A method of processing a signal including a speech signal and an audio signal, the method comprising: receiving the signal, extension information and spatial information indicating an attribute of a plural channel signal (L, R) to upmix a downmix signal; decoding the audio signal in a frequency domain according to an inverse modified discrete coefficient transform; decoding the speech signal in a time domain by using a linear prediction coding scheme; reconstructing the downmix signal by expanding the decoded audio signal and the decoded speech signal to a whole band downmix signal using the extension information, the downmix signal being generated from the plural channel signal (L, R); obtaining an inter-channel phase difference, IPD, coding flag (bsPhaseCoding) indicating whether an IPD value is used in the spatial information from a header of the spatial information; obtaining an IPD mode flag (bsPhaseMode) based on the IPD coding flag (bsPhaseCoding) from a frame of the spatial information, the IPD mode flag (bsPhaseMode) indicating whether the IPD value is used in the frame of the spatial information; obtaining the IPD value of a parameter band (pb=0,..., pb=4) of a parameter time slot (,,...) in the frame, based on the IPD mode flag (bsPhaseMode); smoothing the IPD value by modifying the IPD value by using the IPD value of a previous parameter time slot; and generating a plural channel signal by applying the smoothed IPD value to the downmix signal, wherein the spatial information is divided by the header and a plurality of the frames, wherein the IPD value indicates a phase difference between two channels of the plural channel signal (L, R), wherein the parameter time slot (,,...) indicates a time slot (1, 2, 3, 4,..., N) to which the IPD value is applied, wherein the parameter band (pb=0,..., pb=4) is at least one sub-band of a frequency domain including the IPD value, wherein the IPD value is received when the ratio between the IPD value and an inter-channel level difference, ILD, value exceeds a threshold, and wherein the ILD value indicates a level difference between two channels of the plural channel signal included in the downmix signal. receiving the signal, extension information and spatial information indicating an attribute of a plural channel signal (L, R) to upmix a downmix signal; decoding the audio signal in a frequency domain according to an inverse modified discrete coefficient transform; decoding the speech signal in a time domain by using a linear prediction coding scheme; reconstructing the downmix signal by expanding the decoded audio signal and the decoded speech signal to a whole band downmix signal using the extension information, the downmix signal being generated from the plural channel signal (L, R); obtaining an inter-channel phase difference, IPD, coding flag (bsPhaseCoding) indicating whether an IPD value is used in the spatial information from a header of the spatial information; obtaining an IPD mode flag (bsPhaseMode) based on the IPD coding flag (bsPhaseCoding) from a frame of the spatial information, the IPD mode flag (bsPhaseMode) indicating whether the IPD value is used in the frame of the spatial information; obtaining the IPD value of a parameter band (pb=0,..., pb=4) of a parameter time slot (,,...) in the frame, based on the IPD mode flag (bsPhaseMode); smoothing the IPD value by modifying the IPD value by using the IPD value of a previous parameter time slot; and generating a plural channel signal by applying the smoothed IPD value to the downmix signal, wherein the spatial information is divided by the header and a plurality of the frames, wherein the IPD value indicates a phase difference between two channels of the plural channel signal (L, R), wherein the parameter time slot (,,...) indicates a time slot (1, 2, 3, 4,..., N) to which the IPD value is applied, wherein the parameter band (pb=0,..., pb=4) is at least one sub-band of a frequency domain including the IPD value, wherein the IPD value is received when the ratio between the IPD value and an inter-channel level difference, ILD, value exceeds a threshold, and wherein the ILD value indicates a level difference between two channels of the plural channel signal included in the downmix signal.