Appendix I:
Analog and Digital Transmission Signals

Some researchers have suggested that digital transmission signals, under certain circumstances, may be more likely to produce nonthermal effects with consequences for human health than analog signals. Voice communications systems have traditionally relied on analog signals to transmit messages. Using analog technology, voice messages are electronically reproduced and amplified while being sent to the receiver of the message. A problem with analog amplification is that the voice message is imperfectly reproduced and tends to pick up a "noise" through amplification.

To reduce this noise in voice communications, engineers have developed the technology to convert voice messages into "digits" that is, sound is converted into a pattern of 0's and 1's representing the intensity of sound at points in time. Because the digitized message is repeated exactly as it is transmitted to the receiver, no additional noise is added. In addition to better sound quality, digital technology affords more efficient use of radio-frequency channels. When sound data are converted into digital bits, information is more compact. Thus, capacity is added to the radio waves over which the data are transmitted.

The cellular telephone industry's efforts to implement digital technology have focused primarily on providing multiple access over one frequency, or channel, by either (1) code division multiple access (CDMA) or (2) time division multiple access (TDMA).

• Under TDMA, several users share the same frequency, or channel, by transmitting portions, or bursts, of data in short time intervals. To do this, the signal is turned on and off 50 times per second, causing the signal to appear to pulse.
• Under CDMA, several users share the same frequency, or channel, at the same time. Messages are transmitted digitally as various sequences of 0's and l's with a special code attached so that only the intended receiver is able to decode the message.