Filters

[bsee]

Hi Brad-
 
I didn't mean it that way at all.  I just know that we've had this discussion at least once in the past 6-9 months, and was prefacing my comments as representative of info from that collaboration.  My recollection of that discussion was an admission that the Q switch really doesn't control the Q.
 
Rather than thinking of it in classical terms, though, the description of the impact of the switch is pretty clear.  The switch controls the amount of boost centered at the elbow frequency of the low-pass filter.
 
One thing that I don't think was mentioned in this thread is that a bass that has a filter and no Q switch is hard wired for a +8 dB boost.  
 
Does the content of this thread answer your questions, or are there still things left unsaid?
 
-Bob

[cntrabssn]

OK, I guess I had the Q equation wrong. However, I have seen some stuff that relates Q factor in low pass filters to the peak filter gain, which is expressed in dB, e.g. Q = 1 equals 0 dB resonant peak, while Q = 4 equals a 12 dB resonant peak. I wish I was better at math. I guess I've got some more reading to do  
 
Anyway, the post I mentioned earlier from the other club member is here:  
 
http://club.alembic.com/Images/393/3449.html#POST5376  
 
- nate.
 
(Message edited by cntrabssn on February 04, 2005)

[adriaan]

From what I recall from the discussion, Q is short for quality, and it is an expression for the ratio between the peak level (measured in dB; instead of a peak you could also have a dip) and the distance between the 0 dB points at either side of the resonance (aka bandwidth).
 
So the Q switch really does control the Q, but backhandedly - it actually sets the level of the resonance peak.

[dfung60]

adriaan is right on with regard to the classical discussion of Q and it does stand for quality.  
 
A bandpass filter is selectively favoring part of the full frequency spectrum that is presented to it.  If you drew a frequency response graph, it would be a hump.  The Q of this filter reflects how steeply the sides of the hump rise, expressed as a ratio in dB.  The bandpass of this filter is the width of the top of the hump (the plateau of the mesa!).  If you had a really wide bandpass, then you might not notice that a filter had very high Q.  If you have a narrow bandpass, then you'll perceive the steep sides and narrow top as a resonance peak.
 
These filter concepts are really important for things like TV sets or radios.  Traditionally, the program material is impressed (modulated)upon a carrier frequency and there are many carrier frequencies carrying information in parallel.  A bandpass filter very similar in concept to your tone controls is singling out a single station from the jumble.  The frequencies and bandwidth are carefully defined by the government, and the Q of your receiver affects your ability to receive stations clearly.  
 
A radio tuning circuit is more sophisticated than the tone control in your Alembic, but the mere fact that I just said that will probably cause Ron to develop the first superhetrodyne tone controls for some future Series III bass!  :-)  
 
Of course, the digital age starts rendering a lot of this analog stuff one step closer to antiquity.  Creating filters that would be impossible with physical components is just a matter of software with modern DSPs.  I guess that will be Series IV(!), although I doubt that anybody other than Alembic would invest the effort to make a digital filter that is musically beautiful rather than not too bad.  Maybe it's just me - I have an HDTV and find that the digital artifacts from the conversion to progressive scan much more annoying than the interlaced video that it's trying to correct.
 
David Fung

[adriaan]

David,
 
Couldn't agree with you more about the digital stuff (this coming from one who spends his working day stuck behind a computer). The resonances that an analogue filter produces in electric signals is - well - analogue to the resonances that the body of an instrument produces on the sound waves travelling through the instrument, and that is what makes the audio fingerprint of that particular instrument. With digital sound synthesis you get chopped up resonances - try listening to a digital piano for longer than 1 minute and say your ears don't get tired from the never-ending resonance fits and starts. Try sustaining a single note on that same digital piano and listen how the sound chuckles as it decays.
 
I have been considering getting a digital piano so I could play with headphones, but the drizzle ...

[gare]

Guess I kinda opened a can of worms huh ? LOL
Prehaps Mica or Val will jump in and verify my original question..are the Alembic filters all the same ?  
 
Adriaan..checkout General Musics digital pianos, just recently reading up on them, looks interesting..they're at www.Generalmusic.com. I also really like the Kurzweil grand piano sound.  
 
and thanks everyone for the responses and info.
 
Gary

[rover]

All,
 
My questions may be overly na?ve to some, but I would still appreciate some guidance regarding the possibility of the Q settings on my bass (Anniversary) having an impact on the functionality of the SF2 or vice versa.  For example if the Q switch on the bass is activated to give an 8db boost and the filter is nearly ?closed? thus blocking passage of the lowest onboard frequencies, is there a contradictory ramification of having the SF2 set in low pass mode which introduces frequencies lower than the instrument electronics allow on their own or does one (instrument v. SF2) simply override the other?
 
Thanks,  Rob

[the_mule]

>>> Are the Alembic filters all the same?
 
All filters on Alembic's basses are low-pass filters. Only a few exceptions to this rule, this is one of them: http://club.alembic.com/Images/411/16123.html?1107342100
 
With Alembic's SF-2 Superfilter rackmounted 'box of wonders' you can also use high-pass and band-pass filtering.
 
This is how I explain the difference to interested friends and relatives. Maybe not 100% accurate, but they seem to understand...
 
low-pass = frequencies below the selected frequency are allowed to 'pass'
high-pass = frequencies above the selected frequecy are allowed to 'pass'
band-pass = a certain bandwith around the selected frequency is allowed to 'pass'
 
>>> I would still appreciate some guidance regarding the possibility of the Q settings on my bass (Anniversary) having an impact on the functionality of the SF2 or vice versa.
 
I don't have any technical background, so I can't give an explanation, but hopefully someone else? I can only tell you what my experiences are:
 
I have a EVH 4 carrying custom electronics (2x volume, standby-switch, 2x low-pass filter and 2x 3-position q-switches) and I use it sometimes with, sometimes without a SF-2 Superfilter.
 
I noticed that when both the SF-2's 'direct gain' and 'filter gain' are activated, thus mixing the natural signal of the bass with the signal of the SF-2, it's not hard to find settings which result in a strange (interesting, ugly, out-of-this-world, etc. etc.) interaction between your bass and the SF-2. I can only conclude that it's an interaction, and not a case of one 'overruling' the other...
 
Wilfred

[adriaan]

Rob,
 
The part of the spectrum that you filter out by tuning the onboard filters to a lower frequency (with or without the Q boost) you simply cannot magically reclaim with an outboard EQ of any kind.
 
I've forgotten the niceties of the one time I played through an SF-2 (it was the second time I held an Alembic bass, and actualy the first time I held my Epic, so my attention was on other matters) but it don't think even the SF-2 can reclaim a missing part of the spectrum.

[rover]

Thanks for the insights.  I'm sure I'll have more questions as I learn the basics of the SF2 over the next decade or so.
 
Rob