A Digital Codec Simulation Facility

This paper describes a minicomputer facility developed to aid in the evaluation of the grade-of-service impact of digital codecs. Two minicomputers are interfaced with 15 bit A/D and D/A converters so that an analog signal such as speech can be quantized and stored in real time at sampling rates up to 72 kHz. The acquired digital samples can then be processed by various PCM, ADPCM, and ADM algorithms programmed on the system. After the processing is completed, the facility can automatically generate high quality audio tapes which are suitable for subjective evaluation in an absolute judgment or balance test format. The two minicomputers operate on a stand-alone basis so that the capability exists for simulating either similar or dissimilar tandem encodings in a synchronous or asynchronous mode. A few sample outputs of the facility are also given in the form of subjective test results for single and tandem encodings of μ255 PCM and Nearly Instantaneous Companded (NIC) PCM.