At a glance, the TM134 looks like a fully parametric 4-Band EQ;
however, this module has an entirely different purpose. Using equalization not
in the standard left-right stereo format but in the M/S format, offers a sophisticated
way to modify the stereo image. While a unit like the M/S
direction mixer TM130 can reduce or enhance the stereo base width over
the entire audio frequency band, the M/S EQ makes it possible to apply stereo
base enhancement or reduction in the frequency bands that are defined by the
center frequency and q-factor controls. Here is some basic information on equalizers
in the M/S format.
The most annoying obstacle of just using M/S equalisation is
the need to convert and reconvert the left-right stereo signal to and from M/S.
Unless the input and output format is already M/S (which is rarely the case)
, an additional L/R to M/S matrix is necessary before the equalizer and another
matrix is needed to convert the processed M/S format signal back to L/R stereo.
The TM134 overcomes this problem since the matrix
stages at the input and output are integrated in the module. The M/S EQ can therefore
be used with standard left-right stereo signals without that anything
else is necessary. The input and output matrix stages are independently controlled
by the switches, which allows integrating the M/S-EQ into a M/S signal chain
and use either the input or the output matrix for L/R to M/S or M/S to L/R conversion
independent of the equalizer. The bypass switch, which defaults to enabling
and the matrix stages at a time, can be alternatively configured to remain in
the signal path while the bypass switch affects only the equalizer.
Frequency dependent Stereo Base Width Control
Using M/S equalization always affects the stereo base width and the frequency
response. Unless a stereo
signal has an extreme wide stereo image, the level of the M channel is almost
always 10 to 20 dB higher than the level of the S channel. Using the TM134 in
the S channel alone results in a strong influence on the stereo image with only
a small influence on the frequency response. If the M/S EQ is used in the M channel
only, the effect on the stereo image is lower but the eq setting is a lot more
audible, although the eq itself affects only the center of the stereo system. Each
of the 4 fully parametric bands of the TM134 comes with a 3-position lever switch
that determines the operation mode. In the S position, the particular band affects
only the S channel for effective control of the base width with minor eq effect.
In the M position, only the M channel is affected with minor effect on the stereo
base width and a much higher eq effect. The center position 'M/S' affects both,
M and S channels at a time, but in the way that a boost in S results in a cut
in M and vice versa. This mode can partly compensate the audible equalizer effect;
however, using the TM133 M/S Stereo Mastering
Equalizer for this purpose is the
better solution. The orientation
of these pots applies to the base width;
turning the pot to the right enhances the base width in the particular band while
turning to the left reduces the base width.
All fully parametric equalizer bands are implemented using adt-audio's
fully parametric active 'Vienna Bridge' circuit that combines the flexibility
of an active eq with the sound performance of a passive LC equalizer. The Boost/Cut
ranges of +/- 8 dB are adapted to the use as M/S EQ; however, custom versions
with any range up to +/- 16 dB are possible on order at short notice as well.
Sweep Ranges & Q-Factor
The four bands cover the entire audio range from 20 Hz to 25 kHz. The
LOW-Band starts at 20 Hz and ends at 1 kHz. MID 1 covers the range from 50 Hz
to 5 kHz and MID 2 starts at 200 Hz and ends at 15 kHz. The HIGH band covers
the range from 400 Hz to 25 kHz. Due to the large overlaps, different bands can
be used in the same frequency range. The range from 400 Hz to 1 kHz is covered
by all four bands; 200 Hz to 5 kHz is covered with both mid bands and - from
400 Hz - with the high band as well.
The q-factor control range is the same for all four bands and covers
a range from about 3 octaves for global corrections down to a third for subtle
The headroom is + 30 dBu throughout the entire M/S EQ. At linear settings,
the output noise is less than -94 dBA/-90 dBuRMS with M/S input and output, which results in a total dynamic range of at least 124 dBA / 120 dBRMS.
The TM134 is a 4U ToolMod that fits into two slots of the horizontal 1U
high ToolMod Frames. The vertical version fits into a slot of a 4U
high ToolMod Frame.