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I found this statement on a URL of a major US classical and acoustic music FM station.
I think they are right.
Are they?!
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Follow Ups:
Timbo,
Let's think about this and try to understand what you (and maybe I) mean by multipath, from the "poor" receiver's perspective.
At my antenna terminals I see two identical signals - on the exact same carrier frequency - one slightly delayed in time because it was reflected off of something. Let's call one the "direct" and the reflected, "delayed". The reflected signal should be weaker as it had a longer path to travel. Because they are on the same frequency, they should make it unchanged (although considerably amplified) through my RF and IF stages. So it's what happens once the signals hit my detector stage that we would "hear". I believe the FM detector stage's job is to change an incoming (instantaneous) frequency variation into an analog voltage equivalent. Now my thinking get's hazy - so let's try simplification - what if the transmission was a single 1Khz sine wave - wouldn't a "perfect detector" output two sine waves (if it were fast enough), the "delayed" one at a lower amplitude and "slightly behind" the direct. Having said that I have mo clue what I would "hear" on that simple or with a more complex waveform - Would my hearing mechanism try to mix the two together somehow and, wa-la, produce "jibberish"???
Having gone this far, I would opine that there is no way that my (or any?) detector could distinguish between the direct and reflected signal unless the reflected signal is significantly lower than the direct so it gets "buried in the noise". Even if you could apply some sort of digital processing, I can't see how you could reliably determine and continuously "lock on" to the primary signal. But then I don't (have to) design "spook" communication equipment.
Does this make any sense? New Years Cheers to down under.
Charles
However, some tuners have the uncanny ability to minimize the effect of multipath such as the Philips AH 673. As long as you feed it with strong signal it'll give you a clear view of what the FM stations were using as sources.
If a thing's worth doing, it's worth doing well
(Proverb)
Edits: 01/04/12
Beautiful tuner! Wish I had one. Oh wait, I do but, in black. It is one of those tuners that will give about as good as the best of all tuners out there.
Making it's capture ratio help as much as possible.
My point is that a 'directional enough' antenna will help all tuners even a super-tuner. How much depends on the transmitters/stations, and their paths to the tuner (receiver in RF terms).
That Philips is justly famous for it's good but neutral sound and good RF performance. IIRC the AM stage is worth the used price alone, and a good frame-loop antenna in your loft.
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Developed initially for government and military communications systems, Software Defined Radio is pretty immune to multipath and even fading signals, assuming the signal doesn't fade to zero. Now commonly available even for HAM (amateur radio) applications.
One of the SDR innovators is right there in your home country Timbo!
Fascinating.
But for high-rez audio I like minimal processing.
We also invented Wi-Fi. ? Court case recently.
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Multipath is really a function of environment/location and the antenna sending the signal to the tuner as you know. Mulitipath does however alter the phase after a "bounce" off of buildings and other objects between you and the signal source. Perhaps a circuit can designed to work from this angle. I doubt it though as this has been known for a long while.
Having adjustable sensitivity and selectivity has always helped me in the past. My location is so bad now I don't even try. I'm almost at the base of a narrow valley only 300 yards wide.
I found this:
"""From David Mann Audio:
""Multipath" is a phenomenon that, to a degree, affects all radio communications. Radio frequency energy travels through space at the speed of light. The figure shows a common radio communications situation. The line A-B represents energy traveling straight from the transmitter A to the FM receiver B. Unfortunately, energy bouncing off building C, is causing a problem. Some of the energy travels the line A-C-B to the receiver. Since all of the energy travels at the speed of light, the energy traveling along A-C-B takes longer to arrive because the path length is longer. Hence the name "multipath". Multipath can be caused by a variety of sources including: buildings, mountains, bridges, tall towers, passing airplanes or clouds, and windmills.
On a TV set this late arriving energy will cause a second, distinct picture (Ghost image) offset to the right of the direct picture. Knowing a few details about how TV pictures are transmitted and the speed of light, one can measure the image displacement with a ruler on the face of a TV set and calculate the path difference between A-C-B and A-B. For FM reception, the late arriving signal is not late enough to perceived as an echo, but the recovery of FM Stereo information is disrupted and listening to the station can be an unpleasant experience.
The tuner's plight under a barrage of multipath is similar to your own while trying to understand voice announcements in a large transportation terminal filled with echoes.
Certain FM tuners perform better under severe multipath conditions. The relevant tuner specification is "capture ratio". A fortunate characteristic of FM technology is its ability to ignore a weak station arriving at the tuner on the same channel as a strong station. The stronger station is said to "capture" the tuner. A lower capture ratio usually results in better immunity to multipath problems. A very low capture ratio implies that the weaker station could be almost as strong, yet be completely rejected by the tuner. "Better", however, is not always satisfactory. One should not expect the multipath information to behave nicely and always arrive below the capture threshold.
Your best ally for preventing multipath distortion is a highly directional antenna. If the antenna does not respond to the off axis signal, multipath distortion cannot occur. Unfortunately, directional antennas at FM broadcasting frequencies, come in one size -- large. Unless the diagonal dimension of the antenna is approximately 120 inches, it will not be very effective for eliminating multipath." """
ET
HD radio and my Sony XDR-F1HD reject multipath extremely well, but only in digital mode. Analog broadcasts still suffer from multipath in my location on a non-directional antenna and it's not clear that any amount of digital processing of an analog signal can improve reception very much.
Really severe multipath might disrupt even digital signals, but I haven't experienced that problem.
In the past on other tuners I sometimes got improved reception with an attenuator, but I suspect that had more to do with front-end overload.
Even DAB which ought to be better - being introduced here - is audibly compromised compared to FM at its best. IE live broadcasts of acoustic music simply miked in stereo received with minimal multipath and at high signal strength.
This thread is about whether multipath can be rejected by an analogue FM tuner.
I can't see how it can be.
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
received signal strength is either enough for the 'receiver' or it isn't.
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
No tuner I am aware of will reject all multipath. However some tuners have a more selective RF front-end than others including a capature ratio. Capature ratio is rejection of a less strong signal on the same frequency. A capture ratio specification of 2dB is if the stronger signal is 2dB stronger than a close nearby frequency, the weaker signal will be surpessed 30dB. Usually the 30dB rating is the standard reference. I do not believe capature ratio will have much effect on multipath that is on the same frequency. Capture effect is cool on FM broadcast, totally uncool an aircraft communications as capture effect could stop critical communications. Thus, aircraft uses AM (amplitude modulation).
The best multipath reducer is a highly directional antenna. The next best is a simple rabbit ear antenna. The rabbit ear antenna can take advantage of the common FM station transmitted circular polarization by rotating to exploit deep response nulls being circular right, circular left, horizontial and vertical.
An indoor rotatable rhombic would be cool. Just never calculated the size required.
I use the inexpensive rabbit ear antenna with selective capacitor switch (tuning) as part of the kiss (keep it simple) plan. I am fortunate to have good reception in my area (both locations).
On the horizon are patents that use processing to eliminate some multipath. However, I am not sure if it may end up degrading the sonics.
One can always select mono as it is less disturbed with multipath vs stereo.
I haven't used a tuner for a while but I thought Capture ratio was a factor in rejecting multipath since a good value indicates how well a tuner can use the highest strength signal it receives on a given frequency and reject the others(the reflected mulitpaths).
Or, and in the same sense, a time of arrival issue.Further, for capture ratio to help if it can at all, the direct signal has to be very strong compared to the reflected signals, and only when the reflected signals are considerably weaker than the main signal.
This certainly doesn't apply where I live, The level of the combined reflected signals exceeds the level of the direct signal. It must do because to either side of the known LOSight the signal towards each of the two large hills on either side is just as strong as the direct signal, off a directional antenna. So 3db higher in combination, IF I was using an omni, or a single dipole.
So with an Omni like a whip, what IS the main signal? the strongest!? WE can see now that that will not necessarily be the cleanest.
If the strongest signal is always best, which would need to be true for capture ratio to be a reliable indicator of the MP rejection capability of a tuner, why then do some MFRs add MP indicators?
So, MP is the thing you want to minimise, along with maximising signal strength with low MP. Again, this argues for a directional antenna with considerable gain, and ideally a multipath indicator on the tuner.
Because I listen to real stereo recordings a lot I can hear multipath quite easily on live acoustic broadcasts miked in real stereo. A correctly oriented directional antenna maximises 3D, nuance, interplay and expression.
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Edits: 12/29/11 12/29/11 12/29/11 12/29/11 12/29/11
Because it supposedly doesn't apply in your situation does not mean that "capture ratio" is irrelevant to multipath rejection. If the reflected signal is weaker, which is most often the case, "capture ratio" is definitely a factor in minimizing the effects of multipath.
"Certain FM tuners perform better under severe multipath conditions. The relevant tuner specification is "capture ratio". A fortunate characteristic of FM technology is its ability to ignore a weak station arriving at the tuner on the same channel as a strong station. The stronger station is said to "capture" the tuner. A lower capture ratio usually results in better immunity to multipath problems. A very low capture ratio implies that the weaker station could be almost as strong, yet be completely rejected by the tuner........"
Quoted from audiokarma website
I have several friends who live in flat areas who used to get by with a standard not very long-boomed bandI.5 to bottom of III VHF/UHFIV TV antenna, vertical!
All these live in areas where analogue TV ghosting just isn't an issue and so FM multipath likely isn't either, looking LoS at the one big tall tower on a big conical hill where TV and FM all come from
And some of them who having heard a dedicated FM only YAGI or a dedicated Hills CA8 (a phased double H plus Ch2 reflectors] won't go back to the old combo tenner for FM.
JBTW I may soon need to replace all that RG11 in my place as it may well be a teeny bit damp after 13 or more years. Quad-shield copper RG6 which I am assured will be as quiet, AND let me get digital-TV all the time on our weakest TV company (5 CHs carried). Mostly repeats, but there are jewels in the sewage.
More TV/FM RF outlets too - one for the covered deck, and one each in our his and her dens, and my audio projects room. While I am doing that I'll probably add a stereo line level network, and maybe expand the digital AV signals network.
It will have to wait until autumn / winter as clambering about under the suspended floor of our house is painful enough without it being stifling hot. And I am 61!
Will report honestly when done, Abe. ;-)!
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
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