8.3 Digital Modulation

When digital modulation techniques are used, speech signals must be coded into a format suitable for digital transmission. This can be accomplished by using PCM, DPCM, and delta modulation [6, 7]. A digitally controlled companding scheme [8, 9] can be used to obtain a good telephone link with a bit rate of 32 kb/s, which appears to meet CCITT specifications. For bit rates as low as 20 kb/s, other types of modulation schemes have been reported that are effective in providing intelligible speech signals. Where companded delta modulation is used, a rate of 32 kb/s yields a baseband snr of at least 30 dB over a dynamic range of 45 dB. Figure 8.10 illustrates the snr for various bit rates and input signal levels [8]. Recently, using linear predictive coding (LPC) requires only 2.4 kb/s. The description of LPC is shown in Sec. 14.5.

Figure 8.10: Output signal-to-noise ratio for varying bit rates and input signal levels for delta modulation.

Sampled speech data at a bit rate f _s can be transmitted over a radio channel by either AM, PM, or FM. From the viewpoints of power economy and system simplicity, only two-level systems are considered. Also, the filter shaping is performed at the transmitter, whereas the receiving filter is used only for channel selection purposes.