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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Tue 24, 2020 10:55 pm 
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Location: Utica, NY 13502 (USA)
Receivers need to be designed to capture at least a portion of the transmitted off-channel sidebands to avoid distortion. To prevent interference, this was done through the FCC "Table of Assignments" which did not allow FM stations in any given area to be licensed within 3 channels of each other using a complicated formula for calculating their predicted coverage. This allowed for a much wider receiver bandwidth than 150 kHz. This plan has been somewhat eroded due to the allowed proliferation of FM translators and LP FM stations.

In my experience as a broadcast engineer, there is no intentional sideband filtering in an FM transmitter other than what takes place in its tuned circuits and antenna system. It is the natural distribution of audio sounds that accounts for most of the sideband attenuation although the FCC does require measurements through spectrum analysis to show that off-channel emissions are kept below a certain level.

Dave


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Tue 24, 2020 11:50 pm 
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Location: Boston, MA USA
Dave Doughty wrote:
.....the FCC does require measurements through spectrum analysis to show that off-channel emissions are kept below a certain level.

Dave

IIRC this is done under unmodulated conditions. Is that right?

-David


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Wed 25, 2020 1:26 am 
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dberman51 wrote:
Dave Doughty wrote:
.....the FCC does require measurements through spectrum analysis to show that off-channel emissions are kept below a certain level.

Dave

IIRC this is done under unmodulated conditions. Is that right?

-David
Modulated with normal programming. If it were unmodulated there would be no sidebands.

Here are the FCC rules that allow emissions well beyond 75 kHz either side of the assigned frequency that allow for the existence of sidebands including the IBOC (HD) digital signals:

Attachment:
FCC Rule 73.317 FM  Transmission Standards.JPG
FCC Rule 73.317 FM Transmission Standards.JPG [ 223.17 KiB | Viewed 734 times ]


Before 1960, (pre-stereo) FM stations were allowed to transmit with an audio frequency response up to 20 kHz. 100% modulation at 20 kHz (an unlikely situation) would push the occupied bandwidth of the sidebands well beyond these limits.

Dave


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 28, 2020 3:02 am 
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I am not quite sure that the rules regarding how low sidebands have to be attenuated when beyond the channels allocation implies permission to transmit that far away from the main carrier. 25db below translates to over 300 times less than the carrier power. For instance a 1000 watt transmitter would have a sideband beyond it's permitted bandwidth to be less than 3 watts. That's more like a prohibition rather than permission. It makes no sense that the FCC would permit power to spill over and cause interference to an adjacent channel. Radio stations employ tuned cavities, duplexers and circulators to prevent them going beyond there allocation.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 28, 2020 4:43 pm 
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Location: Boston, MA USA
It is a prohibition for emissions to exceed the limits stated, therefore inherently a permission to conform to them. I have never seen any of the tuning devices you mention in an FM broadcast transmitter. I suspect they are used in UHF TV transmitters, but at FM broadcast frequencies (3-4 meters wavelength) they would be ineffective. FM transmitters I have worked with use capacitors and air-wound coils for tuning.

-David


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 28, 2020 10:01 pm 
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Location: Utica, NY 13502 (USA)
As I mentioned earlier, FM stations are not authorized to operate on adjacent frequencies in the same area except for some FM translators which must accept any adjacent interference that might occur. They must also address any interference to an existing adjacent station if there are collaborated listener complaints about the interference.

I have worked with a good many FM transmitter installations in the US during my career and have never seen the mentioned devices either. A duplexer is a device that allows a common antenna to be used with a transmitter and a receiver and would have no use at an FM broadcast installation. However, diplexers are used when two transmitters on different frequencies feed the same (wide band) antenna. Traps or passband filters are sometimes used at some installations that have multiple transmitter/antenna systems at the same location to correct problems with intermodulation but not for filtering sidebands.

Dave


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sat 28, 2020 10:32 pm 
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To extend what Dave D has said, in my experience we paid much more attention to spurious emissions that were multiples of the carrier frequency and require really serious suppression much more than we did to modulation sidebands. That's because sidebands occur as a natural consequence of the modulation index, determined by modulation level and modulation frequency.

As long as you keep those inputs under control the sidebands will be at the right level. Let the high end get too pronounced by excessive pre-emphasis, or by over-modulation, and the sidebands will be excessive as well. Sideband level and frequency can be mathematically calculated.

-David


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sun 29, 2020 12:40 am 
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I built a couple of decoders over the years with the MC1310 and the MC1800. The 1800's were sold at radio shack for a time. I recall hooking them up to old tube radios. One was a Grundig. The problem I always experienced was some stations had little to no separation while others were over separated with the center channel (L+R)attenuated. The UPC chips and their clones don't seem to have that problem. The Sanyo version clone does have one weird quirk. If the indicator lamp or LED is open or not connected the decoder stays in mono mode. All the others don't care if the indicator is connected or not. The supporting components look very similar. The other difference was the UPC chips draw less than 10 ma lamp off while the 1310 and 1800 would typically draw 25ma.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sun 29, 2020 2:07 am 
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Location: Utica, NY 13502 (USA)
I have built several decoders using the National Semiconductor LM1800. I used them with various solid state receivers and portable radios with multiplex output jacks and they worked well. I never tried them with a tube radio but I suspect their relatively low (20k) input resistance might present a problem if used without a buffer.

Dave


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sun 29, 2020 6:48 pm 
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It's true I never hooked a 1310 or 1800 to a solid state radio. I am thinking that the IF in tube sets have so much more dynamic range. A lot of FM tube sets had no AGC or limiting. Solid state sets on the other hand tend to clip in the last stage IF on strong signals and have agc. Solid state sets don't seem to vary as much between stations in volume just noise level. I don't know if there is a National or Motorola equivalent to the UPC1197 maybe the LM4700? The clones are Mashusita AN362 Rohm BA1320 Hitachi HA12026 Sanyo LA3361 and the NEC UPC1197. You notice they are all Japanese in origin.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Sun 29, 2020 8:06 pm 
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Location: Boston, MA USA
Ummmm...FM receivers are supposed to clip at the last IF stage. That's why it's called the limiter stage. It's what makes FM receivers immune to AM interference. If at least the last IF stage doesn't limit, that means that there is not enough gain in the IF section for a weak signal. And solid-state tuners typically have much more gain in the IF stages than tube-type receivers do, it's one of the reasons they tend to work so much better. There was a thread on this a couple of months ago.

-David


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Mon 30, 2020 4:08 pm 
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Your absolutely right about IF intentionally clipping in FM tuners. Not all tuners are set up this way. Many use the same IF for the AM band. In that case clipping is detrimental to AM recovery. In addition to clipping the FM detector stage is supposed to reject AM and noise found on the carrier. They are set up so any AM variations cancel out while changes in frequency get through.

This thread started with a MPX decoder being employed on a tube tuner. For better or worse there are a lot of people out their who swear by tube radios. McIntosh and Fisher both had equipment in the early 60's that retained tube tuners with solid state amplifiers and they were considered the most advanced at the time. The MAC 1700 comes to mind. In fact Mac 1700's even broken go for thousands on ebay.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Mon 30, 2020 4:51 pm 
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Location: Lincoln City, OR 97367
Greetings to the Forum:

I must respectfully disagree with Robert. You do need response out to 53 KHz; the upper sideband of the L-R signal is important. If you do a vector diagram of various modulation and demodulation schemes, you will find that phase distortion results from truncated or omitted sidebands. Phase distortion in the L-R signal results in a loss of separation. The loss of separation may be acceptable to some listeners, but the most critical will be able to tell the difference.... especially if they are familiar with the material from a recorded media.

A precision demodulator used for stereo measurements most certainly has flat response to 53 KHz.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Mon 30, 2020 5:53 pm 
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Exactly, Jthorusen.

I discovered that while trying to make a Magnavox CR-192 FM tuner good enough for stereo.

The IF bandwidth just wasn't quite good enough to be flat out to 53KHz and that was after spending much time carefully aligning the IF to get it the best possible it could be.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Mon 30, 2020 7:37 pm 
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I would have to agree. The system was supposed to work with a flat recovered composite signal out to 53khz. Unfortunately a lot of manufacturers cut corners to save money.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Mar Mon 30, 2020 11:37 pm 
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Location: Boston, MA USA
robertjt wrote:
Your absolutely right about IF intentionally clipping in FM tuners. Not all tuners are set up this way. Many use the same IF for the AM band. In that case clipping is detrimental to AM recovery. In addition to clipping the FM detector stage is supposed to reject AM and noise found on the carrier. They are set up so any AM variations cancel out while changes in frequency get through.
All FM tuners are set up to limit in the final (at least) IF stage. Some may not have enough IF gain to achieve it. They are bad tuners, likely inexpensive, and require a good antenna and strong signals to work properly. It is absolutely not an issue if the IF stage is common to both AM and FM sections -- the AM AVC circuit will ensure that the IF stage doesn't limit on AM while it is allowed to do so on FM, as it must.

robertjt wrote:
This thread started with a MPX decoder being employed on a tube tuner. For better or worse there are a lot of people out their who swear by tube radios. McIntosh and Fisher both had equipment in the early 60's that retained tube tuners with solid state amplifiers and they were considered the most advanced at the time. The MAC 1700 comes to mind. In fact Mac 1700's even broken go for thousands on ebay.
I agree that there are some very fine tube-type FM tuners. My everyday tuner is a MAC MR65B that I bought pre-owned in 1972 and have used regularly since. It is McIntosh's first MPX stereo tuner, from 1963, and I would put its performance up against anything. However, with a Nuvistor RF amplifier and four IF stages it is not representative of the typical tube-type FM tuner.


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 Post subject: Re: An FM Multiplex stereo decoder board that works!
PostPosted: Apr Wed 08, 2020 7:34 pm 
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Just stumbled on to something. Some one who has one of these board discovered pin 9 disables the decoder forcing it into mono mode. A voltage has to be applied to pin 9 for decode disable. This is verified on the data sheet. Normally the pin floats. He hooked it through a 100K resistor and a switch to 12 volts dc. I tried it and it works. I wanted the light to go out when I selected mono mode. There was a section of the mono switch which grounded a pin on the optional MPX decoder socket on the back of the set. I disconnected it from ground and rewired it to a 12 volt source in the radio. The other contact went to a 100k resistor then on to pin 9. The data sheet recommends a cap to ground to eliminate noise from causing a false trigger in addition on pin 9 but so far with no cap it works just fine. The chip is on a socket so I removed it soldered a wire to the bottom side pin 9 then put it all back together. Just thought I would pass that along.


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