Managing Audio with Pd

Ready to turn your Linux box into the ultimate audio effects machine? With the Pd environment, you can build reverb and more, visually.
Using Messages

Now, let's try a more complex example, making a patch that adds two numbers together and displays the result. Make a patch like the one shown in Figure 7, using two number boxes at the top, an object box in the middle and a number box on the bottom. Next, exit edit mode (Crtl-E again), and try changing the numbers at the top. You can do this by clicking and typing or clicking and dragging; move up to increase the number and move down to decrease it. As you've probably noticed, changing the number on the left makes the sum immediately change, but changing the number on the right does nothing. Why?

Figure 7. Adding Two Numbers

Nearly all objects in Pd treat their left-most inlet as hot, meaning that any changes in their value affects an immediate change in the output. The other inlets are cold. Changes in their values don't trigger any change in the output. The new value simply is put into storage until a computation is triggered by the hot inlet, at which point the new value is used.

But, what if you do want a change in a cold outlet to trigger a change in output? One way to accomplish this is to inset a message box. In Figure 8, I connected the outlet of the number box on the right to the inlet of a message box. The outlet of that message box then is connected to the hot inlet of the addition object box below it. When message boxes receive any message at all on their inlets, they send all their contents as a new message on their outlets. So when the right-most number box is changed, it sends a message to the bang message box, which then sends a bang message to the addition object box. Bang messages mean “Do something!”, so any object box receiving one on its hot inlet immediately performs whatever computation it's told to do. We use this behavior here to make our addition object box act as though it has two hot inlets.

Figure 8. The Naïve Way to Deal with Hot and Cold

But wait, doesn't that mean the bang message has to arrive after the number? If it doesn't, the patch won't work, right? Well, yes; depending on the order in which you made your connections, you already might have noticed that it doesn't. Indeed, in Figure 8 the numbers don't add up precisely because of this problem. So, we need a way to ensure that the bang message arrives after the number. An easy way to do this is to insert a delay, as shown in Figure 9. Interestingly, a delay of 0 actually works. The message simply is delayed by one DSP cycle, thus ensuring that the bang message arrives second.

Figure 9. Dealing with Hot and Cold the Right Way

Audio Processing

The most basic audio function is input and output. The adc~ tilde object, standing for analog-to-digital converter, performs the first task; and the dac~ object, digital-to-analog converter, performs the second. Both objects operate on the first two channels by default. If you want to change this—for instance, if you have a multichannel sound card such as a Hammerfall HDSP—you can enter channel numbers as arguments, and the respective channels then are mapped to their respective inlets or outlets. Figure 10 shows a simple example where stereo input is flipped and routed to output. Because stereo input is the default, the channel numbers in the example are redundant, but we've included them anyway for demonstration purposes.

Figure 10. Flipping Left and Right in a Stereo Signal

Sound data is a sequence of numbers at a specific sample rate, so it's possible to apply arithmetic operators to sound data. All you have to do instead is add a tilde to the end of the operator you want to use. For instance, Figure 11 turns a stereo signal into a mono one by adding the left and right channels together. Another useful operator is multiply, *~, which acts as a gain control. Remember, though, signals clip when output to hardware if they are beyond the values –1 and 1.

Figure 11. Turning Stereo into Mono

Figure 12 shows a more complex example. You might want to turn your speakers down some before you run this one, because it's loud. First, the osc~ object at the top is a sine-wave generator, in this case running at 440Hz. This signal is split into two, the left side going directly to the addition and the right side first going through a multiplication.

Now try entering –1 into the number box. The sound stops, why? If you remember your wave physics from high school, you know that waves can cancel one another out. In this case, the –1 creates a perfect inverse of the original signal. So where the original is at 1, the inverse is at –1. When you add these two signals together you get 0, silence. You also can try holding down the Shift key and dragging on the number box; the numbers should change slowly enough that you can hear the sound getting quieter as you approach –1 and then finally stopping.

Figure 12. Using Inversion to Cancel Two Signals