ounds is usually less than those lower in frequency. Frequency shift keying Many signals heard on the bands will employ a system called frequency shift keying or FSK to carry digital data. Here the frequency of the signal is changed from one frequency to another, one frequency counting as the digital one (mark) and the other as a digital zero (space). By changing the frequency of the signal between these two frequencies it is possible to send data over the radio. FSK is widely used on the HF bands. To generate the audio tone required from the receiver, a beat frequency oscillator must be used. Accordingly to obtain the correct audio ones the receiver must be tuned to the correct frequency. Often at frequencies in the VHF and UHF portion of the spectrum a slightly different approach is adopted as frequency stability or accuracy may be a problem. An audio tone is used to modulate the carrier and the audio is shifted between the two frequencies. Although the carrier can be amplitude modulated, frequency modulation is virtually standard. Using audio frequency shift keying (AFSK) the tuning of the receiver becomes less critical. When the data signal leaves the receiver it is generally in the form of an audio signal switching between two tones. This needs to be converted into the two digital signal levels. This is achieved by a unit called a modem. This stands for MOdulator/DEModulator. Audio tones fed into the modem from the receiver generate the digital levels required for a computer or other equipment to convert into legible text. Conversely it is able to convert the digital signals into the audio tones required to modulate a transmitter to transmit data. Modulation 63 The speed of the transmission is important. For the receiver to be able to decode the signal it must be expecting data at the same rate it is arriving. Accordingly a number of standard speeds are used. The unit used for this is the baud. One baud is equal to one bit per second. Codes are used to enable the series of marks and spaces or ones and zeros 