Hi everybody! I am looking to an answer on this topic. Here's how it goes: A radio producer (in FM band, mono) moves on holiday to Austria. However, he still wants to break the news so he decides to present his news bulletin through the telephone. What is the bandwidth needed?

 

Rather strange question.
What does FM band, mono, holiday to Austria, have to do with the problem?

The voice bandwidth of POTS is 300-3400Hz.

 

Rather strange question.
What does FM band, mono, holiday to Austria, have to do with the problem?

Not much. I suppose it's just for completeness sake.
Anyway, this is just an exercise I encountered once. I forgot to mention that there was a hint to use the Carson rule. Any ideas?

 

Did you google Carson rule?

 

I know what Carson's rule is about. It states that bandwidth requirements of communications system components for a carrier signal that is frequency modulated by a continuous or broad spectrum of frequencies

where CBR is the bandwidth requirement,

is the peak frequency deviation, and

is the highest frequency in the modulating signal [check: http://en.wikipedia.org/wiki/Carson_bandwidth_rule]. As far as I know,

is 75 KHz

is 25 KHz so CBR=200 KHz for a radio station. However, I still can't figure out what this has to do with the telephone service

 

I think it is a trick question that has nothing to do with FM broadcast.
The reporter is going to submit the news via telephone, 3400Hz max.

 

Yes, it's possible but I still wonder. If BW is 3100 Hz without any modulation as you say wouldn't that degrade the quality of the transmission in comprison with the FM? And as I said before, there was this hint about Carson's rule

 

Yes, it's possible but I still wonder. If BW is 3100 Hz without any modulation as you say wouldn't that degrade the quality of the transmission in comprison with the FM? And as I said before, there was this hint about Carson's rule

As far as the radio station or the producer is concerned,the phone system is a baseband audio system with 300-3400Hz bandwidth.
This is despite any form of modulation which may be used by the Phone company.
In fact,one of the aims of a phone system is that it will not exhibit any artifacts which make it sound any different to a direct physical connection from phone to phone.

The question possibly wants to know:
(1) What bandwidth signal would the radio station transmit during the time that he was talking with the limited bandwidth audio.(the fact that they refer to mono may be a hint--you don't have to allow for subcarrier,etc.)

(2) What bandwidth special program line would he have to hire from the phone company to cover the normal bandwidth audio required for the station.

(3) He's the producer,who normally isn't an on air personality,so he can probably present the program--in other words,organise what the on-air people & production staff do,just by having his phone patched into the talkback at the station.

 

Well, I think that I have come up with possible solution. First of all, I would rephrase the problem this way. The producer makes a call to the radio station and then breaks the news through the telephone while somebody holds the earphone in front of the station's microphone so that the producer's voice is transmitted with FM modulation. In that case, considering that transmission is mono, we know that FCC's standard for

is 75 KHz and fm is 3.4 KHz for a telephone. So, using Carson's rule we obtain B=2*(3.4+75)=156.8 KHz which is a "good" result because it's smaller than the 200 KHz that are needed normally so there won't be any interference with neighbooring stations. I also think that that's why one can call a radio station and talk live during a broadcast.

 

In the real world,it wouldn't matter,even if the source was incredibly wideband,as any signal above the allowed program audio bandwidth would be removed by filtering before it reached the input of the Transmitter.

Also remember that peak deviation is dependent upon modulating signal amplitude as well,but this would not exceed the requirement either,as it would be amplitude limited.
The question was probably devised by an academic who doesn't know anything about real Broadcasting!

 

Also remember that peak deviation is dependent upon modulating signal amplitude as well,but this would not exceed the requirement either,as it would be amplitude limited.

I don't understand this very well. How is peak deviation dependent upon signal amplitude? I woud also like that you suggest me any link, pdf etc. on "real" fm broadcast

 

For just voice, the bandwidth requirement is only about 3 KHz, and this is still commonly done for temporary programming.

 

I think you may have arrived at the figure being asked of you there toumbous.
To expand in vk6zgo's real world scenario, in a studio, a phone line would be patched through to the mixing desk via a device known as a telephone hybrid rather than using acoustic coupling.
One of the most expensive items in the transmission chain in a typical broadcast system is the compressor / limiter and this will make sure that at nearly all times, the amplitude is such that the deviation will be approaching the maximum allowed which for a stereo station with RDS / SCA is 270KF9EHW.

 

I don't understand this very well. How is peak deviation dependent upon signal amplitude? I woud also like that you suggest me any link, pdf etc. on "real" fm broadcast

Just think for a moment on the term "Frequency Modulation"
What does it suggest to you?

As a hint,at the TV transmitter where I used to work,one of the tests was as follows (I will refer to the old mono TV sound system so as not to confuse things):

If a 1KHz signal at +8 dBm is fed to the sound exciter,the deviation on a deviation meter is 20 kHz.
If the same frequency is now input at +16 dBm.the deviation reads 50kHz.

What has changed between these two tests?

The maximum deviation allowed, by the regulations for TV sound in Australia is 50kHz at any frequency of input signal.
The transmitter exciter obviously can go higher in deviation,but then it is out of Specifications.

"Peak Deviation" is not some esoteric Engineering concept,it is simply the deviation caused by a given modulating signal level.
It may be,& often is,due to the use of limiters,Optimods,etc,very close to the legal limit for the FM broadcasting service,but may also be
much lower due to the nature of the program material.

 

I think it is a trick question that has nothing to do with FM broadcast.
The reporter is going to submit the news via telephone, 3400Hz max.

Agreed.

But in the real world the radio station have a lot of high tech auto equipment, and even on a live broadcast received by 'phone (very common) they use audio processing equipment, graphic equalisers, and even things like an "audio exciter" (I have one) that artificially generates higher frequencies that were absent in the original signal.

A good sound engineer can do a respectable job of making a 'phone signal sound quite good.

So it's a "double trick" question.