John wrote: > How will the modulation be determined from any SSB transmitter when the > source of the modulation is via the microphone audio input of that > transmitter? > > Simply stated, how would any digital mode create anything other than some > form of FSK simply by inputting a tone at the microphone input? > > Regardless of the software being used to generate the tone(s), at any given > time there is nothing more than the absence or presence of a tone at the > audio input of the transmitter. This is true of HRD's DM780, MixW modes, > MMSSTV, or many other sound card driven software packages. They all have one > thing in common, they generate a sequence of tones which is then processed by > the very same transmitter in the very same way. The maximum output bandwidth > is supposed to be somewhat limited in the bandpass of the transmitter > circuitry (which is NOT being altered). Again, NO transmitter circuitry is > being altered in any way that I am aware of. >
That's indeed the question! Given the way (legal) rigs filter the SSB to a voice grade bandwidth, it's really hard to see the many modes as anything but variants/nuances of AFSK, PSK, etc. And for what it's worth, spread spectrum is nearly always BPSK, just spread across many frequencies via some pseudo random or direct sequence. There are some technical descriptions of spread spectrum, and they nearly always consist of: 1- signal bandwidth to information bandwidth ratio significantly greater than one. (usually 100 or more) 2- Frequency shift driven by something other than the information itself. Item 1 actually is indicative of gain, the larger the ratio, the more effective the circuit gain is in terms of S/N. Given the low ratio in ROS, most definitions would exclude it based on this alone. A corollary to this is that a single bit must be spread across multiple frequencies. The larger the bandwidth expansion factor, the less of a "bit" is ever present on any single frequency. Item 2 is referring to sequence generators driving RF vco's directly, along with the signal in an adder mode. It is not referring to randomization of the in band audio signal, or even dividing the signal into multiple bands and sending information in parallel. Based on these measures, most ham grade transmitters would be incapable of spread spectrum operation. Pretty much all SS is a form of "code division", as opposed to frequency division or time division. Code division is the clock signal (direct or random) which shifts the carrier frequency unrelated to the information modulation. IE: Even sending all zero's, ones or random data, the base frequency shift sequence will be the same. NTIA has two definitions for spread spectrum: 1. Telecommunications techniques in which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information. Note: Frequency hopping, direct sequence spreading, time scrambling, and combinations of these techniques are forms of spread spectrum. [INFOSEC-99] 2. A signal structuring technique that employs direct sequence, frequency hopping or a hybrid of these, which can be used for multiple access and/or multiple functions. This technique decreases the potential interference to other receivers while achieving privacy and increasing the immunity of spread spectrum receivers to noise and interference. Spread spectrum generally makes use of a sequential noise-like signal structure to spread the normally narrowband information signal over a relatively wide band of frequencies. The receiver correlates the signals to retrieve the original information signal. [NTIA] ROS appears to me to be a neat audio j3? mode using spread spectrum type techniques. But does not meet the technical definition of spread spectrum by FS-1037C or subsequent NTIA definitions. You could say it does have some aspects of the non-technical SS NTIA definition. IE: interference & noise immunity. All that said.... neat idea, nice to see a new mode! Have fun, Alan km4ba
