Leif Asbrink wrote: > Hi Jose and all,
> There is another alternative which is implemented in Linrad. The frequency > band where the carriers are contained is filtered out with a narrow filter. That is a sensible solution in that case, similar to the "pseudo-synchronous" detection implemented in in some analog TV (AM) IC demodulators. > In your example it would have to be 5 Hz wide. Then the phase of whatever > signals that passes the filter is used as the carrier for synchronous AM > and FM with the AM modulation to one ear and the FM modulation to the other > ear. Seems that the instantaneous phase shall be a vector built up with the superposition of all "carrier contributions". Why did you do that AM in one ear and FM on the other? What are the adventages? I see one you may not have thought of...monitoring of homebrew transmitters. Here it is important. With a good, clean transmitter with a stable and well isolated VFO the spurious modulation output (either AM of FM) shall be zero. While building my old homebrew heterodyne phasing transmitter back in 1972, I checked the VFO for pulling listening with a receiver tuned to the VFO frequency, which should remain stable and unmodulated. > I would be interested in a recording where the ECSS detector has > difficulties. Linrad currently does not provide the corresponding > function with a filter to remove one sideband but it would be a fairly > easy add-on in case it would give any benefit. I don't have any at hand now. Such audio files can be large. > > Regarding to the LF response, I would like to have the lowest possible > > frequency cutoff available, 20 Hz might be a good choice, if it is > > possible to go that low. > Do the BC stations really transmit anything that low (close to the > carrier)? Yes, at least the studio equipment can go down between 20 and 50 Hz. Whether the studio - transmitter links can support that is another story. Nautel transmitters chop the large low frequency peaks to avoid overheating with sustained large peaks. But pulse width modulators and pulse step modulators can have excellent low frequency responses, almost impossible to achieve with the old modulation transformers and shunt feed chokes. > What is "lowest possible" is determined by where you want the AGC time > constant. The purpose of the AGC is to remove the variations in signal > amplitude due to the the interference between signals that travel > different distances. Sometimes they are in phase and create a strong > signal, sometimes they are out of phase and nearly cancel out. Taking that into account, maybe 50 Hz could be a reasonably low limit, imposed by the real performance of small subwoofer systems. I have one of those, and sounds very well. I worked in broadcasting for some years and quality audio can get to be very addictive. > When carriers are spaced at 5 Hz, the QSB rate will be 5 Hz and to > make the AGC tune up the gain fast enough during a dip it may be > reasonable to allow the AGC to absorb a substantial part of the AM > modulation at 20 Hz. Another thing is that the fading may be selective > for the amplitude of the entire signal to vary less rapidly or not > much at all. > > I had no responses on the Linrad AM detector so far. I do not know > whether it is because starting to use Linrad is too difficult > or whether Linrad does not provide any improvement over existing > AM detectors. You may download linrad.exe and run it on a wideband > .wav file under any Windows system. Linrad is free and open source > and it compiles under Linux Windows as well as under Windows. I have not downloaded any recent versions after 2.34. One of the hiccups of my mail provider seems to have caused the suspension of my subscription to the Linrad list. Being busy and forgetful is not a good combination. > http://www.sm5bsz.com/linuxdsp/linrad.htm > <http://www.sm5bsz.com/linuxdsp/linrad.htm> > > 73 > > Leif / SM5BSZ 73, Jose, CO2JA
