made with coincident
cardioid microphones. The sense of space can be improved by increasing the
difference signal below about 700Hz. This can also lead to an improvement in stereo
imaging away from the 'stereo seat'.
When shuffling techniques are applied to recordings made with near-coincident
microphone techniques or techniques in which out-of-phase images can occur when
using non-coincidental microphones, shuffling should be restricted to frequencies
below about 300Hz, ie to frequencies which arrive in phase at both microphones.