The SHS-D400 IR headphones are designed to be used with Alpine's TMX-310U overhead monitor. The SHS-D400 uses digitally modulated IR transmission, unlike the classic IR Transmitter/IR Headphone pair where the signal is transmitted in analog form. Thanks to the new technology the sound transmission quality has improved significantly, in particular the signal-to-noise ratio has increased from 55 dB to almost 90 dB. Another advantage of digital wireless transmission is the ability to transmit more audio information, with the SHS-D400 receiving up to four independent stereo channels. With the TMX-310U, this allows you to select, with the headphone button, either the audio corresponding to the video played on the screen by the monitor's built-in player or any of two external audio sources connected to the AUX1 or AUX2 input.