Basement Archaeology: Acoustic Coupled Modem

Novation CAT Acoustic Coupled Modem

Briefly

The Novation CAT 300bps acoustically coupled modem was very popular in the late 1970's and into the early 1980's. To use it, you would pick up the phone and manually dial the number for the computer (actually, another modem) to which you wanted to connect. Then, you would listen for a squealing noise indicating the remote modem had answered. Once you heard this "carrier tone," you would press the handset into the rubber cups on the CAT, being careful to orient it the proper way. The two modems would then start talking to each other, moving data to and fro at a whopping 300 bits/second. When you were finished with your session, you had to remove the telephone handset from the modem and hang up.

A Little More

Communicating with remote computers has been going on for a long time. Using the public telephone system to do so, however, had special rules.

Prior to 1968, the AT&T monopoly on the telephone system was complete. You paid for the connection to the phone company, the AT&T-owned Bell System, you leased your telephone, which was manufactured by Western Electric, another AT&T subsidiary, and you were not allowed to electrically connect any non-AT&T equipment to the phone system. Sweet deal for AT&T.

Fortunately for modem-makers at the time, Western Electric's telephones had very standardized handset designs. Novation and others used this by connecting their equipment to the telephone network acoustically, not electrically. The modem's speaker would "talk" into the handset microphone, and its microphone would "listen" to the handset's speaker. Because of the relatively poor bandwidth of the voice telephone network and the inherent distortion of the acoustic coupling method, the rate at which data could move back and forth was very limited.

As of this writing, through methods of compressing and encoding data as well as the ability to electrically connect to the public phone network, data rates of up to 56 Kbps are common. That said, dial-up connections through the phone system are rapidly disappearing with the spread of higher speed technologies using DSL, cable, fiber, and even satellite communications. Data rates based on modern technologies and media are measured in millions of bits per second Mbps, not hundreds or thousands.