a vehicular audio system receives audio inputs from audio sources and a radio receiver . analog audio is converted to digital , and digital audio remains natural digital . the receiver front end converts a radio signal to an intermediate frequency then an adc converts that to a digital signal . the inputs that are converted to digital are selectively mixed with each other and with the natural digital signals . this allows for sounds from multiple sources to be heard simultaneously so that a telephone ring may be provided without requiring background music to be interrupted and for uses such as voice by microphone over a music tape . a reference frequency to the receiver front end of 7 . 2 mhz is particularly beneficial for noise reduction and consequent mixing of digital audio at 48 khz , the standard frequency for typical digital audio inputs .