The 3-Valve Switching System

 

Tutorial home

 

 

                                                    

 

 

 

Most brass horns use a 3-valve switching system.  Although not immediately obvious, this system is closely related to the trombone slide.

 

Our example will be the baritone, pictured above.  This is basically a trombone that contains a 3-valve switching system instead of a slide.

 

The valves are numbered 1, 2, and 3.  Each valve is connected to a small length of additional pipe, and acts as a switch.  When a valve is up, air bypasses the additional pipe.  When a valve is depressed, the air is redirected through the additional pipe.  This makes the entire bore a little bit longer, and lowers the instrument’s pitch.

 

 

 

Valve details

 

In the baritone pictured above, valve 1 is on the right, valve 2 is in the center, and valve 3 is on the left.

 

 

The reason a baritone was chosen (as opposed to a trumpet or tuba) is because a baritone is tuned exactly like a trombone, which we covered previously.  When all valves are up, the pipe’s fundamental is Bb1, and Bb2 is the lowest normally-played pitch.  When all valves are down, the pipe’s fundamental is E1, and E2 is the lowest normally-played pitch.

 

So it turns out that the various valve combinations correspond exactly to the positions on a trombone slide.  The chart below compares them.

 

Trombone Position

Valve 1 Position

Valve 2 Position

Valve 3 Position

1

Up

Up

Up

2

Up

Down

Up

3

Down

Up

Up

4

Down

Down

Up

4

Up

Up

Down

5

Up

Down

Down

6

Down

Up

Down

7

Down

Down

Down

 

You’ll notice that position 4 on the trombone slide can be attained through two different valve fingerings.  The top-most one (Down/Down/Up) is preferred because the other (Up/Up/Down) is slightly flat.

 

 

 

Tuning problems

 

If you’re familiar with equal temperament, you may be skeptical that 7 equally-tempered notes can be obtained with only 3 lengths of pipe.  And your skepticism is warranted.  It turns out that the bottom two notes are sharp, the last one notoriously so.  The trombone doesn’t have this problem, because its slide can be adjusted to any position.

 

To correct this, many 3-valve instruments have a finger-ring attached to valve 3’s pipe.  Since all of the valve pipes are removable for cleaning, the musician can put a finger through the ring, and extend the length of the pipe slightly, lowering the pitch until it’s correctly tuned.

 

 

 

Let’s make a baritone

 

Let’s start with the trombone patch, and make a list of the things that can remain unchanged:

 

 

Now, let’s make a list of the things we have to change:

 

 

It sounds like a tall order, but we’ll take it one step at a time.

 

 

 

Creating the valves and pipes

 

Below is a patch fragment that contains a complete baritone bore.  It begins with a fixed pipe tuned to Bb1, then three valves with switched pipes, and finally a reflection filter and gain control.  The fixed pipe, reflection filter, and gain control are identical to the trombone.  It’s the valves that are different.

 

 

 

 

Each valve contains three modules:

 

  1. A pan module, which acts as a switch.
  2. A comb filter module, which models the length of pipe attached to the valve.
  3. A mixer module.

 

The valve’s up/down position is controlled by the modulation input on the pan module.  When the input is zero, the pan module sends its signal directly to the mixer, bypassing the comb filter.  When the input is 64, the pan module sends its signal through the comb filter, increasing the length of the pipe.  The reason we’re using an analog switch like the pan module is so we can have a smooth transition between the two states.

 

How can we determine the pipe lengths for the valves?  It’s not easy, especially because we’re using comb filters instead of simple delay lines.  Comb filters were chosen because they have pitch-modulation inputs, which we can use to tune our model to the outside world.

 

Here’s the process of determining the length of valve 1’s pipe (which lowers the main pipe by two semitones):

 

  1. Calculate the length needed to play Bb1 (when the valve is up).  Bb1 is 58.270 Hz, and the period is the reciprocal of that, so the length is 1 / 58.270 Hz, or 0.01716 seconds.
  2. Calculate the length needed to play Ab1 (when the valve is down).  Ab1 is 51.913 Hz, and the period is the reciprocal of that, so the length is 1 / 51.913 Hz, or 0.01926 seconds.
  3. Subtract the shorter length from the longer length (finds the length of the additional pipe needed).  0.01926 seconds minus 0.01716 seconds is 0.00210 seconds (about 2 milliseconds).
  4. Figure out what frequency this is, so we can tune the comb filter.  The frequency is the reciprocal of the length, so the frequency is 1 /  0.00210 seconds, or about 475 Hz.  Since the G2’s comb filters can’t be tuned with that kind of precision, we’ll fine-tune the filter with an attenuated constant module.

 

 

 

Creating a 3-valve switching system

 

There are a lot of ways to do this on the G2.  Given the affinity these patches have had for SeqCtrs, you can probably guess they’ll come into play.

 

The SeqCtr module not only has an analog output for each stage:  it also has a digital Gate output.  So, we’ll simply expand the technique used in our trombone.  Instead of one SeqCtr per note (representing the slide position), we’ll have three.  The gate output on each one will correspond to a valve.

 

The patch fragment displays how they’re used.

 

 

 

 

If you look carefully, you’ll see that we’re using the alternate fingering for trombone position 4, Up/Up/Down.  We’ve taken a little liberty here.  The reason is to make the instrument a little easier to precisely tune.  The chart below explains the strategy.

 

Trombone Position

Valve Positions

Tuning Strategy

1

Up/Up/Up

Tune Bb bore

2

Up/Down/Up

Tune valve 2 to A

3

Down/Up/Up

Tune valve 1 to Ab

4

Up/Up/Down

Tune valve 3 to D

5

Up/Down/Down

Fine-tune valve 3 to Db

6

Down/Up/Down

Fine-tune valve 3 to C

7

Down/Down/Down

Fine-tune valve 3 to B

 

 

 

Correcting valve 3’s length

 

According to the chart above, we’ll tune valve 3 to D.  The other notes that use valve 3, Db, C, and B, will each need a little additional tuning correction.

 

The patch fragment below will do this.  It accepts the gate outputs from the SeqCtrs, detects D, Db, C, and B, and supplies a tuning correction to the length of valve 3.

 

 

 

 

Let’s see it all

 

The patch here is a functioning baritone.  It’s pretty big, so there’s no picture posted.  Variation 1 is controlled by an envelope generator, and variation 2 by a breath controller.  Notice that the note-to-note transitions are much smoother than for the trombone patch.