> One of the great advantages of FSK RTTY is you can amplify it with > anything, linear or not and no harm is done. I believe there may be > other digital modes which may be capable of this but I haven't seen > anyone come right out and say so. Are there? > Bill, W6WRT
Hi Bill, Some of the various "MFSK" digis are similar to FSK RTTY, in that they have nearly continuous carrier, shifted only in frequency. Of course, with a continuous full-amplitude carrier, frequency modulated, it is possible to use transmitter chains that have less than linear amplification, such as Class C, etc. But some digi waveforms contain keying or symbol transitions that cause spectrum occupancy to widen significantly when non-linear amplification is used. I should point out that the Multi-Frequency Shift Keying designation may be somewhat of a confusing name for this modulation technique, since many of the MFSK digis used on HF have only a single frequency present at any given instant of time. The M for Multi means "multiple possible" instead of "multiple simultaneous". They may also be more discriptively called "n-ary FSK" signals, where "n" is the number of possible frequencies that the tone may be shifted to in the signal. This is often abbreviated to "n"FSK (example 8FSK, 16FSK, 32FSK). Using this designation, common RTTY might be "2FSK" or BFSK, but it traditionally is simply called FSK. The ALE standard [MIL-STD 188-141 or FED-1045] signal is 8FSK (8aryFSK), which has the symbol frequency shift transition at the peak of the tone cycle. This type of ALE was designed to be used with common SSB transceivers that have speech compression, unknown ALC action, and somewhat-less-than-linear amplification. For a view of this ALE 8FSK waveform, including spectrum analysis images, please see one of my ALE articles on the HFLINK.COM website: http://www.hflink.com/articles/#signal The Olivia MFSK signals (an some others), commonly a 32FSK or 16FSK or 8FSK signal, includes amplitude modulation of the tone waveform. At the beginning and end of each transition between tone symbol intervals, the amplitude level is reduced instantaneously to near zero. This technique eliminates the transition glitch between different tones, and has the effect of narrowing the extra bandwidth artifacts caused by keying, and reducing inter-symbol interference. But this keying method requires linear amplification to maintain minimal occupied spectrum. On-Off-Keyed (OOK) A1 CW and OOK Feldhell or FM/FSK Hellscrieber can be used with non-linear amplification, such as Class C bias, etc. BPSK waveforms are normally transmitted in HF amateur radio using very linear amplification, for mimimal spectrum width. However, it is possible to use non-linear amplification for PSK while sacrificing narrow spectrum occupancy (normally not advisable on HF amateur radio). Class C bias and similar, has been used for BPSK amplification in UHF/microwave applications where the wide bandwidth of the sidebands generated is not a major concern. OFDM or QAM waveforms, with varying amplitude-phase shifts, multiple simultaneous tones, and complex time/frequency domains, require linear amplification to preserve the symbol constellation. Sometimes, modulation techniques are used in OFDM to bias the constellation toward the higher amplitudes, making the intersymbol amplitude space narrower, while raising the bottom amplitude parts of the constellation higher above the ambient channel noise floor. These techniques require very linear amplification. Normally the OFDM or QAM signal cannot survive much non-linearity or distortion. 73---Bonnie KQ6XA ------------------------ Yahoo! Groups Sponsor --------------------~--> Check out the new improvements in Yahoo! Groups email. http://us.click.yahoo.com/6pRQfA/fOaOAA/yQLSAA/ELTolB/TM --------------------------------------------------------------------~-> Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
