fish@ihu1g.UUCP (Bob Fishell) (03/09/84)
(oo)
Owing to the number of requests I've had for this article, and
also to some mail problems I had, I'm posting this to the net.
I normally wouldn't put such a long article on the net, but there
seems to be substantial interest. To all of you who have responded,
thanks for you interest. If I mailed you a copy, don't bother
reading this; it's the same one. If you requested a copy and did
not get it, I'm sorry to have kept you waiting. Here's your chance.
I'll still honor any mail requests or answer any additional questions.
Bob Fishell
ihnp4!ihu1g!fish
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Comments_on_the_dbx_120_Subharmonic_Synthesizer
The dbx 120 is a recent addition to dbx's line of signal
processors. Signal processors in general fall into two categories,
compensators (such as frequency equalizers) which reduce the
effects of deficiencies in the reproduction process, and enhancers,
which attempt to reduce the effects of deficiencies in the
recording, manufacturing, and/or broadcasting processes.
The dbx 120 is both a compensator and an enhancer. It can do a
good job of compensating for a comparatively high woofer rolloff
(55 Hz and up), merely by boosting the amount of low frequency
energy in the signal. However, any good equalizer can do the same
job, and there are low-priced rigs called "speaker equalizers"
which merely boost the bass. The latter variety is more likely to
find its way into the hands of an audio owner who doesn't want to
spend the big bucks needed for speakers with good low bass
response. However, a good Baxendall type tone control is almost as
effective, and you get it for free with the amplifier.
The 120's principal value, then, is as an enhancer. The extent to
which it succeeds depends on numerous factors which I have
attempted to identify.
The dbx 120 is electronically three units in one. The simplest
unit is a sharp-rolloff infrasonic filter which effectively cuts
out frequencies below 30 Hz. It is very effective at removing
rumble and the near-DC abberations that sometimes occur from record
warp and tonearm resonance. The filter feeds two independent
circuits whose output is mixed at the 120's final output stage.
One of these is a rather modest low frequency amplifier which
boosts signals between 30 and 150 Hz. It has a maximum gain of
+6dB at 55 Hz, and rolls off gently above and below that frequency.
Its gain is controlled by single slide pot on the 120's front
panel.
The third element of the dbx 120 is its most interesting feature.
Input information between 55 and 110 Hz is fed through a frequency
divider and mixed into the output. The amplitude of this
synthesizer is variable from infinite attenuation to +9dB relative
to the fundamental, depending on the position of another slide pot
on the front panel. In addition, a push-button cutoff switch is
provided to defeat the synthesizer.
The only other features of the 120's control set are a power switch
and an array of four LEDs which indicate that the synthesizer is
operating. They are useful if you like flashing lights on your
stereo system (something I've got lots of, these days), but don't
really help you with control settings.
The synthesizer and LF amp don't feed each other, so the panel
controls operate independently. Their interactions are only
significant at the output jacks, where they are summed. The 120
has a maximum output level of 6.5 volts ( p-p, I think), and is
rated at .05% THD, all controls flat. The dbx 120 retails for
about $200.00; mail-order prices may be a bit less. However, it's
probably a good idea to get one from a local dealer, unless you're
absolutely sure you want one. Most dealers will take back
equipment for any reason, within a reasonable period of time.
Objective Comments: The dbx 120 claims to restore bass which was
present in the original program material, but which has been lost
in the recording or cutting process. The degree to which it can
succeed in this depends mainly on the actual extent to which this
loss of information has in fact occurred, and on the information
which *does* make it onto the recording. If the second harmonic of
a low-bass fundamental (such as an organ pedal) has survived the
recording process, the dbx 120 will, indeed, restore the
fundamental to its original volume -- if the subharmonic level is
set right. In practice, such exact compensation would be extremely
difficult to obtain, as it is with dbx's expanders. However, the
ear's ability to distinguish differences in volume at frequencies
below 100 Hz is much less than at midrange frequencies, so this
should not be regarded as critical.
Unfortunately, the dbx 120 makes no distinction between the second
harmonic of a lost fundamental and a fundamental that's not
*supposed* to have a subharmonic. This drawback is immediately
apparent when listening to a deep-voiced announcer with the 120
engaged. The *objective* view of the dbx 120 must, then, be highly
skeptical. It can do what it's supposed do do, but it can't tell
exactly when to do it. Therefore, the 120 is *not* for the audio
purist who insists on equipment that delivers sound with as much
accuracy as the state of the art permits.
Subjective Appraisal: Despite the 120's nonlinearities, it rarely
produces effects which are objectionable, and often does add a
"realism" to recorded music which is not obtainable with simple
equalization. The 120's effects are most pleasing in material
which ordinarily contains a good deal of low bass. When I first
plugged my newly-acquired dbx 120 into my system, I chose as my
first listening selection a dbx-encoded 7" reel recording, made at
7.5 ips, of Saint-Saens' famed "Organ Symphony" (Charles
DuToit/Montreal Symphony, digitally mastered). I ordinarily don't
listen to the LP because I'm afraid of LF feedback. Needless to
say, the 120's effect was magical. The double basses in the string
section stood out with the authority one usually misses in recorded
music. Likewise, the tympanis had much more of a presence.
However, it was on those lovely pedal notes, which predominate in
the second movement, that I truly appreciated the presence of the
added bass. I actually FELT the 16' diapasons in my chest cavity,
the way I do when I hear a real organ, and yet, there was no sense
of booming or heaviness, an effect I had obtained when I tried to
bring out the pedal tones with an equalizer. The sforzando that
begins the fourth movement of this work just about pasted me to the
wall, and sent my 80-w/ch amplifier close to its dynamic limits.
The overall effect was breathtaking. I felt like calling the
salesman to tell him how much I liked the unit, but common sense
and my engineer's skepticism prevented me from wholeheartedly
endorsing this unit just then.
Switching to rock music, I chose Supertramp's "Paris" live album.
Here, the effect was less profound, but nonetheless pleasing. The
dbx 120 does a lot to bring out the bass passages, which are for
some reason often understated on a lot of rock music. Some rock
bassists, e.g., Chris Squire, are quite articulate, and the 120
allows the listener to follow the bass note by note, again, without
the sense of booming or heaviness that often comes from simply
boosting the low bass. In short, the 120 brings out the *presence*
of the bass, without overwhelming the listener. Since rock fans
usually like a lot of bass, this component is definitely for them,
given a system that can handle it (see SIDE EFFECTS, below).
On some types of music, the 120's effects are minimal. Since it
operates on program material below 110 Hz, music which does not
fall into this frequency range is largely unaffected by the 120.
Further, on some types of music, the 120's effects are unwanted.
String quartets, for example, sound better without the 120; indeed,
the subharmonic synthesis adds unnatural and spurious subharmonics
to the cello. Opera fans would also not appreciate the 120, since
it adds unwelcome subharmonics to the basso or baritone voice,
although the effect is much less noticable in the sung voice than
it is in the spoken voice.
The dbx 120's effect on piano music is unexpected and pleasing.
Whether a listener would really appreciate it depends on whether
he/she is used to listening to piano passively, or to playing one.
For the most part, the bass fundamentals of the piano don't carry
very far, so the pianist has a unique perspective. Since I play
the piano, I appreciate the 120's restoration of the low bass, but
a person used to listening to piano music on records might find the
effect unrealistic.
Side Effects: As mentioned previously, the dbx 120 cannot
distinguish between stuff it's supposed to act upon and stuff it's
not supposed to act upon. As a result, it adds unwanted low
frequency information to such sources as male announcers' voices,
turn-on and switching transients, and musical instruments which
normally range the midbass (e.g, the cello). On such *musical*
sources, the sound is much less objectionable, because of the
psychoacoustical phenomena which cause the listener to hear the
higher octave of a diad as the primary tone. However, the purist
would rightly complain about subharmonics that patently do not
belong.
The 120 has another side effect which is more of interest from a
technical viewpoint than from a subjective one: The dbx 120 can add
up to +15dB of power-demanding low frequency signals to what's
already available from the program source. As such, a fairly hefty
amplifier should be used, say, 45w/ch minimum, if the 120 is to be
used to its maximal effect. Furthermore, no other bass-affecting
controls should be used in conjunction with the 120. Equalizer
controls below about 150 Hz. should be set to mid-position (0db
gain), bass tone controls should be set flat, and "loudness" should
never be used. For folks who routinely use "loudness" at low
listening levels, the 120 will more than compensate for the bass
response, and the treble can be touched up with a tone control.
The dbx 120 also makes a system much more susceptible to damage
from transients. The 120 should either be switched on before and
turned off after the power amp, or switched on and off
simultaneously via a switched outlet. Even at that, it's a good
idea always (120 or not) to turn the volume all the way down when
powering up or powering down a system. Stylus drop should also be
avoided.
Overall appraisal: I like the dbx 120 and it is a welcome addition
to my audio system. I particularly enjoy its effects on rock
music, large orchestral works, and pipe organ music. However, I
find that it is more persnickety than the other processors I own
(an expander, an equalizer, and a rear-channel time delay system).
It is not a set & forget device; its optimal settings vary with the
program material, and there are many instances where I don't use it
at all. Fortunately, there are few controls to set, and a bypass
switch cuts out the subharmonic synthesis ( but not the modest LF
boost) in those instances where its effects are not wanted.
I find that the dbx 120 produces its most pleasing effects when it
is used subtly. The subharmonic synthesis is controlled by a
single slide pot whose leftmost position represents no synthesis;
the center position corresponds to a subharmonic level equal in
amplitude to the fundamental. For the most part, I listen to the
120 with its subharmonic level set *below* the center position.
This adds subharmonic effects that complement, rather than
overwhelm the existing bass. As with all processors, the rule to
follow is: If the effect is obvious, turn it down. However, the
dbx 120's effect is, by nature, less subtle than that of other
processors; its presence in my system is immediately apparent at
all effective levels. It's something you have to live with for a
while, experimenting with different program material, before you
arrive at settings which are most pleasing to individual tastes. I
like my music to have full-bodied bass, so I might use it a little
more than average. But then, if I hadn't hungered after more bass
in my system, I wouldn't have bought it in the first place.
The dbx 120 is not for everybody. Self-proclaimed "golden ears" or
other purists would likely object to anything that adds material to
a program source that was not intended by the engineer or producer.
Furthermore, it requires a host system which is capable of
producing very low bass, and it requires a fairly big amplifier.
The psychoacoustic effects of low bass also have to be considered.
Although I've grown accustomed to it now, at first I found the
added bass rather disconcerting. I think that it's been
demonstrated that in many individuals, very low bass tones produce
mild anxiety. Cathedral organists are well aquainted with this
effect, and often use the pedal notes to soften up the sinners
before the preacher comes in.
And then, there are the neighbors to consider. The term, "wall-
shattering bass" aptly applies to the dbx 120, and if the walls or
ceiling or floor are shared with somebody else, their nerves -- and
tempers -- may get shattered as well.
On the other hand, I can think of no other component which
specifically deals with the problem of anemic bass in many kinds of
program material. If the dbx 120 is less than perfect in its
effects, at least it gives an audiophile a chance to appreciate
those big, expensive speakers sitting in the living room, which
were bought for their good low bass response.
Perhaps, with the digital age dawning on us, signal processors will
become obsolete. Most of dbx's products are designed to compensate
for deficiencies in the recording and reproducing processes; the
120 is one such. With a program source that already contains an
adequate helping of bass, its use is unnecessary. However, I for
one have a lot of older records that I like to listen to a lot.
All my signal processors enhance my enjoyment of these, and,
perhaps more importantly, they give me a sense of control over what
I hear that is most satisfying. For all their supposed drawbacks,
my dbx 120 and its companions free me from being entirely at the
mercy of the recording industry.
--
Bob Fishell
ihnp4!ihu1g!fish