a system and method are provided which allow a user to obtain a plot of the amplitude response of a channel such as a telephone line in the band occupied by a modem signal without interrupting or affecting the data traffic through the modem . no test tones are required for m e spectral measurements , i . e ., the system extracts spectral data from the actual received modem signal that has been shaped by the amplitude characteristics of the line . the amplitude characteristics are displayed on an external oscilloscope using a modem eye pattern display .