Nuts and Volts - February 2009

shown in Figure ³.

The Home Screen displays are:

USB Status: The “cactus” — six characters from the end of the first row is my low-res attempt at the USB symbol. It is displayed when mistralXG is connected to your PC. The next five symbols show the status of the user options (discussed later).

Error Status: The space to the left of the USB symbol shows the global error flag and is normally blank. If an error occurs, this changes to an “!” and the error screens become available.

Real-time MIDI Activity Monitor: The second row shows MIDI IN (top line) and MIDI OUT (bottom line) activity. Figure ²shows the data being transmitted from the PC on channels 1, 3, 5, 6, 7, and 10 (left to right). No data is being transmitted to the PC (all channels are low on the top line). The MIDI IN monitor remains active even when mistralXG is not connected to your PC.

The remaining screens allow you to access user options and are displayed by pressing the SELECT button. The SET button changes settings or performs some other useful action. From the Home and Splash screens, SET provides quick access to useful features (see Figure ⁴). No matter which screen is being displayed, after a few seconds without a button being pressed, the display returns to the Home Screen.

Using mistralXG

SELECT cycles through the various screens, as shown in Figure ⁴. Pressing SET toggles or otherwise modifies the setting for each particular screen. I’ve already discussed the Home Screen, so now let’s take a look at the others.

User Options

The first two screens allow you to select which MIDI stream is transmitted to the synthesizer. The first option flag on the Home Screen (“U” in Figure ³) shows which stream has been selected. Screen 1 lets you choose between the MIDI IN and WX IN inputs (Switch 1 in Figure ¹), while Screen 2 selects between the output of Switch 1 and USB data from the PC (Switch 2 in

Figure

¹). The display shows “M,” “W,” or “U,” depending on your choices. If USB is selected and mistralXG is not plugged into your computer, Switch 2 automatically selects your M/W choice, reverting to U when the USB connection is reactivated.

Screens 3 and 4 control the MIDI THRU and MIDI OUT streams respectively. When enabled, MIDI THRU receives the M or W stream coming from Switch 1. MIDI OUT echoes the MIDI stream coming from the PC. Selecting Disable in these screens switches their respetive streams off.

Screen 5 controls MIDI Running Status. This feature of MIDI reduces the amount of data transmitted. It works on the principle that if a status byte would be repeated, it can be omitted. So, if consecutive MIDI commands are all “Note on, channel 5” (0x94 as already seen), then the command byte is omitted for subsequent notes until a new status byte is required. This reduces the amount of data transmitted by 10% to 15%. Normally, Running Status should be enabled, but you could disable it if you think it may be causing problems for a receiving device.

Certain byte sequences are invalid MIDI commands. A receiving device should ignore these, so filtering shouldn’t be necessary. If you think invalid messages may be causing problems, however, the filter can be enabled in Screen 6. Invalid sequences are reported as errors by mistralXG, whether or not the filter is active.

Screen 7 is not normally displayed. It shows the error types that have occurred, and only becomes available when an error has been registered. In this case, the Home Screen global error flag is displayed (see Figure ⁴). Pressing SET while on Screen 7 shows the individual error screens. A second press of SET resets the specific counter.

After zeroing a counter, pressing SET once more allows you to clear all error counters and return to the Home Screen. If you’d rather examine the individual error

RESOURCES

Here are a few links that I have found useful. Links come and go, of course. If that happens, just load a few key words into your favorite search engine and you’ll find lots out there.

MIDI

MIDI specifications: www.midi.org (unfortunately, these have to be purchased, but web sources are almost as good). There are hundreds of MIDI-related sites on the Internet. These are just a couple of the top hits from Google:

http://ccrma-www.stanford

.edu/~craig/articles/linuxmidi/misc/ essenmidi.html www.computermusicresource.com/MIDI.Commands.html

USB

USB Specifications: www.usb.org. Here you can find both the USB 2.0 spec (650 odd pages!) and more specific documents, including the USB-MIDI spec.