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FM Pre-Emphasis and De-Emphasis

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Papers » Basic principles of radio technique » FM Pre-Emphasis and De-Emphasis
           
Dietmar Rudolph
Dietmar Rudolph
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26.Jan.11 17:23
 
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FM systems use a pre-emphasis for the audio signals. That means, all audio  frequencies above a corner frequency are emphasized.

The corner frequencies are:

  • fc = 2.1221 kHz for region 2 (the Americas)
  • fc = 3.1831 kHz for Region 1 (Europe and Asia)

This gives the time constants:

  • τ = 75 μsec for region 2
  • τ = 50 μsec for region 1

In the beginning of the FM area even a time constant τ = 100 μsec was tested.

Pre-emphasis with 100 μsec gives a too much treble boost, and the transmitter may be overdriven. 

At the receiver's side a corresponding De-emphasis has to be applied in order to get a flat frequency response for the transmitter - receiver chain.

In this diagram also the reason for the pre-emphasis / de-emphasis can be seen. With frequency modulation the higher audio frequencies suffer more and more from noise (hiss). To alleviate this the pre-emphasis is applied at the TX, and the de-emphasis is applied at the RX. Together with the compensation for higher audio frequencies also the hiss is reduced by de-emphasis.

Radio collectors sometimes have a problem with the different values of time constants. Often collectors in US have European sets in their collection, and European collectors may have some US sets. What is the consequence of the different time constants or corner frequencies?

The Matlab graph shows what happens:

  • red: an European set receives an US  transmitter:  trebble boost of approx. 3.5 dB
  • blue: an US set receives an European transmitter:  bass boost of approx. 3.5 dB

However, this can be settled by the tone control of the receiver, and so no change of the receiver's time constant is really necessary. It is only to be recognized that the neutral position of the tone control knobs are different now.

Furthermore today very many broadcasters use a dynamic form of pre-emphasis and some other sophisticated tone processing, done by an Optimod® processor. That is why the correct value of the de-emphasis time constant plays no major role any more.

Regards,

Dietmar

Dietmar Rudolph
Dietmar Rudolph
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28.Jan.11 12:08

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The Pre-Emphasis and De-Emphasis Bode Diagrams shown in post#1 may be simplified for the sake of easier understanding of the effect due to the different corner frequencies used in US and in Europe/Asia.To this end, the Bode diagrams are approximated by straight lines.

  • Pre-Emphasis gives a horizontal line up to its corner frequency fc_Pre and then ascending by a slope of + 20 dB/Decade.
  • De-Emphasis also gives a horizontal line up to its corner frequency fc-De and afterwards a descenting line with a slope of - 20 dB/Decade.
  • Because logarithmically multipication changes to addition, these lines can be added and provide the resultant amplitude.

Because 20 dB is a factor of 10 in amplitude, and also 1 Decade is a factor of 10 in frequency,  +20 dB/Decade means that amplitude is direct proportional to frequency, the resultant amplitude due to the mismatch of the corner frequencies gives 20*log10(3/2) = 20*log10(75/50) ≈ 3.5 dB.

For practical cases, these results only apply for HIFI sets without any twisted tone control, and which have an absolut linear frequency range. Therefore, for most sets they are no longer relevant today.

In former days, when the transmitters still had a linear frequency range (excluded pre-emphasis), a sonorous bass volume of the receiver called for a big bass boost in the receiver. And most tube receivers do have that, and so have the typical "wooden tube radio sound".

Nowadays broadcasters use sopisticated audio processing (mostly Optimode), thereby boostig bass and treble. Furthermore, the dynamic of the sound signal is greatly reduced (sometimes to 3 dB). These stations sound very loud (or strong?) what is benefical for mobile car reception and advertizing.   

Together with Optimod bass boost at the transmitter's side and the bass boost incorporated in an vintage tube FM set, a thump sound will  occur. However, this is not a problem of the mismatch of the Pre-emphasis and De-emphasis time constants.

Regards,

Dietmar

This article was edited 28.Jan.11 12:44 by Dietmar Rudolph .

Dietmar Rudolph
Dietmar Rudolph
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31.Jan.11 14:56

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FM tube radios from Europe were designed at times when FM transmitters did not use audio processing (compressing, bass and treble boost) in such amount like today where extensive Optimod sound processing is usual.

European tube sets often have a built in treble and bass boost possibiliy, sometimes called "Wunschklang-Register". However, boosting in the receiver like in those sets and additionally boosting at the transmitter as it is usual today may bee to much, and no good sound quality will result any more.

Nowadays small FM transmitters are available for few $ cost, and they don't have audio processing.

left: some examples of FM TXs; right: FM TX from ELV, whos spectra are shown in RM. 

With such a small FM TX a FM signal like in former radio days can be provided. In this way a separation of the influence of pre-emphase time constant and audio processing will be possible.

Regards,

Dietmar

Paul Reid
 
 
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05.May.11 08:07

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> ...the reason for the pre-emphasis / de-emphasis can be seen. ... the hiss is reduced by de-emphasis.

Yes, but there is another factor.

We "can" boost the highs before the transmitter because in speech or music the high frequency sounds are weak compared to the mid and bass sounds.

I've known this for years, from books and observations. However your essay inspired me to re-check the idea with familiar sounds.

I ran spectrum analysis on several recordings of sounds we might hear on radio.

For speech I used a news-radio broadcast, WCBS-AM news-brief. (Yes, this is AM but speech is not very different AM or FM; also I used WCBS's internet signal.)

Spectrum WCBS

There is a wide choice of popular music. The general spectrum does not vary a lot. I used ABBA's 'Dancing Queen' because it may be familiar to many and because it was handy.

'Classical' or 'good' music can vary a lot. And analysis of a full symphony takes a long time. I used a sample of baroque organ (*), which happened to have little bass content. Some heavy organ works on boomy 20th century organs will fill-up the bass, though not as much as ABBA.

After capturing the spectra, I plotted a line across the tallest frequency peaks, breaking at 2KHz (US de-emphasis) to 6db/octave.

As you can see, if the stronger sounds below 2KHz are not over-modulating the channel, then sounds above 2KHz will not over-modulate even if boosted 6db/octave above 2KHz.

So why not 1KHz or 3KHz?

For speech and many selections of older music, a 1KHz corner would work fine. And it would give significantly less channel hiss.

However if you look at the ABBA sample and discount the artificial boom at 55Hz, then shift the red line to fit, both the 440Hz peak and the 2600Hz peak would touch the line. The US 2KHz pre-emphasis just barely clears ABBA. There are many less-common musical styles, even before elaborate studio tricks, which if pre-emphasized too soon may over-modulate in the highs before the bass or middle.

Most music will pass 1KHz boost fine, but some tracks will distort. Far fewer tracks will distort with a 2KHz boost, enough so the 6db higher hiss is justified. A small number of tracks will distort for 2KHz, not for 3KHz. To pass these cleanly you must accept 3db more hiss.

I do not know why the US picked 2KHz and Europe picked 3KHz. It may be because the long distances in the US mean more fringe listeners who have increased hiss levels due to weak signals. There are also economic influences: US stations needed large audiences for advertising income and favored "light" music over long telephone lines; in Europe much radio was operated by national organizations, and pressure for listenership was instead a sense of pride in quality signals.


(*) St. Johannis Church in Luneberg, Germany
Bach's Toccata, Adagio and Fugue in C-Major
M. Schneider

 

Paul Reid
 
 
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05.May.11 18:52

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Correction:

> 1KHz boost ... 2KHz boost, .. 6db higher hiss
> 2KHz, ...3KHz. ... 3db more hiss.

Should read:

1KHz boost ... 2KHz boost, ... 3 db higher hiss
2KHz, ...3KHz. ... 1.5 db more hiss.

  
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