Part 1: Setup

In order to understand the rest of the lab, it is important to first understand the signal that is generated by the monochord assembly and interpreted in Pure Data (pd). The monochord consists of a tightly stretched string which passes between two magnets. As the string vibrates, the magnets produce an electrical signal along the string. Physically, the signal corresponds to the velocity at which the string passes between the magnets. Thus, if the string vibrates more quickly, the electrical signal will oscillate more quickly. Similarly, if the string is plucked so as to generate a louder sound, the electrical signal will be stronger. (If the optical pickup is used instead of the electromagnetic pickup, then the measured signal corresponds to the string displacement instead of the velocity and similar statements may be made.) This electrical pickup signal is amplified by the sound card input and read into the computer as a sound wave. Roughly speaking, the electrical signal that is visible on the computer corresponds to sound wave received by your ear.

1. Begin by opening pd if the software is not already open. In Windows, this will likely involve clicking the Start button and selecting Start $\rightarrow$ Programs $\rightarrow$ Pure Data $\rightarrow$ Pure Data.

2. Download the lab patch 1-1 (1-1.pd) and save it to a local directory. Then open 1-1.pd with pd. One way to do this is to select Open from the File menu.

   Check to make sure that the patch is not in ``edit mode'' by clicking on the Edit menu in the lab patch. If there is a check mark for edit mode (at the bottom of the edit menu), click Edit $\rightarrow$ Edit Mode in order to clear the check mark.

3. Patch 1-1 contains three displays, but depending on the resolution of your monitor, you may need to scroll down to see the third display. The top two displays allow the user to record a 10 second sound clip. The bottom display represents a 200 ms sample that is automatically updated to reflect the signal currently being received by the sound card.

   Enable the sound input by checking the ``compute audio'' box in the upper right corner of the main pd window, or by selecting Media $\rightarrow$ audio ON in the main pd window. Sound can also be toggled easily on Windows or Linux by pressing Ctrl-/ to turn sound processing on and Ctrl-. to turn sound off. Click the ``Begin Display'' button to enable the patch. At this point, pluck the string to confirm that pd is receiving an audio input. This will be confirmed by a waveform appearing in the waveform display window (bottom). If a waveform still does not appear, the pd patch is not receiving a signal.