Adding More Pipes
A single pipe isn’t a very
interesting sound. An organ’s richness
comes from playing multiple pipes when a key is pressed.
We’ll follow this path in
two steps. First, we’ll add a single 2’
pipe to the patch. Second, we’ll build a
patch with five pipes per key.
What Are Those ‘ Markings, Anyway?
Let’s take a moment to talk
about how pipes are named. A “rank” of
pipes is typically a group of 61 pipes, tuned to the chromatic scale. 61 pipes are 5 octaves, and most organs have
5-octave keyboards.
A typical 5-octave organ
keyboard plays the notes between C2 and C7.
A piano has two additional octaves, one below and one above, and plays
the notes between C1 and C8.
It turns out that if you
build a pipe that plays the C2 pitch, it’s about 8 feet high. And that’s where the 8’ name comes from: it’s the length of the longest pipe in the
rank. There’s more to it than that, but
that’s the historical background behind the naming convention.
The rich sound of a pipe
organ is usually achieved by playing multiple pipes at once, tuned apart by
octaves, or fifths. It’s like having
multiple oscillators on a synthesizer, and tuning them apart by octaves: it makes a full sound.
An 8’ rank of pipes is the
“standard” length. That is, the pitches
correspond to the middle 5 octaves of a piano keyboard. Many pipe organs have other ranks available: 16’, 4’, 2’, 1’, and 2 2/3’ are common
lengths. These names also correspond to
the longest pipe in the rank. That is, a
16’ rank of pipes will play pitches that are one octave lower than an 8’
rank. A 4’ rank will play pitches that
are one octave higher than an 8’ rank. A
2’ rank will be two octaves higher, a 1’ rank will be 3 octaves higher, and a 2
2/3’ rank will be one octave and a fifth higher (between the 4’ and 2’ pipes).
Hey, these names match the
drawbar names on a
Adding a 2’ Pipe
Our first change will be the
addition of a 2’ pipe. This is a pipe
that’s tuned two octaves above the first pipe.
It makes a distinctive pipe organ sound, without requiring a lot of
additional DSP power. Notice that the
comb filter delay for the 2’ pipe is tuned two octaves higher than the 8’ comb
filter delay.
The patch is below. The second picture is the FX area which
contains a pipe mixer and a reverb module.
Creating a decent-sounding
pipe sound across five octaves presents some challenges. These are:
- High-pitched pipes won’t play unless the lowpass
filter is opened up. But the
lowpass filter needs to be closed down at the bottom of the keyboard, so
the notes don’t sound too “clarinetty”.
This is why the lowpass filter has its own SeqCtr. Remember, any manipulation of this
filter’s frequency requires re-tuning.
- The highpass filter is necessary to remove the
DC “thunk” at the start of a note.
This, too, needs some tracking.
If its cutoff frequency is too low, the note will “thunk”. But if it’s too high, the note will
sound thin. Ideally, another SeqCtr
could be used to fine-tune the filter, but that would eat up more DSP.
A Five-Pipe Patch
Now, we’re going to be more
ambitious, and attempt a five-pipe patch.
These pipes will be tuned in octaves:
16’, 8’, 4’, 2’, and 1’. Creating
the basic patch is as simple as cutting and pasting one of the existing
pipes. The time-consuming work is that
each pipe must then be filtered and tuned until it sounds acceptable across the
keyboard.
Here is where we’ll run into
the G2’s polyphony limitations. A single
one of these five-pipe notes requires an entire DSP. That means a standard G2 can play only four
notes at a time. If we add an FX area
containing reverb, the polyphony is reduced to three notes. Ugh.
That’s not enough for any serious organ music. This patch will benefit greatly from the G2’s
expansion card.
A portion of the patch is
shown below. We’ll mix the pipes using
the 6-channel stereo mixer with pan.
That lets us pan each pipe in the stereo field, if we wish. All 8 variations are programmed with
different combinations of pipes.
The above patch contains the
reverb module, so is limited to 3 notes on an unexpanded G2. For those with outboard reverb units, here is the same patch without the reverb. It provides an additional note of polyphony.