> Well, IR is RF in the terahertz range, so what was described was > amplitude modulation of a signal using a frequency modulated signal as > input? ;-) Yes, it's FM subcarriers on the terahertz "RF." This is the similar to the DSB-SC subcarrier on FM used to carry the stereo difference signal. The original definition of FM stereo used this frequency division multiplex description. But, you get the same thing if you use time division multiplex spending a little time on the left channel and a little on the right. It's amazing how you can look at the same signal in different ways. Thinking about the direct conversion AM receiver, I'm thinking there will be problems due to the presence of both sidebands unless the local oscillator is phase locked to the received carrier. If the local oscillator is phase locked at 0 degrees, you will get DC plus the audio (plus two times carrier that is taken out by the low pass filter). If the local oscillator is 90 degrees out of phase with the received carrier, you will get nothing. If the local oscillator is not the same frequency as the received carrier, you'll get a tone corresponding to the difference in the frequencies plus a mix of the audio shifted up a bit in frequency and the audio shifted down a bit in frequency. If the frequencies are very close (say 0.1 Hz apart), I suspect you will hear the audio disappear and reappear over a 10 second period as the phase between the two rotates. The null at 90 degrees is an interesting situation. You can transmit two signals on the same frequency without interference by setting the carriers 90 degrees apart. This is the basis of QAM or Quadrature Amplitude Modulation. This is a common digital modulation system where, for example, 8 levels are transmitted on the I (in phase) carrier and 8 are transmitted on the Q (quadrature) carrier. For each transition of the carrier (a baud), 3 bits are transmitted on I and 3 bits are transmitted on Q for a total of 6 bits per baud. It turns out that when you add sine waves that are 90 degrees apart and vary the amplitude of each, the result is a sine wave whose amplitude is the square root of the sum of the squares of the amplitudes and the resulting phase angle is the arcsine of the Q amplitude over the I amplitude. You can plot the resulting signal on a "vectorscope." With the 8x8 QAM mentioned before, you get dots in 64 different positions (an 8x8 array). This is "64QAM." But, the QAM signal could carry analog signals. In NTSC color television, the "baseband" carries a linear combination of red, green, and blue that produces a good picture on a monochrome television. Another linear combination of red, green, and blue modulates the I subcarrier at 3.58MHz. Another linear combination of red, green, and blue modulates the Q subcarrier at 3.58MHz. Through the baseband, I, and Q signals, we are transmitting three signals that are "linearly independent." In the receiver, we can multiply each of the signals by different constants, then add them to yield the original red, green, and blue. Very clever! But then, a monochrome signal does not modulate the I and Q subcarriers at all. As such, we can adjust the gain of the I and Q signal to vary the color saturation (no gain gives monochrome). Further, adjusting the phase of the local I and Q signals relative to that at the transmit end (and transmitted in the color burst for reference) varies the "hue" of the resulting color. So here we have a QAM signal, but the amplitude and phase have their own meaning. When you put up color bars, you should see dots in specific locations on a vectorscope. Modulation... It's fascinating! Harold --=20 FCC Rules Updated Daily at http://www.hallikainen.com Not sent from an iPhone. --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .