by Charles Bachand
One of the most exciting possibilities open to computer users is in the field of telecommunication, the passing of data back and forth over a telephone line. Unfortunately, Ma Bell is not set up to handle the transmission of binary data. It has to be converted to a more suitable medium, in this case that of sound waves. The little black box that handles this magical process is known as a modem.
Sending data over a phone is very similar to sending data to a cassette recorder. Both procedures require the generation of different audio frequencies. With a cassette recorder, only two different frequencies are needed to represent the one's and zero's of binary data. A high bit is represented by a 5326 Hz tone while a low bit uses a frequency of 3995 Hz. The data to and from a cassette recorder gets transfered at a speed of 600 baud (600 bits/sec). This works out to 60 bytes/sec, though in actuality the speed is somewhat less, mainly due to interrecord gaps, the silent passages recorded between records on a tape.
A modem operates a little differently. Instead of using two separate frequencies, a modem requires four. This is because a modem has to have the ability to send and receive data simultaneously. It also needs two separate sets of frequencies: one set for transmitting and one for receiving. This is necessary, since your telephone incorporates an audio feedback circuit. You might have noticed that when you talk into the transmitter end of a telephone, you are still able to hear your own voice, although attenuated, in the receiver. The psychological reinforcement this produces allows one to use a telephone with much more confidence. Otherwise, you might start thinking that the line was dead, or that the other person might not be able to hear you. With this in mind, if we were to use a system with only two tones, our computer might interpret data we were sending out as coming from the other computer. To alleviate this problem, we use two sets of frequencies. One group of tones is sent by the modem originating the call. Here we use audio tone of 1070 Hz to represent a logic 0 and 1270 Hz to represent a logic 1. The modem that is answering the call on the other end of the line sends back data using 2025 Hz for logic 0 and 2225 Hz for a logic 1.
Most modems are designed to connect to a computer through an interface using the RS-232C standard, though some of the new dedicated modems connect through the serial I/O connector or through one of the joystick ports. The use of the RS-232C standard can be traced back to the computer's dark ages, some 20 years ago, when telecommunication was in its infancy. No one then had heard of present day TTL circuitry, which is based on a 5 volt power supply. Back then all they had were computers which used two power supply voltages (+12 and -12 volts). So it's no wonder that the voltage levels they decided upon for their interface standard were based on the voltages +12 and -12. Most standard modems need an 850 Interface Module to work with an Atari computer. The 850 converts the +12 to -12 volt signal from the modem to one of a zero to +5 volt level that the computer can handle.
Now let's discuss connecting your modem to the telephone. There are two basic ways to do this. The easiest method, from the manufacturer's standpoint, is the use of an acoustical coupler. This is merely a microphone and speaker combination that is placed in close proximity to the telephone's handset and allows the modem to actually talk and listen through it. The second method is to connect the modem directly to the phone line, bypassing the telephone altogether. This is by far the preferred method since no outside noise will be picked up by the telephone receiver and it also makes available the ability to have the computer dial the phone number for you.
One drawback of direct connect modems, as far as the manufacturer is concerned, is the government certification that each model has to go through in order to legally be attached to your phone line. Ma Bell understandably does not want anything catastrophic happening to their equipment and has specified that any third party hardware must be tested by the FCC as to its design worthiness. In other words, the modem should not be designed in a way that it might short circuit, sending 117 volts of house current into the phone line or acting as a radio transmitter turning the phone line into a giant broadcast antenna.
A less involved method of hooking up a modem is by tying it directly into the telephone. As you will notice I did not say telephone line, but telephone. Your telephone already incorporates the circuitry needed to isolate it from the phone line. Instead of including isolation circuitry in the modem, we can let the telephone's electronics do the isolation work for us. The manufacturing costs are reduced and the need for certification is eliminated since we are not connecting directly to the phone line. The modem is patched into the circuit between the handset and the phone base unit using modular connectors. This necessitates having the phone off the hook while the modem is in use since the phone considers the modem as another receiver.
Now to sum up. Cost-wise, the handset direct connect modem is the cheapest way to go. Performance-wise, the true direct connect modems are the most reliable and feature packed of them all. And when the need for portability arises, the acoustic modem cannot be beat. So it really boils down to what you want your modem to do, and what you can afford. Striking a happy medium is the hard part.