 the transmission may hop a hundred times a second, although at HF this will be much less. The transmitter will remain on each frequency for a given amount of time before moving on to the next. There is a small dead time between the signal appearing on the next channel, during this time the transmitter output is muted. This is to enable the frequency synthesizer time to settle, and to prevent interference to other channels as the signal moves. To receive the signal the receiver must be able to follow the hop sequence of the transmitter. To achieve this both transmitter and receiver must know the hop sequence, and the hopping of both transmitter and receiver must be synchronized. Frequency hopping transmissions usually use a form of digital transmission. Even when speech is used, this has to be digitized before 72 Newnes Guide to Radio and Communications Technology being sent. The data rate over the air has to be greater than the overall throughput. This is to allow for the dead time while the set is changing frequency. Direct sequence spread spectrum Direct sequence spread spectrum (DSSS) is a form of spread spectrum modulation that is being used increasingly as it offers improvements over other systems, although this comes at the cost of greater complexity in the receiver and transmitter. It is used for some military applications where it provides greater levels of security and it has been chosen for many of the new cellular telecommunications systems where it can provide an improvement in capacity. In this application it is known as code division multiple access because it is a system whereby a number of different users can gain access to a receiver as a result of a different 'code' they use. Other systems use different frequencies (frequency division muptiple access - FDMA) or different times or time slots on a transmission (time division multiple access - TDMA). Its operation is more complicated than those that have already been described. When selecting the required signal there has to be a means by which th