A digitizing instrument is used for modifying pattern data and jitter and noise components of a communication signal. In a typical implementation, the midpoints of a rising edge slope and horizontal portion of the communication signal are determined and multiple digital data records are acquired at the midpoints. The data sample records are transformed to frequency components and the random jitter and noise, and periodic jitter and noise components are determined. A correlated pattern data and the jitter and noise components are matrix elements in a simulated signal channel having communication system elements. Each correlated pattern data and jitter and noise component may be modified for each of the communication system element. The selectively modified correlated pattern data and jitter and noise components are combined to produce a modified communication signal that is displayed as a numeric table, eye diagram or bit error rate presentation.