Active vs passive tone control

[hieronymous]

OK, here's where some of the confusion lies for me. I never heard of "low pass filters" until I got into Alembic and Moog. I had never heard of the passive tone controls of a Fender bass (for instance) being referred to as low pass filters - until recently.

In the manual for the SF-2 (here's the recent post linking to the manual), it states the following:

Why We Choose to Filter

Natural acoustic instruments behave in large part as filters. All that statement means is that these filters tend to resonate in response to some signal - and acoustic instruments certainly do resonate. Low-pass filters are the closest electronic equivalent to these natural acoustic sounds.

This is precisely the reason we chose to use low-pass filters in our Alembic instruments. As a result, our instruments preserve the natural sound of the string's vibration and a variety of pleasing sounds are achieved. They were, in fact, the first active electronics ever to be used in guitars and basses.

Now I understand that this passage isn't written in a technical manner. But to me it implies that Alembic's use of low-pass filters is a significant, new advance over the traditional method. It doesn't say what that traditional method is, but I would assume the standard tone control. So to me, this implies that the traditional tone control is not a low-pass filter, it is a different electronic method of sound manipulation that could be seen as similar to a low-pass filter, but different. Then again, in the final sentence they point out that this system is "the first active electronics" so you could read it as the active filter is the advancement.

There is also the issue of "resonance" which is emphasized in the manual. Resonance isn't adjustable in the standard passive tone control - maybe that's the key?

I wish I could blame this on it being the morning and my brain not working yet but if my brain's not working then it's been this way for 20 years! It could just be semantics, but I personally see the recent (?) use of "low pass filter" to describe the standard passive tone control as misleading, if not incorrect. Not sure that I'm correct in that though.

[JimmyJ]

David's simple description of how the cap works in a passive tone control is really great.  Thanks for that!

Hieronymous, low-pass = treble roll off = high-cut, yes?  That much is semantics.  The use of the word  "resonance" here is a bit more ... interpretive?  There are a few meanings in the dictionary but I'm pretty sure they mean like an acoustic instrument's resonant cavity, as in an acoustic guitar's hollow body.  And as sound travels through air the dampening effects first happens on higher frequencies. The further away you are from a sound source the more natural "low-pass" filtering happens.  Does that make any sense? 

So passive low-pass and active low-pass generally do the same exact thing to the sound.  But the passive version is more reactive to resistive and inductive loads, pickup volume levels, etc, whereas the active version has built in "buffering" which isolates the circuit so that it always works as expected.

Alembic being the "first active..." could certainly be seen as an advancement because of what I just described.  But it also doesn't claim to be the "only" way to do it.  It's certainly MY preference though.   


Jimmy J

[hieronymous]

Hieronymous, low-pass = treble roll off = high-cut, yes?  That much is semantics.  The use of the word  "resonance" here is a bit more ... interpretive?  There are a few meanings in the dictionary but I'm pretty sure they mean like an acoustic instrument's resonant cavity, as in an acoustic guitar's hollow body.  And as sound travels through air the dampening effects first happens on higher frequencies. The further away you are from a sound source the more natural "low-pass" filtering happens.  Does that make any sense? 

So passive low-pass and active low-pass generally do the same exact thing to the sound.  But the passive version is more reactive to resistive and inductive loads, pickup volume levels, etc, whereas the active version has built in "buffering" which isolates the circuit so that it always works as expected.

Alembic being the "first active..." could certainly be seen as an advancement because of what I just described.  But it also doesn't claim to be the "only" way to do it.  It's certainly MY preference though.   

Jimmy J

Thank you Jimmy san! I appreciate the response. I guess the other word is "filter" and a passive tone control I grudgingly admit could be considered a "low pass filter" - maybe "resonant filter" is the important part?

I guess part of my just doesn't want to grant "low pass filter" status to the lowly passive tone control! In a way I feel like it takes away from the achievement of Alembic electronics, but then again, most people won't get it anyway, and those that know, know.

[edwin]

Resonance is usually created by feedback at a certain frequency. While the hollow cavities in an instrument do this to a degree, a filter's resonance is much simpler than that and is centered around one frequency.

Formants, multiple resonances in speech, are an interesting example. Each formant is created by the resonance of various parts of the head, throat, etc. It defines the vowel characteristics of a voice When you move formants around, you are moving multiple resonances at the same time and that's really the way to make voices sound like different voices. It would be very interesting, and probably more accurate in making a bass sound like different instruments, to have a formant filter onboard, but that's a massive increase in complexity.

Alvin Lucier did an interesting piece which was originally inspired by his trying to smooth out his stutter by playing a recording of his voice in a room and recording the results over and over until the room and the formants of his voice and the room interact and create a very interesting and haunting rhythmic sound.

https://en.wikipedia.org/wiki/I_Am_Sitting_in_a_Room

[pauldo]

Inserts mind blown emoji.