ues are SO familiar that we convert them by inspection, namely, 2 and 4 mW. The sum is 6 mW. Now we take 10 log 6 to convert back to dBm again and the answer is +7.78 dBm. Remembering that there is an error when we work "3s" (3, 6, 9, 1, 4 and 7 values), we recalculated using a scientific calculator throughout. The answer was +7.76 dBm showing a 0.02-dB error. On occasion, we will have to combine a large number of input/outputs where each is of the same level. This is commonly done with frequency division multiplex equipment or with multitone telegraphy or data. Suppose we have an FDM group (12 voice channel inputs), where each input was -16 dBm. What is the composite output? This is stated as Composite power 16 dBm + 10 log 12, = 16 dBm + 10.79 dB, -5.21 dBm. The problem of adding two or more inputs in a combiner is pretty straightforward if we keep in the power domain. If we delve into the voltage or current domain with equivalent dB values, such as dBmV (which we cover in Section 15.3.2), we recommend returning to the power domain if at all possible. If we do not, we can open Pandora's box, because of the phase relationship(s) of the inputs. In the next section we will carry out some interesting exercises in power addition. C.7 dB APPLIED TO THE VOICE CHANNEL The decibel is used to quantify gains and losses across a telecommunication network. The most common and ubiquitous end-to-end highway across that network is the voice channel LEARNING DECIBELS AND THEIR APPLICATIONS (VF channel). A voice channel conjures up in our minds an analog channel, something our ear can hear. The transmit part (mouthpiece) of a telephone converts acoustic energy emanating from a human mouth to electrical energy, an analog signal. At the distant end of that circuit an audio equivalent of that analog energy is delivered to the receiver (earpiece) of the telephone subset with which we are communicating. This must also hold true for the all-digital network. When dealing with the voice channel, there are a number of special aspects to 