A mixing console has, for example, eight signal processing channels each for processing audio signals, and indicator units respectively for the signal processing channels. Each indicator unit is to display the channel number or the note name or another short message about the channel. Each channel is provided with a control element for commanding a control such as setting equalizer parameters. When a control element is actuated and held for more than one second, for example, such an actuation causes the designation of a channel about which a long message is displayed by using the indicator units for the eight channels in series.