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FM technology
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Old 1st July 2019, 02:38 AM   #1
revenant is offline revenant
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Default FM technology

Hi to u all, Last time i was reading about FM and its inventor,major E.Armstrong in an American History magazine.He introduced FM to the USA and thereafter to the whole planet.His superhet system is still used to this day in almost all communication equipment worldwide.FM was better than AM as far as noise performance was concerned althoughit was a more complex system .FM stereo which was developped in the late fiftees brought a new dimension to audio and shrewed maufacturers proposed equipment to get the most out of it.

Because of channel spacing,the federal authorities had limited the sound of FM stereo broadcasting to 15Khz instead of the full20Khz bandwidth.The main reason was to preventovercrossing to adjacent channels and also because the healthy human cannot perceive sound beyond 10Khz.yet some experts are saying that this 15Khz limit is detrimental to the sound and harmonics which contribute to the beauty of the audio are lost in the process.

1)Is that affirmation true?
2)by multiplexing the audio to the stereo decoder,the sound is further deteriorated .Is that also true?
3)By building two mono transmitters to do the job,can one obtain a better sound? 4)Lastly.Are there new technologies that can dispense the use of multiplexing the sound? Awaiting for your comments and observations,

Regards,
Revenant.
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Old 1st July 2019, 02:48 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by revenant View Post
this 15Khz limit is detrimental to the sound and harmonics which contribute to
the beauty of the audio are lost in the process.1)Is that affirmation true?
2)by multiplexing the audio to the stereo decoder,the sound is further deteriorated .
Is that also true?
There is a 19kHz pilot tone, used in decoding the stereo information,
that is notched out by a filter, which limits the useful audio bandwidth.
The stereo information is encoded in AM (not FM), on the 38kHz DSBSC
subcarrier, so there is more distortion and worse noise for stereo FM.

Last edited by rayma; 1st July 2019 at 02:58 AM.
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Old 1st July 2019, 01:12 PM   #3
DF96 is offline DF96  England
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People argue about whether we need more than 20kHz, yet few people can hear much beyond 16kHz. If we were starting from scratch now, FM might have a slightly wider bandwidth but 16-17kHz is what we have and that is enough for good fidelity.

FM stereo is a compromise, but a very successful compromise given a good tuner and a suitable antenna. Not all tuners and antennas are good enough.

Using two mono transmitters would almost certainly give worse sound than FM stereo because you could not guarantee identical paths so channel balance would vary.

Multiplexing (for the reason I just said) is the right way to do it. How you do the multiplexing could change but there is no good reason to do so.

In theory a good digital system could be better than FM. In reality the broadcasters are mostly not interested in good sound so digital systems typically give worse sound than FM.
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Old 1st July 2019, 09:25 PM   #4
MarcelvdG is online now MarcelvdG  Netherlands
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Quote:
Originally Posted by revenant View Post
Hi to u all,
Last time i was reading about FM and its inventor,major E.Armstrong in an American History magazine.He introduced FM to the USA and thereafter to the whole planet.
Actually the Dutch radio pioneer Hans Henricus Schotanus a Steringa Idzerda already used a form of FM modulation from his very first broadcast onwards, which was on 6 November 1919 at 8 PM Dutch time. It was a form of narrowband FM, though, that didn't have any of the advantages of Armstrong's system. Idzerda's FM system allowed a very simple transmitter construction.

Quote:
Originally Posted by revenant View Post
His superhet system is still used to this day in almost all communication equipment worldwide.
Superheterodyne reception was invented almost simultaneously by the Frenchman Lucien LÚvy, leading to a series of legal procedures that were eventually mostly won by Lucien LÚvy. I must say that the description in Armstrong's patent is a lot clearer than the one in LÚvy's patent.

Quote:
Originally Posted by revenant View Post
FM was better than AM as far as noise performance was concerned although it was a more complex system .FM stereo which was developped in the late fiftees brought a new dimension to audio and shrewed maufacturers proposed equipment to get the most out of it.Because of channel spacing,the federal authorities had limited the sound of FM stereo broadcasting to 15Khz instead of the full20Khz bandwidth.The main reason was to preventovercrossing to adjacent channels and also because the healthy human cannot perceive sound beyond 10Khz.yet some experts are saying that this 15Khz limit is detrimental to the sound and harmonics which contribute to the beauty of the audio are lost in the process.
1)Is that affirmation true?
I've read some very contradictory things about that. In a 1950's article from the BBC, they conclude after a controlled listening test that only a few of their most experienced people could hear any difference at all between audio band limited to 12 kHz and not deliberately band limited. On the other hand, a 1990's Japanese article indicates that signals above 26 kHz affect the brain waves of Japanese gamelan players listening to a gamelan recording.

I think the consensus is that 15 kHz is a bit low for the bandwidth of an audio chain, even though most people won't notice the difference.

Quote:
Originally Posted by revenant View Post
2)by multiplexing the audio to the stereo decoder,the sound is further deteriorated .Is that also true?
3)By building two mono transmitters to do the job,can one obtain a better sound?
4)Lastly.Are there new technologies that can dispense the use of multiplexing the sound?
Awaiting for your comments and observations,
Regards,Revenant.
The main issue with FM stereo is the increased noise when you have weak reception. The reason for this is a phenomenon known as triangle noise: an FM demodulator converts white noise in the channel into noise that increases with 20 dB/decade at the demodulator output.

For FM mono, at the transmitter side, the audio signal is passed through a pre-emphasis filter that amplifies the higher audio frequencies more than the low frequencies. In the USA, the gain is essentially flat up to 2122 Hz and then increases by 20 dB/decade. In western Europe the corner frequency is 3183 Hz. You can get away with this because the high frequencies are usually much weaker than the low frequencies in audio signals.

After demodulation in the receiver, the FM mono signal is passed through a de-emphasis filter that does the opposite: attenuate the high frequencies from 2122 Hz or 3183 Hz onwards. As a result, it also much attenuates the triangle noise.

For FM stereo, left and right are first passed through pre-emphasis filters (and sharp low-pass filters at 15 kHz), then the difference between left and right is double-sideband modulated on a 38 kHz subcarrier. The average of left and right (that is, the mono signal) and a relatively weak 19 kHz pilot tone are added to the DSB signal, producing the so-called multiplex signal. The multiplex signal is then FM modulated onto an FM carrier.

This means that the whole left minus right signal is centred around 38 kHz before FM modulation and right after FM demodulation. As a result, it suffers much more from triangle noise than an FM mono signal. The pre-emphasis/de-emphasis is also not nearly as effective as it is for FM mono.

I think that if FM mono had not been invented first, every FM stereo transmitter would consist of two separate FM transmitters, one for left and one for right, or of some construction that produces the same output signal. Both transmitters would have to use the same aerial (antenna) for the reasons DF96 mentioned, but that could be arranged. It would give a far lower noise during poor reception than the present system. However, FM mono had been invented first and the new stereo system had to be compatible with the equipment and channel allocations that were already in use.

Last edited by MarcelvdG; 1st July 2019 at 09:31 PM.
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Old 2nd July 2019, 10:42 AM   #5
DF96 is offline DF96  England
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There is more to having a common path than sharing an antenna. You also need to use the same carrier frequency and modulation method, hence some form of multiplexing is unavoidable. This is because path disturbances can be frequency-dependent.
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Old 2nd July 2019, 11:29 AM   #6
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by MarcelvdG View Post
However, FM mono had been invented first and the new stereo system had to be compatible with the equipment and channel allocations that were already in use.

Those were the good old days !


Best regards!
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Old 2nd July 2019, 06:11 PM   #7
MarcelvdG is online now MarcelvdG  Netherlands
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Quote:
Originally Posted by DF96 View Post
There is more to having a common path than sharing an antenna. You also need to use the same carrier frequency and modulation method, hence some form of multiplexing is unavoidable. This is because path disturbances can be frequency-dependent.
I'm pretty sure that two carrier frequencies close to each other will work well enough in practice, because that's what was used for Dutch stereo television until everything went digital. The (L+R)/2 was frequency modulated on a carrier at 5.5 MHz offset from the picture carrier and the (L-R)/2 on a carrier at 5.74 MHz offset from the picture carrier. They had to use sums and differences for compatibility with mono televisions that only received the signal at 5.5 MHz offset, I don't know whether they also did it to minimise the subjective impact of multipath interference.

In any case, the main thing is that they avoided the 22 dB(A) noise penalty of FM stereo by not putting anything on an ultrasonic subcarrier and putting that into an FM modulator.
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Old 3rd July 2019, 01:11 AM   #8
PRR is online now PRR  United States
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Quote:
Originally Posted by revenant View Post
...........Are there new technologies that can dispense the use of multiplexing the sound?....
Digital. MP3 and similar. You can put several programs on one carrier with "little" loss. "Digital FM". It has been done in Europe. Not everybody is a fan. Perhaps because with new digital flexibility the broadcasters get greedy and put more than a few signals on one carrier and let the quality slide. (Also takes sum+difference and often "simplifies" the difference to the detriment of stereo.)

5KC 7KC 10KC 12KC 15KC and 20kHz have all been "good enough" to different generations. A lot depends on signal to noise. When we could barely get 40dB S/N, anything over 7KC was hiss, and better to cut it. Experienced listeners preferred this to wide-range with hiss. In AM the S/N is all about transmitter power to local lightning and cross-mod power, and even 40dB in local reception requires HIGH-power transmitters. The biggest would throw signal halfway around the world, causing international friction. FM separates S/N from raw power by taking a wider bandwidth. The 15KC in 150KC selection (not practical in the 500-1600KC band) gives a reasonable balance of S/N and audio bandwidth and spectrum space. Going to 20KC audioband implies 3/4 as many channels in the same spectrum space. While spectrum use may be light in Mauritius, in most of US/EU it is extremely tight, signals shoulder-to-shoulder interfering with each other.
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Old 3rd July 2019, 09:49 AM   #9
DF96 is offline DF96  England
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Two carrier frequencies quite close together could work, but you are doubling the bandwidth used by each station.
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Old 3rd July 2019, 09:48 PM   #10
MarcelvdG is online now MarcelvdG  Netherlands
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Of course. I don't know whether that would have been considered a big deal when FM broadcasting started, but assuming that the bandwidth per FM signal would have to be reduced to compensate for that, it is still better than the system we have now.

According to Carson's rule, you need about 180 kHz for FM mono and 256 kHz for FM stereo with the present standards.

Now suppose you have a bandwidth of 256 kHz at your disposal to distribute between two FM signals, one for left and one for right or one for (L + R)/2 and one for (L - R)/2, depending on what sounds best under multipath conditions. Each signal could then use 128 kHz. In reality you need some distance for filtering, so make that 100 kHz, leaving 56 kHz of distance. (You don't need much suppression because the signals are about equally strong.)

With 100 kHz bandwidth and 15 kHz maximum audio frequency, each FM signal can have 35 kHz peak deviation according to Carson's rule. Compared to the 68.25 kHz we have now for audio (the rest is used for the pilot tone), that's a loss in SNR of about 5.8 dB. (With separate left and right channels, the loss for mono reception would only be 2.8 dB due to the averaging of left and right.)

With the present multiplex system, the noise floor comes up by 22 dB(A) assuming 50 us de-emphasis when switching from mono to stereo. The difference only decreases to around 5 dB when the reception is very good indeed, so LO phase noise and demodulator noise dominate over channel noise.

One issue that might have been a problem with such a system is the required tuning accuracy. Then again, it is not that much worse than for the FM system we have now.

Last edited by MarcelvdG; 3rd July 2019 at 09:50 PM.
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