The same technology that makes it possible for you to view this Web site is also being used on the air. Digital modes can organize information into packets that contain address fields, information about the transmission protocol being used, error detection code, a few hundred bytes of data, and bits to indicate where each packet begins and ends.
Instead of transmitting messages in continuous streams, packet modes break them into packets. At the receiving end, the different packets are re-assembled to form the original message. If a packet is missing or received with errors, the receiving station can request a retransmission of the packet. Packets can be received out of sequence or even from multiple sources (such as different relaying stations) and still be assembled into the original message by the receiving station.
While packet modes have mainly been used to send text, any information that can be converted into digital form---sound, graphics, video, etc.---can be transmitted by digital modes.
Another advantage of packet modes is that packets can be addressed to specific stations in the address field of each packet. Other stations will ignore packets not addressed to them.
The big disadvantage of packet modes is the complexity of the necessary receiving and transmitting gear. The frequency space occupied is directly proportional to the speed at which messages are transmitted, and radio digital modes are very slow compared to their Internet equivalents. The slowest Web connection via the Internet is 14,400 baud (14.4K), while the maximum practical digital mode rate via radio is 9600 baud (9.6K). On frequencies below 30 MHz, it is even slower; rates are usually restricted to just 300 baud (0.3K)! As a result, digital modes via radio today deliver performance far short of their potential.
Special receiving adapters for packet modes are available, and these usually work in conjunction with personal computers. Most offer FSK receiving capabilities as well.
Another form of digital modulation is known as spread spectrum. Most other modulation methods pack all of the transmitter's output power into a bandwidth of only a few kHz. (Even in FM, the carrier doesn't occupy much bandwidth, although its frequency may be deviated over a wide range.) Spread spectrum literally "spreads" the carrier over a frequency range that may be as much as 10 kHz on frequencies below 30 MHz. (Spreading over 100 kHz or more is common on the VHF and UHF bands.) This spreading is usually done via a "spreading code" contained in an internal microcontroller chip.
When heard on a conventional receiver, spread sprectrum sounds like random noise or "gurgling" water. A receiver equipped with a microcontroller having the matching "spreading code" is necessary to properly receive the spread spectrum transmission. Advantages of spread spectrum include a high degree of privacy and freedom from intereference, since the spread spectrum receiver will reject any signal not having the proper spreading code. Almost all users of spread spectrum below 30 MHz are various military and government services.
Digital mode Sound Samples from site "Worldwide Utility News"