1. Field of Invention
The present invention relates to radio communication equipment. In particular, the present invention relates to a system for converting voice and music signals into their digital equivalents, transmitting this digital information over air, receiving the digital signal, translating the digital information into FM (frequency modulation) format, and re-transmitting the information as an FM radio signal.
2. Description of the Related Art
Typical prior AM (amplitude modulation) and FM (frequency modulation) broadcast systems generally comprise a main RF (radio frequency) transmitter with either local facilities or remote studios which control inputs to a transmitter. By using an audio mixing apparatus, an operator typically selectively combines the desired analog inputs from turntables, microphones, tape decks, or disk players. All these analog signals are subsequently used to modulate the RF carrier of an AM or FM transmitter.
FM is a method by which the frequency of the carrier wave is made to change in accordance with an audio voltage. During AM, the carrier frequency remains constant, but its amplitude is made to change in accordance with the audio signal. The essential difference between FM and AM is that in the AM modulation period the amplitude of the RF carrier rises and falls in accordance with an impressed audio frequency signal; whereas in FM, during the modulation period, the frequency increases and decreases as the audio frequency changes, but the amplitude of the RF carrier remains constant. Both AM and FM are inherently analog modulation systems.
There are numerous prior methods to modulate AM and FM transmitters. But for AM or commercial FM, all prior systems use an analog signal input to modulate and transmit.
Every radio transmitter requires a certain amount of the radio spectrum for its operation. The electromagnetic spectrum has been broken down into "channels". The standard AM broadcast band extends from 535 kc to 1,605 kc. Accordingly, there are one hundred and seven AM broadcast band channels, each occupying 10 kc of bandwidth. The commercial FM broadcasting band extends from 88 mc to 108 mc on the electromagnetic spectrum. One hundred FM broadcast band channels each occupy 200 kc of bandwidth.
Many (AM and FM) channel assignments are duplicated across the Country. Any overlapping of shared channels within a common broadcasting area produces interference. Also, any deviation from assigned operating frequency produces interference to adjacent channels.
A transmitter modulates an RF carrier frequency. The modulated RF carrier frequency is subsequently transmitted over air from an antenna. The transmitted (AM or FM) signal is received by a receiver, which then converts the signal back to the original (analog) form which had been supplied to the audio mixer.
Although the broadcast area of AM is greater than FM, AM is inherently noisier than FM due to the narrower bandwidth of any given AM channel and the inherent susceptibility of the receiver to amplify modulated signals that are produced by electrical discharges such as lightning and high power transmission lines.
Although FM typically offers a clearer signal upon reception than does AM, FM requires a wider bandwidth and, accordingly, occupies more of the electromagnetic spectrum per channel.
Noise can also be introduced by the equipment itself (i.e. the RF amplifiers, the mixers, etc.), by background noise from the microphone, or by other RF generators.
The broadcast range of FM is inherently limited to local area coverage. FM signals tend to fade when there is a physical barrier between the transmitting antenna and the FM receiver.
Although both AM and FM can be relayed, neither can be relayed without introducing noise to the signal.