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The Bell 103 modem used Audio Frequency-shift Keying to encode data. Different pairs of audio frequencies were used by each station:
Although original Bell 103 modems are no longer in common use, this encoding scheme is referred to generically as "Bell 103 modulation", and any device employing it as "Bell 103-compatible" or "a Bell 103 modem". For many years, higher-speed modems retained the ability to emulate the Bell 103, allowing a fallback method for data to be communicated at low speed if channel conditions deteriorated. APPLICATIONS Bell 103 modulation is still in use today, in Shortwave Radio , Amateur Radio , and some commercial applications. Its low Signalling Speed and use of audio frequencies makes it suitable for noisy or unreliable narrowband links. For example, the CHU shortwave station in Ottawa , Canada transmits a Bell 103-compatible digital time code every minute. Bell 103 modulation is also the standard for amateur Packet Radio in the HF (shortwave) bands. Trials RELATED TECHNOLOGY The ITU-T V.21 communications standard defines a very similar modulation scheme. Commercial 300 baud modems typically support both formats. |
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