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 Post subject: FM receiver sensitivity question
PostPosted: Jun Thu 15, 2017 1:36 pm 
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I've noticed something having repaired an earlier FM tuner recently.

It seems like the older the FM tuner or device with FM tuner in it is the more sensitivity the tuner has.

That of course only applies to quality devices as we all know the cheap things with FM tuners in them aren't going to be very good regardless of age.

Now I have a theory as to why which is because there weren't as many FM stations in the earlier days, FM tuners had to be more sensitive and as more FM stations started to exist there was no need for FM tuners to be quite as sensitive.

Does that seem reasonable or is there some other reason?


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Thu 15, 2017 3:12 pm 
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My experience is the opposite. The most sensitive FM tuners I have seen are the classic, high quality silver-face and black-face receivers from the 1970s and 1980s. A stereo tuner must be far more sensitive than mono for equivalent S/N ratio, because the L-R subchannel is transmitted at only 10% modulation IIRC.

-David


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Thu 15, 2017 4:43 pm 
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Seems to me like a lot of the digital tuners commonly available in the 80's aren't as sensitive as a receiver from the 60's, yet I got a Magnavox CR-192 FM tuner that blows all other tuners I've had out of the water far as sensitivity is concerned.

That said I've never had any of the high end receivers of the 70's and 80's to compare anything to.


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 2:38 am 
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Tube Radio wrote:
Seems to me like a lot of the digital tuners commonly available in the 80's aren't as sensitive as a receiver from the 60's......
Yes, most of the "commonly available" black-faced digital tuners are no match compared to many tube tuners available in the '60's. My favorite is H. H. Scott. But there are some less-commonly available high-end solid state tuners from the 80's that are outstanding and totally blow away any of the quality post war sets from post-war era. My estate sale find Carver CT-Seven tuner/preamp never ceases to amaze me for signal pick-up, adjacent channel rejection and noise-free stereo reception from distant signals.

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It is the 19 kHz pilot frequency used to synchronize the stereo decoder with the station's stereo generator that is transmitted at 10% of the overall modulation. L-R is part of the composite base-band signal. The fact that L-R is configured as a double-sideband, suppressed carrier AM signal in the supersonic region of the FM passband contributes to the increased noise reception in stereo.

Dave


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 2:56 am 
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Take a look at sold prices for '60's-'70's Marantz receivers on The Bay...

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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 4:33 am 
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Dave Doughty wrote:
It is the 19 kHz pilot frequency used to synchronize the stereo decoder with the station's stereo generator that is transmitted at 10% of the overall modulation. L-R is part of the composite base-band signal. The fact that L-R is configured as a double-sideband, suppressed carrier AM signal in the supersonic region of the FM passband contributes to the increased noise reception in stereo.

Dave

I don't believe that's true. IIRC the L-R sidebands (the 38kHz carrier is suppressed as you say) are limited to 10% modulation. The pilot is another 10% modulation. That leaves 80% modulation (assuming no SCA) for the L+R main carrier. This was by design -- the FCC wanted a scheme that would detract as little as possible from the performance of existing mono FM receivers when receiving a new stereo signal.

-David


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 11:39 am 
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From what I gather the L-R sidebands have to be at the exact same level as the normal audio in order for the stereo decoder to provide proper separation.


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 12:16 pm 
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I agree that most L-R information averages about 10% or so but it is much more than that when there is program material that contains extreme left and extreme right information. When L = -L for example, 45% is main channel modulation while 45% is subcarrier modulation. When the 10% pilot is included, 100% total modulation is achieved. While there is a mathematical limit, there is no FCC mandated limit for L-R modulation as long as the carrier does not exceed + or - 75 kHz deviation (80 kHz if an SCA is used).

Dave


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 5:49 pm 
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Dave, I just re-read Part 73.322 (it's been a very long time) and you are right. The L+R main channel components and L-R subcarrier components share the 90% modulation not occupied by the pilot (assuming no SCA and only one stereo subcarrier).

The modulation limits of 45% for an L or R signal, and 90% for main channel + stereophonic subchannel, are specified in 73.322.

To your point, for a practical matter with common source material, L+R will be transmitted at significantly higher modulation than L-R. Hence, better receivers and better antennas needed for stereo.

-David


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 6:54 pm 
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dberman51 wrote:

To your point, for a practical matter with common source material, L+R will be transmitted at significantly higher modulation than L-R. Hence, better receivers and better antennas needed for stereo.

Actually a lower end stereo multiplex receiver can work adequately; you just need to adjust the balance control more toward L.

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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 7:35 pm 
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No, you need to set it to mono to eliminate L-R. Many of the better receivers will do that automatically if signal strength is too low for good stereo reception.

-David


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 9:37 pm 
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I have a lower end stereo receiver that I bought new in 1969, and just recently started using again.

The selector switch, in addition to AM, Phono and Tape, has positions for FM, then AFC, then MPX. When on a stereo signal, the FM Stereo lamp lights, on either the AFC or MPX position, but I often have to adjust the balance one way or another to get balance between both speakers.

Sometimes I can't get clear audio on MPX, and have to use AFC instead. The audio, when on MPX, sounds different than AFC or FM.

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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 10:55 pm 
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The audio will sound different on FM and AFC than it does on MPX. FM and AFC (Automatic Frequency Control) are mono, MPX is stereo. If you are tuned to a station with less than a strong signal you will get much less hiss using FM or AFC than on MPX.

There is also less sensitivity to multipath distortion using FM or AFC settings than on MPX.

If the audio channels are not balanced that is an entirely different issue not related to tuner sensitivity. There is a failure in one of the output circuits or possibly just a dirty selector switch. Better to start a new thread for this.

-David


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Fri 16, 2017 10:59 pm 
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dberman51 wrote:
The audio will sound different on FM and AFC than it does on MPX. FM and AFC (Automatic Frequency Control) are mono, MPX is stereo.

Why then would the stereo indicator light, when tuned to a stereo transmission, with the selector on AFC?

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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Sat 17, 2017 1:36 am 
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I've had a number of better black face SS tuners and receivers. Nearly all of them out-performed any tube or early SS units. My Luxman R117 receiver is one of the best I've ever used equaling or beating Pioneer Elites and Sony ES units. But for black face models, it's all about the ICs and their implementation. I had a cheap little bpc HH Scott with good ICs but cheap implementation that could have worked well but didn't.

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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Sat 17, 2017 3:08 am 
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MPX and AFC are totally unrelated. Some tuners labeled their selector switch for receiving FM stereo as "FM MPX" to differentiate it from the earlier AM-FM stereo scheme. AFC (automatic frequency control) can be used in mono or stereo reception to help in tuning and to prevent local oscillator drift. It has nothing to do with the multiplex stereo system other than to ensure that the tuning remains spot-on for best detector response.

Dave


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Sat 17, 2017 3:42 am 
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dberman51 wrote:
... The most sensitive FM tuners I have seen are the classic, high quality silver-face and black-face receivers from the 1970s and 1980s.. David
I tend to agree. All my semi-vintage s/s AM/FM stereos have a better FM sensitivity than my earlier tubes ones. The later need at least some sort of dipole while the s/s state gear will receive FM in the basement on a short length of wire (but I use a dipole if they are in service.)
Cheers.
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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Mon 19, 2017 1:39 am 
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fifties wrote:
dberman51 wrote:
The audio will sound different on FM and AFC than it does on MPX. FM and AFC (Automatic Frequency Control) are mono, MPX is stereo.

Why then would the stereo indicator light, when tuned to a stereo transmission, with the selector on AFC?


The stereo indicator shouldn't be lit.

Probably an issue with the switch as something should be either disabling the MPX circuit or killing the pilot tone.


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Tue 27, 2017 12:23 pm 
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The early FM tuners from the mid to late 1940s were not sensitive at all. My Magnavox FM tuners do not perform nearly as well as tuners from later on. They are all recapped and aligned. I was always very impressed with my 1972 JVC receiver which has wonderful sensitivity; that was until I bought a new Polk Audio HD receiver from Radio Shack in 2008 for about $140. The regular FM performance is outstanding. Sensitivity is unbelievable and adjacent channel rejection is the best I've ever heard.


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 Post subject: Re: FM receiver sensitivity question
PostPosted: Jun Tue 27, 2017 1:56 pm 
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DarrenWGaransi wrote:
The early FM tuners from the mid to late 1940s were not sensitive at all. My Magnavox FM tuners do not perform nearly as well as tuners from later on. They are all recapped and aligned. I was always very impressed with my 1972 JVC receiver which has wonderful sensitivity; that was until I bought a new Polk Audio HD receiver from Radio Shack in 2008 for about $140. The regular FM performance is outstanding. Sensitivity is unbelievable and adjacent channel rejection is the best I've ever heard.


I found with the Magnavox CR-192 tuner the alignment has to be exactly perfect in order for it to be sensitive. Took me at least 2-3 weeks of fooling around with the tuner to get it just right while troubleshooting other minor issues it had, but once done it is easily one of the most sensitive tuners I have ever used.

I also found that using a 75 ohm to 300 ohm balun and adjusting the antenna trimmer with the balun connected does seem to help sensitivity when using a regular antenna or just a wire given the balun makes the signal balanced and the antenna input is 300 ohms balanced.

Mine is even good enough for proper stereo reception using an MPX decoder.

That said for maximum sensitivity the antenna trimmer really should be adjusted with the antenna connected that is going to be used with the tuner given each antenna is different and will require a different setting of the antenna trimmer for best performance.


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