MFSK
 
 

Introduction

The Professionals Professional communications engineers are always interested in sending information faster and more reliably than was previously possible, if necessary by using complex technology, greater bandwidth or higher power to achieve this. In addition, commercial and military users for whom this equipment is designed, are generally interested in short distance communications with high reliability and relatively high signal strengths. These types of communications are also generally bi-directional (full duplex) or unidirectional (simplex or broadcast) rather than one way at a time (half duplex).

The Amateurs

Although some Amateur Radio applications have similar goals, especially on VHF, most Amateur Radio HF, MF and LF data communication has limited bandwidth available, and usually involves half duplex operation, while the available transmitter power is definitely limited, both by legislation and by cost. Fortunately, Amateurs have lesser expectations of reliability, and can always postpone operation until conditions improve, or simply talk to someone else! Amateur Radio operators also have an almost unique interest in real-time communications, now abandoned by almost all other HF services in favor of automated message techniques.

Radio Amateurs wishing to transmit data or text rather than voice (digital modes) are often interested in very robust transmissions over very long distances, such as from one side of the world to the other. Bandwidth needs to be kept to a minimum, since all the Amateurs in the world share the same limited space, and power requirements should be modest. Fortunately transmission speeds can often also be modest, which is helpful since speed can be traded off for improved reliability, lower power, or narrower bandwidth. This concept is the logical concept of early work in communications theory by Claude Shannon (1947).

DX Conversation Modes Thus there is always an interest in improving the communication performance of low power long distance links, typically casual person-to-person conversations where each person takes a turn to type on the keyboard and transmit what he types, while the others receive and print what is sent. Radio Teletype (RTTY) was used in this way for many years. Hellschreiber has recently been revived for the same purpose, and has proved to be very effective. New techniques such as PSK31 by Peter G3PLX have extended the performance of narrow-band links considerably. The recent trend in these new modes has been to use differential PSK (DPSK) transmissions, since DPSK offers very high sensitivity and rejection of noise. Such modes are therefore ideal for low power. However, the biggest problems facing very long distance (DX) communication on HF are generally selective fading and ionospheric modulation of the signal, rather than sensitivity, and the PSK modes do not handle these problems very well. The MFSK Option Few Amateurs have even heard of MFSK, while some that have might be dismissive of MFSK as being "old fashioned". As has been clearly demonstrated by the recent successful revival and acceptance of Hellschreiber, old ideas combined with modern techniques such as DSP can be very effective. In the past, MFSK was used successfully by the British Foreign Office, the Belgian and French military and others, using such systems as Piccolo and Coquelet. Such systems were designed for high communications reliability in the days of electromechanical equipment. These old MFSK systems provided very good performance for the time - robust, sensitive and reliable, with good results in fading and poor ionospheric conditions without requiring error correction. There are some modern military systems of a broadly similar nature used for similar reasons. The opportunity has now arrived to modernize the MFSK technique, creating a new high performance yet inexpensive mode that will benefit from the advantages of MFSK, plus the simplicity of the PC and sound card, and the advantages of many associated DSP techniques, since PCs are now fast enough to perform this type of processing. MFSK Overview for Beginners MFSK is a technique for transmitting digital data using multiple tones, extending the RTTY two-tone technique to many tones, usually, but not always, one tone at a time. MFSK means Multi - Frequency Shift Keying, and should not be confused with MSK (Minimum Shift Keying). There are a number of different techniques, using concurrent (or parallel) tones, sequential (one after another) tones, and combinations of tones. MT-Hell can be either concurrent or sequential, DTMF tones used for telephone signaling are concurrent tone pairs, while Piccolo and Coquelet, although using tone pairs, are decidedly sequential. MFSK transmissions have a unique sound, almost musical, which is why Piccolo and Coquelet received their names (Coquelet means rooster). MFSK uses relatively narrow tone spacing, so remarkable data rates are achieved for a given bandwidth - 64 bps in a signal bandwidth of 316 Hz is typical. The following picture is a spectrogram of an MFSK16 signal (16 carriers) with a spacing of 15.625 Hz and operating at 15.625 baud. The transmission operates at 62.5 bps (about 80 words per minute!) and occupies about 316 Hz of bandwidth. The two black horizontal lines in the picture are at 1000 Hz and 1300 Hz, and the horizontal scale is about 20 seconds. This short transmission contains about 120 letters. MFSK16 is always operated with FEC, so the text throughput is actually only about 42 WPM (31.25 bps). MixW Version 2, by Nick Fedoseev, UT2UZ and Denis Nechitailov UU9JDR. Help system by Scott Thile, K4SET

 
   
 
   
 
 
 
 
我们的世界--数据通信--MFSK

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