A DSL transmitter and receiver are disclosed that are capable of accurately measuring crosstalk in G.vector-compliant DSL lines resulting from a legacy line. During crosstalk measurement, the legacy line can be placed in a showtime mode. Once in showtime mode, a data signal designated for the legacy line is reduced in power spectrum density and/or spectrum to reduce its effects on crosstalk measurements. An O-P-Vector-1 signal is then superposed on the reduced data signal, and the resulting test signal is transmitted over the legacy line while sync signals are transmitted over neighboring G.vector lines. The receiver can then accurately measure the crosstalk on the neighboring lines, and report back to the transceiver for adjustments. A similar configuration can be performed in a diagnostic mode of the legacy line.