[AH] [REVIEW] MOTM-450 Fixed Filter Bank

From konkuro
Sent Fri, Oct 24th 2003, 08:09

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Yes, I know: the MOTM-450 Fixed Filter Bank from Synthesis Technology isn't=20
shipping yet.=A0 But it will be shipping ere long, and what good is reading=20=
review after you've bought the damn thing?=A0 Since I had the good fortune t=
o test=20
the prototype and am in a mood to dish, perhaps you will humor me and actual=
read this review.=A0 But before I grace the world with the opinion it so=20
deserves, let all be reminded that my synth of choice is Synthesizers.com an=
d my=20
only MOTM module thus far is a sub-octave mux (love it).=A0 Fortunately, the=
systems use identical power supplies and are pretty much compatible, so I wa=
able to test the FFB with no problems.=A0=20

I have heretofore reviewed the dotcom FFB, so we won't rehash that again.=
However, some comparisons will be made.=A0 How could I not, what with two Te=
FFB's sitting side-by-side begging for a shootout?=A0 So, armed with my trus=
Scopemeter and my trustier ears, I set about putting the 450 through its pac=


The MOTM-450 follows the venerated Moog 907A filter bank in that it includes=
a LP filter, a HP filter and eight bandpass filters.=A0 But whereas the Moog=
used BP filters based on an LRC topology (inductance, resistance,=20
capacitance), the MOTM=A0 FFB uses FDNR filters (frequency dependent negativ=
e reactance).=20
This is the way to go if you want to emulate an LRC filter without using=20
inductors, which are expensive and bulky and a general bitch to deal with no=
How much the MOTM filter sounds like Moog, I don't know.=A0 I'm not even sur=
that's important.=A0 What is important is that the FFB does what it sets out=
 to do=20
and does it well-namely, providing a choice of fixed frequency bands for=20
spectrum enhancement and formant synthesis.

Controls on the MOTM-450 include a Wet/Dry mix pot and a manual/remote bypas=
switch with LED.


When all of the controls on an FFB are set to zero, you should hear nothing,=
ideally.=A0 I guess that makes the 450 ideal in that regard.=A0 With all con=
set at minimum and the mix set to wet, there was no detectable output, even=20
with my amplifier cranked as loud as I dare.=A0 Bugbears such as bleedthroug=
hiss, and line noise must have been out having coffee or something because t=
sure weren't at the output.=A0 The filter was also nicely resonant, lending=20=
excellent instrumental quality to otherwise electronic sonorities.=A0=20

I didn't take a measurement, but my ear detected no attenuation of the=20
filtered signal, other than what would be expected from the lack of overall=20
spectra.=A0 What you dial is what you get, and at the output level you expec=

The mix control was nice, but I hardly used it, given that my interest was i=
purely filtered signals.=A0 The LED was also a nice touch, as I like knowing=
a glance whether something is active or not.

The LP and HP sections performed as expected.=A0 The HP section gave=20
particularly crisp results.

THE BAD STUFF (not all that bad, really)

The module was a real power hog.=A0 For some bizarre reason I neglected to=20
measure the amperage, but suffice it to say that when I hooked the FFB up "h=
ot" it=20
created a noticeable spark at the connector.=A0 I actually had to disconnect=
six of the eight dotcom VCOs just to run the thing.=A0=20

The .com oscillators output a 10v p-p signal, which caused clipping in the=20
MOTM FFB unless attenuated. The LP section was particularly prone to clippin=
and would not handle a signal greater than about 3V p-p without distortion.=
=A0 The=20
max input for the remaining sections was about 3.5V p-p, beyond which was=20
obvious clipping.=A0 This deficiency was not serious, however, because a 3V=20=
signal still represents a lot of juice.=A0 If the filter were noisy, this wo=
have been more of a problem because SNR would be lessened.=A0 But given that=
noise is essentially zero, having to deal with lower than normal signal leve=
(by .com standards) really wasn't an impediment.

My ear told me that some of the MOTM FFB center frequencies were off the=20
mark, and measurements with a frequency counter confirmed this.=A0 At a targ=
center frequency of 350 Hz, for example, the dotcom FFB resonated at 353 Hz.=
=A0 The=20
MOTM FFB weighed in at 422 Hz.=A0 At a target of 700 Hz, the dotcom was reso=
at 701 Hz (!).=A0 The MOTM FFB was resonant at 802 Hz.=A0 Paul had some=20
explanation as to why the MOTM FFB was so far off the mark, but the brain ce=
ll with that=20
memory is now dead. He did say that the frequencies in the production versio=
would probably be changed to reflect the center frequencies called out on th=
panel.=A0 I hope that change was made because not to do so would impact the=20
usefulness of the FFB.=A0 To get the primary formant for an oboe or bassoon,=
example, you need to be able to dial up 500 Hz.=A0 Any other value won't qui=
te cut=20

One more thing:=A0 I think the switched bypass should be attenuated, so as n=
to be so jarring when switched from filtered to non-filtered output.=A0 But=20=
is a minor nit that can be worked around easily enough


So how did the MOTM FFB stack up against the dotcom FFB?

The MOTM FFB offers bands that are more resonant than what the .com FFB=20
offers, but the difference-while noticeable-was not dramatic.=A0 A sawtooth=20=
at 1 kHz with a touch of reverb sounded virtually identical on both systems=20
(when the .com FFB Aid module was used).

The MOTM FFB offers 7 bands only, while the .com offers 12.=A0 From the=20
standpoint of synthesis, this isn't a major advantage for the .com because t=
he ear is=20
less sensitive to the higher and lower frequencies offered.=A0 All of the=20
"business" frequencies are covered on the MOTM FFB, and the HP and LP filter=
s can=20
be used to take care of the rest.=A0=20

The dotcom FFB costs less than the MOTM FFB and the center frequencies calle=
out on the panel were far more accurate than on the MOTM FFB (see above).=
However, my dotcom FFB has a serious bleedthrough problem that spoils the=20
filtering effect. Roger Arrick claims to have a fix for this bug, but I have=
gotten around to taking him up on it.=A0 Since I don't know how quiet the do=
tcom FFB=20
would be with the patch, I can't make a comparison on the bleedthrough.=A0 B=
I doubt that ANY design could match the low noise floor of the MOTM FFB.

Interestingly, the MOTM FFB reminded me a lot of the Doepfer FFB, which was=20
also nicely resonant.=A0 But while the Doepfer FFB offers far more ranges an=
costs much less than the MOTM FFB, it is rather noisy-making the MOTM FFB a=20
better choice, even at the higher price.


The MOTM FFB is expensive and offers fewer bands than other designs, but wha=
it does it does quite well.=A0 Fans of "CD quality" analog circuitry will be=
sent into an ecstasy at the astonishing quietude of the 450.=A0 This is an=20
excellent filter bank that MOTM users with some cash to spend will certainly=

Konkuro Rating is K:4 (on a scale of 1 to 5)