The DS0 test feature enables testing on each of the 24 channels of the T1 line. The receive channel is decoded, and the VF level and frequency are measured and displayed. The signal is also connected to a speaker, enabling the testing technician to make an audible assessment of the signal quality of the circuit. If the circuit is out-of-service, the tester can insert a test tone on the transmit channel for measurement either at a remote location with another test set, or locally with a loopback. VF is displayed above 100 Hz and up to 3000 Hz.