A signal processing device comprising: an audio signal acquisition portion that is adapted to acquire an audio signal representing stereo audio content; an external signal acquisition portion that is adapted to acquire a stereo external signal representing external sound obtained by collecting ambient sound; an output signal generation portion that is adapted to generate, from the audio signal acquired by the audio signal acquisition portion and the external signal acquired by the external signal acquisition portion, an output signal representing stereo audio provided to a user; a mode setting portion that is adapted to set, as an operation mode of the output signal generation portion, at least an external mode corresponding to the generated output signal; and a fade control portion that is adapted to control an operation to generate the output signal by the output signal generation portion, in accordance with the operation mode set by the mode setting portion, a direction specification portion that, when the external mode is set, specifies a direction of an emission source of the external signal with respect to the user, based on the stereo external signal acquired by the external signal acquisition portion, wherein when the external mode is set, the fade control portion is adapted to cause the output signal generation portion to generate the output signal for one of the right ear and the left ear of the user from at least the external signal, and also to generate the output signal for the other ear from at least the audio signal in accordance with the direction of the emission source specified by the direction specification portion, and wherein the mode setting portion also is adapted to set, as the operation mode in place of the external mode, at least an audio mode corresponding to the output signal, and when the audio mode is set, the fade control portion causes the output signal generation portion to generate the stereo output signal from the stereo audio signal. an audio signal acquisition portion that is adapted to acquire an audio signal representing stereo audio content; an external signal acquisition portion that is adapted to acquire a stereo external signal representing external sound obtained by collecting ambient sound; an output signal generation portion that is adapted to generate, from the audio signal acquired by the audio signal acquisition portion and the external signal acquired by the external signal acquisition portion, an output signal representing stereo audio provided to a user; a mode setting portion that is adapted to set, as an operation mode of the output signal generation portion, at least an external mode corresponding to the generated output signal; and a fade control portion that is adapted to control an operation to generate the output signal by the output signal generation portion, in accordance with the operation mode set by the mode setting portion, a direction specification portion that, when the external mode is set, specifies a direction of an emission source of the external signal with respect to the user, based on the stereo external signal acquired by the external signal acquisition portion, wherein when the external mode is set, the fade control portion is adapted to cause the output signal generation portion to generate the output signal for one of the right ear and the left ear of the user from at least the external signal, and also to generate the output signal for the other ear from at least the audio signal in accordance with the direction of the emission source specified by the direction specification portion, and wherein the mode setting portion also is adapted to set, as the operation mode in place of the external mode, at least an audio mode corresponding to the output signal, and when the audio mode is set, the fade control portion causes the output signal generation portion to generate the stereo output signal from the stereo audio signal.