KV9U wrote: > I did some more research and found out that according to the > Wikipedia (which I find to be the most incredible resource on the > internet for general encyclopedic information),
I can agree on that....8-) > PSK (Phase Shift Keying) can be considered to be a subset of QAM (Quadrature Amplitude > Modulation). I told before that it all depends on the abstractions you make. I do not exactly agree on PSK being a subset of QAM. For those who can understand spanish, there is a poem by Calderon that goes like this: En este mundo traidor nada es verdad ni es mentira todo sera del color del cristal con que se mira I used spanish to keep the rhyme. More or less it can be translated as: In this treacherous world, nothing is false or true, it all will be tinted the color of the crystal you look thru. But PSK can be thought as an ASK of two out of phase sine waves. > That rather surprised me since QAM is using two 90 > degrees out of phase carriers that convey data by changing the > amplitude of these carriers (tones). PSK is only the changing of the > phase but keeping the amplitude constant. > > One reference considers QAM to be a combination of ASK and PSK, where > there is at least two phases and at least 2 amplitudes. > > And QAM doesn't have to be digital in nature. No, it NEVER is digital. Whatever modulates a transmitter with digital information is analog... that MAY convey digital info. All those slow envelope or frequency transitions...the use of LINEAR amplifiers is needed for ANALOG signals. Once again, the message, the content, may be digital or analog. > PAL is a type of analog QAM and perhaps by extension, so are SECAM and the NTSC color TV > generation? PAL and NTSC use QAM with analog messages, two color difference signals (or I/Q in the case of NTSC, a variation of color difference signals) SECAM is not. SECAM is a sequential analog color system that uses FM to transmit color difference (R-Y/B-Y) signals, and that cannot send two components at once, as NTSC and PAL can, using two quadrature displaced sinewaves (some may prefer the mention of a sine wave and a cosine wave) that are linearly independent and can send two messages simultaneously. In practice, it translates into a chroma vector whose phase (referred to the burst phase) conveys hue and its amplitude conveys saturation. Some jokers qualify SECAM as : "System Essentially Contrary to the American Method". It is really too complicated in the studio, and yes, it does not suffer the problems of NTSC, but suffers some new worse ones. (For those who are not aware, NTSC is from the > organization that came up with the U.S. color TV system that was sort > of compatible with black and white TV, the:National Television System > Committee. There are some who insist it really means Never The Same > Color:) On its beginnings it was nicknamed Never Twice Same Color...now it seems it is not justified. Feedback solid state video amplifiers made great differences with regards to its original performance. The same system with open loop tube circuits with "AGC" and "reactance tube" property was really asking for trouble, as it made differential gain and phase distortions inherent to those old circuits. > Apparently, one of the main attributes of QAM is the significant > savings in bandwidth. Yes, it can save half the bandwitdth, just by itself. > Electronic Design pointed out at: > > http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=2372 > <http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=2372> > > That 4-PSK (same as QPSK) having a single amplitude level is actually > the same thing as 4QAM. Yes and no. The PSK constellation is on the axes, and the QAM constellation is 45 degrees displaced. > In terms of the coding types, I was thinking that pactor used Viterbi > and Clover II used Reed-Solomon. But if Pactor uses both, then this > makes it difficult to compare directly, and apparently the > combination is better. To be true, I cannot assure by heart that the block coding method that pactor 2 uses is RS. I have to "reread the fine manual" once again. But one of the properties of block codes is that they can detect and CORRECT errors without retransmission. How many bits can be corrected at once, depends on the code used. As far as I understand, Pactor II outperforms Clover. I have never owned Clover equipment, but all I have read so far indicates that. It had a fundamental weakness negotiating speed in worsening propagation conditions, it lost the link without being able to negotiate a speed fallback. Pactor II and III seemingly do very well on that front. Jose, CO2JA -- MSc.Jose Angel Amador Fundora Departamento de Telecomunicaciones Facultad de Ingenieria Electrica, CUJAE Calle 114 #11901 e/ 119 y 127 Marianao 19390, Ciudad de la Habana, Cuba Tel:(53 7) 266-3352 Email: [EMAIL PROTECTED]
