Development of Circuit Switched Digital Capability (CSDC) in the United States and the general evolution toward an integrated digital services network worldwide has created interest in the development of 56 or 64 kbps digital coders for audio signals with 6 or 7 kHz bandwidth. To date there have been a number of coders proposed but no formal testing of them. Furthermore, a method for testing them has not been proposed. In this paper we discuss the problem of testing such coders and describe a series of both objective and subjective tests which can be performed using a real-time coding system. We discuss the results of these tests when performed on a selection of coders which we have implemented.