Choosing Speech Codecs for Mobile Communications
- To maximize the numbers of users on system it becomes very important to compress the speech. Bandwidth which is available to us is very limited so in design of digital mobile communication choosing the right speech codec becomeimportant.
- A balance must be struck between the perceived quality of the speech resulting from this compression and the overall system cost and capacity
- Depending on the technique used, different speech coders show varying degree of immunity to transmission errors
- Speech signals are represented by fewer and fewer bits, the information content per bit increases and hence need to be more safely preserved.
- Depending on their perceptual significance, bits in each group are provided with different levels of error protection through the use of different forward error correction (FEC) codes
- The choice of the speech coder will also depend on the cell size used
- Cellular systems operating with much larger cells and poorer channel conditions need to use error correction coding, thereby requiring the speech codecs to operate at lower bit rates.
- The type of multiple access technique used, being an important factor in determining the spectral efficiency of the system, strongly influences the choice of speech codec.
- The US digital TDMA cellular system (IS-54) increased the capacity of the existing analog system (AMPS) threefold by using an 8 kbps VSELP speech codec
- The type of modulation employed also has considerable impact on the choice of speech codecCDMA systems, due to their innate interference rejection capabilities and broader bandwidth availability, allow the use of a low bit rate speech codec without regard to its robustness to transmission errors
- Table 15.1 shows a listing of the types of speech codecs used in various digital mobile communication systems
Table 15.1 Speech Coders Used In Various Mobile Radio Systems