A Subjective Comparison of Selected Digital Codecs for Speech

In recent years, a great amount of interest has been expressed in the literature on the subject of efficient encoding of voiceband signals. Much of this interest stems from the economics of bandwidth reduction t h a t are possible with efficient codecs. Waveform codecs such as differential PCM (DPCM), adaptive differential PCM (ADPCM), delta modulation (DM), and adaptive delta modulation (ADM) are considered for general use in telephone networks because, among other reasons, they often are a reasonable compromise between the bandwidth required of the transmission channel and the terminal complexity at the ends of the channel. Aside from the economics, there is the issue of degradations introduced by a codec in speech and voiceband signals. In fact, a signal may undergo a number of encodings by codecs of different types as it progresses through the network. An example of this is the hypothetical network of Fig. 1. Here, a mixture of analog and digital switching and transmission facilities are represented. The toll portion of the network incorporates analog toll switches such as the No. 4 crossbar and the digital No. 4 ESS switch. T h e D channel banks and VIF terminals are shown in order to point out where analog-