Tutorial #2 - The Basic Patch : Lowpass Filtering
            w/Resonance

            Overview
            --------

            In the last tutorial, we saw how to change the level
            (loudness) of a synthesizer tone over time.  This gives us
            the ability to control certain perception of the sound, like
            "punchiness" or "smoothness".  However, there was nothing
            that we could do over time that will change the "darkness"
            or "squeakiness" of the sound; this is the job of
            filtering.  Without filters, a synthesizer's character would
            only be as good as the oscillators (or samples) used to
            create the sound.

            As I mentioned before, one of the most desirable elements of
            older analog synthesizers (particularly Moog `boards) was
            their beautiful filters.  The Kurzweil K2000 has exceptional
            filtering that, while digital, maintains much of the warmth
            of the famous analogs of the past.  The sounds in this
            tutorial will feature analog emulation - I'm not going to do
            much orchestral work here.  However, the concepts and
            examples given will be usable regardless of the type of
            sound you are programming.

            The "Patch Run" section of this tutorial will utilize
            patches from the file TUTOR_02.KRZ (sometimes called
            DDGT0002.KRZ, available at various Internet resources.


            Technical Discussion
            --------------------

            In the previous tutorial, I compared filters to tone
            controls - they allowed the synthesist to vary the frequency
            emphasis of the oscillator's output.  However, this image is
            a little misleading.  Most tone controls on a stereo (or, as
            Dad called it, the hi-fi) have a set frequency range, and
            only allow adjustment of how much (or how little) emphasis
            was applied to that tonal range.

            The K2000 filters, on the other hand, allow you to set the
            frequency range to affect, and allow you to control the way
            the frequencies are affected.  Additionally, an important
            control - filter resonance - allows a tonal "peak" to be
            added to the frequency range, perfect for the "squawk
            factor" we so desperately desire.  This is something Dad's
            hi-fi never had...

            Our basic analog synth had the following signal path:

            [oscillator] --> [filter] --> [amplifier] --> *output jacks*
                                                ^
                                                |__[envelope generator]

            You will notice that the oscillator feeds directly into the
            filter.  This allows the sound to be processed with no
            regard to its volume or envelope.  In later tutorials, we
            will learn to utilize envelopes to control filtering; but
            for now, we will explore the filters in their own right.

            A typical analog filter has the following controls:

            Cutoff Frequency
                 The point (roughly) where frequencies will begin to be
                 reduced.

            Resonance
                 The amount of emphasis that will be applied at the
                 cutoff frequency.

            Key Tracking
                 Whether the cutoff frequency will "follow" the notes
                 being played, and to what extent.

            In addition to these, the K2000 adds:

            Velocity Tracking
                 Whether the cutoff frequency is adjusted based on the
                 velocity of notes, and to what extent.

            The K2000 provides a number of "types" of filters,
            including 1-pole, 2-pole, 4-pole, lowpass, hipass, bandpass,
            notch and resonant filters.  For this tutorial, we will be
            limiting the discussion to a 4-pole, resonant, lowpass
            filter.  But just what does that mean?

            Well, let's rip the name apart.  First, it is a lowpass
            filter.  As you might expect, low tones are allowed to pass
            through the filter, while higher tones are affected by it.
            But how much?  For all filters, the point that tones are
            affected is the cutoff frequency.  Above that frequency, the
            filter reduces the frequency content of the tone.  The
            amount varies, and can be graphed as a slope.  Since ASCII
            character text isn't very good at displaying graphics, I'll
            just refer you to the Musician's Guide, chapter 14.  Under
            DSP functions, the various lowpass filters are listed, with
            a graph of their cutoff slopes. What you should notice is
            the rate of change - a four-pole filter has a much
            "steeper" slope than a one-pole filter.  The amount of
            frequency reduction (also called attenuation) is listed in
            dB/octave - the amount of audible change for each octave
            above the cutoff frequency.  The simple guide:

            1-pole filters  = 6db/octave attenuation
                 A gentle effect.  You would typically use a 1-pole
                 filter for slightly modifying an already acceptable
                 tone, like cutting a bit of high-end from a string
                 sample.

            2-pole filters  = 12db/octave attenuation
                 A moderate effect.  2-pole filters were the standard on
                 older Oberheim synthesizers, and are noted for
                 "warming" the oscillator sound.  I normally use 2-
                 pole filters for analog string or horn pads, where I
                 want to affect the sound without making it sound
                 totally synthetic.

            4-pole filters = 24db/octave attenuation
                 A more drastic effect.  This was the domain of Moog.
                 4-pole filters are the tools of choice for growling
                 basses and popping synth lines.  But, used in
                 moderation, 4-pole filters can be successfully applied
                 in more traditional sounds.

            Finally, resonance.  In the Musician's Guide, the graph for
            a 4-pole lowpass filter w/resonance (page 14-11 in my copy)
            shows a peak just before the attenuation slope begins.  A
            slight peaking is a result of any 4-pole filtering, but the
            peak displayed is the result of resonance.  Resonance
            "pushes" a peak at the cutoff frequency - in essence
            emphasizing the cutoff, since all frequencies above it are
            attenuated.  This peak can be used to emphasize various
            tonal characteristics, and can be varied from +24db (an
            extreme peaking) to -12 dB (anti-peaking!).

            Resonance is heavily used on synths like the Roland TB-303
            and MC-202 for their characteristic sound.  And, as you
            might have already guessed, this is the control lever that
            Acid House guys always break off of their synths in
            excitement!

            While I've just written a bunch of text on the subject,
            filtering is best understood by playing with it.  So, let's
            get to the keyboard!



            Patch Run #1 - Cutoff Variation

            This "Patch Run" will utilize the first three patches,
            each prefixed with "[ddg]02/01".  You should load the
            patches into your K2000 (typically in the 200's bank) in
            order to work along with this tutorial.

            The first patch, labeled "[ddg]02/01 fil 1" is a common,
            sequence-oriented low synth patch, with a broad chorus
            effect.  Select this patch as current, and begin editing it
            (by hitting the [EDIT] button).

            You will notice that I'm using a simple algorithm that looks
            very similar to our standard analog model.  The second
            algorithm "block" is set as a "4-pole lowpass w/sep".
            What it doesn't say is that this is a resonant filter.  What
            is separation?  Well, it's a method for making a 4-pole
            filter act like two tightly-knit 2-pole filters.  For now,
            we will ignore the separation, and concentrate on standard
            4-pole effects.

            Press the [more>] soft button until you see an option for
            [F1 FRQ].  Hit this key, and the controls for frequency
            cutoff will be displayed.  To hear the effect of the cutoff
            frequency, play some notes while adjusting the "Coarse:"
            field's content.  I've used an oscillator that has a lot of
            high frequency content, so the effect of increasing and
            decreasing the cutoff is enhanced.  Playing octaves while
            adjusting this control can easily induce a trance on the
            listener, so beware!

            You should notice that the display of the course cutoff
            frequency is shown in both "note value" as well as
            "frequency".  This can help you adjust a sound
            specifically, either in the expected frequency content (like
            if you were dropping the high end of a string patch), or in
            emphasizing a note range (when making a bass patch shake the
            walls, for example).  The "Fine:" field allows you to
            further adjust the frequency, in cent (100ths of a semitone)
            range.

            Return the coarse and fine controls to their original values
            (A#4 466Hz / 0 ct).  Play octave C's across the keyboard.
            Did you notice the sound getting quieter as you go high on
            the keyboard?  As soon as the note exceeds the cutoff
            frequency (A#4 in this case), attenuation kicks in on the
            *fundamental* frequency - the base frequency of the note
            itself.  I won't get into a diatribe about audio physics;
            suffice to say that a note is made of a frequency and some
            number of higher harmonic frequencies - called overtones.
            When a filter's cutoff is greater than the note you are
            playing, you are attenuating these overtones - which will
            affect the timbre of the sound.  However, if a note's base
            frequency is greater than the filter cutoff, you will
            attenuate the entire tone, thereby making it sound quieter.
            This isn't a bad thing, it's just something to remember!
            This effect is why some really nice bass patches sound
            terrible when played high, and some high sounds sound
            horrible at the lower end of the keyboard.

            Suppose you want this patch to follow your playing,
            adjusting the cutoff frequency to roughly match the note you
            are playing?  This is what the KEYTRK (Key Tracking) field
            is for.  The amount of key tracking is listed in cents per
            key.  Since 100 cents = 1 semitone (i.e. one key), setting
            this value to 100 will cause the filter to change in direct
            relation to the key you are playing.  Patch "[ddg] 02/01
            fil 2" applies this principle to our synth patch.  You
            should now hear the high frequencies "track" with your
            playing, giving a brighter sound as you move higher on the
            keyboard.  Most lead synthesizer sounds will use key
            tracking to provide "in your face" filtering regardless of
            the range that you are playing.

            In some cases - especially when you are dealing with
            samples, you may not want to track the note played exactly,
            since the higher cutoff my expose graininess or sample
            distortion.  Applying a KEYTRK parameter less than 100
            ct/key will allow the filter to increase somewhat as you
            play higher, but will tend to cut more high frequencies as
            you play higher on the keyboard.  Conversely, you can apply
            a KEYTRK parameter greater than 100 in order to cause the
            sound to get brighter as you play higher - especially useful
            with radically distorted (on purpose) samples.  Try several
            KEYTRK options to get a feel for its effect.

            Return to the program selection list, and select the patch
            labeled "[ddg]02/01 fil 3".  Play a bass note with varying
            velocities.  Notice the filter "expanding" as you hit the
            note harder.  This is an example of velocity tracking.
            Enter the patch editor, and select the [F1 FRQ] controls.
            Notice that I've set the VELTRK field to 4800 cents - that's
            four octaves!  This means that, based on the velocity of
            your key hit, the filter will "open up" - increase in
            value - by as much as four octaves.  This gives you the
            ability to control the filter in real-time, which is great
            for techno sequencing.  However, there is another very
            important use for velocity tracking.  Many acoustic
            instruments "speak" differently based on how hard you
            blow, hit or strum them.  Velocity tracking is great for
            this effect - apply some VELTRK to make a horn patch
            brighter on a hit than on a legato passage, or a lead guitar
            patch bite harder on slammin' lead sections than when
            chording.  Again, experiment with the range of VELTRK values
            to become better acquainted with its effect.



            Patch Run #2: Resonance Variations

            Select the patch labeled "[ddg]02/02 fil 1".  This is an
            oddball, hollow sounding honk, with a bright gated reverb to
            enhance its ugliness.  This is just the sort of thing a good
            blipcore band might use as a Kraftwerkian cymbal.  Just
            don't play "Feelings" on it - this could hurt your K2000!

            In this Patch Run, we are going to explore resonance, and
            its relation to the cutoff frequency.  Edit this patch, and
            select the [F2 RES] soft key.  In this case, I have the
            resonance "cranked" to its maximum level of 24db.  This
            means that the filter will peak at the cutoff frequency with
            a 24db boost to that frequency.  In order to hear the effect
            of this radical resonance shift, vary the AMOUNT: field
            while banging on a middle C - notice the increase in the
            "hollowness" of the sound.  This is one of the effects
            that filter resonance has on a sound, especially when you
            are emphasizing the fundamental (primary) frequency of a
            sound.  Now, jump to the [F1 FRQ] page.

            Notice that I have the COARSE: field set to C5 532Hz.
            However, we were hitting the C4 (middle C) key, so why is
            the fundamental being emphasized?  Well, this is one of the
            joys of working with a sample-based instrument.  The sample
            I'm using is the internal "Waterphone" sample, and it
            appears to have a weak fundamental frequency, making the
            perceived fundamental one octave higher than played.  Often,
            when you are working on specific tonal filtering options,
            you will find that the samples do not seem to match the key
            being played.  This is often done to allow the maximum
            playable range of an instrument fit on a 61-note keyboard.
            What it means for a synthesist is "Use your ears, not your
            eyes!"

            To best hear the effect of resonance on the various
            frequencies of a tone, start banging on middle C again, and
            adjust the COARSE: setting upward slowly.  As you do, you
            will hear the resonant peak as it "hits" each harmonic;
            first at G5, then at D6, and so on.  Play the filter to its
            entire range.  You will notice a slight emphasis at C4 (the
            actual fundamental), and lower cutoff frequencies will have
            little effect other than to lower the overall volume.

            This patch brings up a point : that resonance is not a
            parameter that lives on its own, but is closely related to
            the cutoff frequency.  Again, if you look at the graphs in
            the Musician's Guide, you will notice that the peak created
            by resonance affects a small portion of the frequency range
            near the cutoff point.  When you are using resonance, make
            sure you are also paying attention to the cutoff - if you
            aren't getting the sound you imagined, it is probably a
            result of the *combination* of these parameters, and not
            just the resonance.

            Exit the editor, and select patch "[ddg]02/02 fil 2".
            This patch, as saved, is a typical rock synth-organ (there
            is a similar sound all over Van Halen's 5150 album), created
            using a saw wave.  Jump into the patch editor and look at
            the [F2 RES] page.  You should notice that I've created the
            patch with no resonance, but am adding up to 24db of
            resonance based on velocity (using the VELTRK: field).  This
            allows for a subtle organ-ish sound when played lightly, but
            a more nasal, synthy sound when hit harder.

            This patch is a real chameleon.  Move to the [F1 FRQ] page,
            and adjust the COARSE: cutoff frequency.  You will hear the
            patch change drastically, and you should be able to identify
            a number of synth sounds you've heard on records.  A saw
            wave, with appropriate filtering, is the basis of any number
            of popular sounds; but the quality of the filter is
            responsible for the success (or failure) of the sound.

            Patch "[ddg]02/02 fil 3" is an 808 kick drum emulation, as
            heard through a passing car.  All you can hear is the boom
            of the kicker boxes as the rappin' kids drive by.  It is
            created by using a sine wave, and pounding the resonance,
            with the filter cutoff right at the fundamental tone.  This
            patch brings up an unfortunate limitation of the K2000.  If
            you have a chance, get a real analog-head to show you an old
            beast with self-oscillating resonance.  On some instruments,
            an extreme level of resonance will actually feed back into
            itself, literally washing the entire sound in a squall of
            noise.  The K2000 filters can approach this wail, but never
            go "over-the-top" into self-oscillation.  Many popular rap
            and techno sounds are based on an analog filter at or near
            self-oscillation.  While you can (and this patch does) come
            close, it is sometimes not enough...

            Another aspect of the 808 kick drum is its variable filter
            cutoff and decay time.  You can control this patch to suit -
            filter cutoff is the COARSE filter variable, while the decay
            is part of the AMPENV variables.



            Patch Run #3 : Sound Effects

            "[ddg]02/03 fil 1" is a concept patch.  I wanted small
            alien voice in a crowd, with the ability to make the aliens
            get "excited".  Go figure.  To play the patch, fumble your
            fingers around the low end of the keyboard, then excite the
            aliens by moving higher on the keyboard.  Somewhere, there
            is a film to be made here...

            First, a bit of explanation.  I've set up the patch so it
            does not output the entire keyboard range - the notes are
            scaled to about 1 1/2 octaves over the range of 61 notes.
            You won't get any chords out of this baby!  However, this
            allowed me to control the voices without constraining myself
            to a few keys.  If you edit this patch, and look at the [F1
            FRQ] page, you will see that the key tracking is 25ct/key -
            a quarter of the normal value.  This allows the cutoff
            frequency to match my constricted note range.

            If you look at the [F2 RES] page, you will see that I've set
            up the ADJUST: field to a negative value, an "anti-
            resonance".  This means that there is a dip in frequency
            response at the filter cutoff.  However, I've also added key
            tracking at 1.50 dB/key - a healthy amount.  The end result
            is that the patch starts off with "anti-resonance", but
            resonance is added as I increase the MIDI note value.  This
            is what, along with the pitch change, give a timbre change
            to the voice as I move up the keyboard.  Reduce the key
            track value to 0.00, and notice the lessening of the change
            as you move up the keyboard.  Key tracking resonance is one
            of the subtle changes that can really liven up a patch.

            Play with the filter (as well as AMPENV) options of this
            patch to create a number of wild and pointless sound effect
            noises.  By the way - there's good money in this come
            Halloween!




            That's all for Tutorial #2.  I hope you enjoyed it.  As
            always, you can e-mail me at:

            ddg@wi.rr.com

            Thanks, and I'll type at you later!