A transmitting accessory couples with a portable electronic device such that audio signals or data from the device can be digitally transmitted to a remote receiver. The accessory includes an input port for receiving audio data from the portable electronic device. A digital transmitter transmits digital data corresponding to the audio signals or data from the portable electronic device to the remote digital receiver. If the audio signal is an analog signal, the accessory uses encoding circuitry to digitally encode the analog audio signal for transmission. The audio signal may be encoded into an HD radio format for reception by an HD radio receiver. The user inputs of the portable electronic device can preferably be used to control functions of the accessory and the device's display to display the selected transmission frequency or channel.