Media encoding is not an Internet technology. Hence, it is different from most of the technologies described so far. While many of the technologies we discussed earlier were originally developed for the Internet and later adapted to work in the IMS, media-encoding technologies were developed for other environments. For example, speech coding was developed with circuit-switched telephony in mind.
Owing to this we do not follow the same structure as in previous chapters, where we described how each technology is used on both the Internet and in the IMS. In this chapter we describe how speech-encoding technologies evolved from the simple codecs used in the fixed PSTN to the advanced codecs used in today’s cellular networks. In addition, we look at video-encoding technologies.
15.1 Speech Encoding
Audio encoding consists of converting an analog audio signal into a digital signal. This digital signal is transmitted over the network and decoded at the receiver’s side, as shown in Figure 15.1. If the audio signal consists of human speech, the process just described is referred to as speech encoding. We focus on speech encoding in this section.
The algorithm used to encode and decode the digital signal is referred to as a codec. Two important characteristics of any codec are its speech quality and its bandwidth. ...