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G8MNY  > TECH     26.03.21 10:21l 228 Lines 10753 Bytes #999 (0) @ WW
BID : 28546_GB7CIP
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Subj: AM Broadcast Radio Principles
Path: IW8PGT<IZ3LSV<I3XTY<I0OJJ<GB7CIP
Sent: 210326/0920Z @:GB7CIP.#32.GBR.EURO #:28546 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                  (Corrections Jan 10)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

AUDIO
There are 3 main parameters to Audio quality.

  1/ FREQUENCY RANGE
  Here is the approximate frequency plot for some audio sources. Note that the
  old disk system was not really upper limited like CDs & special equipment can
  do Quad audio with high frequency sub-carriers!

 ³Sub Sonic³< - - - - - - - Human ear response - - - - - - - - >³Ultra Sonic>
 ³I             BASS                MID              TREBLE
 ³N    < - - - - - - - - - - - - Disk H i F i - - - - - - - - - - - - - >
 ³F     < - - - - - - - - - - C.D. H i F i - - - - - - - - - - - - - >
 ³R         < - - - - - - - - FM Broadcast HiFi - - - - - - - - - >
 ³A         < - - - - - - - * Mpeg/D.A.B. Radio - - - >...........>
 ³              <- - - - - - - A.M. Radio - - - - - - - - >
 ³  F E E L I N G              < - - - COMMS - - - ->
 ÀÂÄÄÄÄÄÂÄÄÄÂÄÄÂÄÄÄÄÄÂÄÄÄÄÄÂÄÄÄÂÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÂÄÄÄÂÄÄÄÄÂÄÄÄÄÄÂÄÄÄÂÄÂÄÄÄÂ>Freq
 10    20  30  50   100   200 300  500    1k    2   3    5    10  15 20 30kHz

  * Mpeg/D.A.B. & D.R.M. systems the quality is quite variable depending on the
  data rate chosen (same as internet broadcasting).

  Broadcast AM (EU) uses 9kHz channel spacing so in theory 4.5kHz should be the
  upper limit, but in practice 7kHz is the top limit (-40dB@ 9kHz) to make it
  sound a bit better.

  Comms Audio is the smallest bandwidth that can easily be understood, but not
  having any treble there is confusion over sounds of F S, B P T D E C G, M N
  letters etc!

  2/ SIGNAL TO NOISE RATIOS
  This is the measure of unwanted noises below the wanted sound..
  e.g. Hiss & Hum, or windage/engine noise, Neighbours/street noises etc.
   COMMON S/N LEVELS
   0dB´ Noisy Conversations
   10 ´ Poor Comms, NORMAL CAR
   20 ´ Fair Comms
   30 ´ VERY QUIET CAR                            AM RADIO (GOOD PHONE)
   40 ´ Typical Cassette Tape, Quite Living rooms
   50 ´ Reel-Reel tape, Dolby Cassette            TV SOUND
   60 ´ New Vinyl Record                          FM RADIO
   70 ´ Mini disk (unmasked noise)
   80 ´ Dat tape.                                 DAB*, TV NICAM
   90 ´ Perfect Digital CD, apparent Minidisk & Mpeg*
  100 ´
  110 ´
  120 ´ Ear Threshold Noise/signal pain

  3/ HARMONIC DISTORTION
  This the amount of unwanted signals generated in harmonics of the wanted
  signal in the audio pass band of interest. It is usually very dependent on
  the level, except for digital systems where it is a constant mathematical
  design feature.

  It is measured as a % of the signal, so 10% = -20dB in harmonics.
    %    dB
   10 Å -20  Cheap AM Radio at high Volume, Comms Audio
    5 Å -26  Film Optical Sound (density type)
    3 Å -30  Cassette & Reel Tapes                 Low bit DAB
    1 Å -40  Quality Valve Amps                    AM BROADCAST
   .5 Å -46  High Quality Disk                     FM BROADCAST
   .3 Å -50  Most Loudspeakers??
   .1 Å -60  Most AF Amps                          High Bit DAB, NICAM
  0.05Å -66  Good modern AF Amps
  0.03Å -70  Most Digital AF sources (not too compressed).

  Sometimes the above parameters are joined together in a Signal In Noise
  And Distortion (SINAD) rating for measured RF signal level of a Rx.

EMPHASIS
With AM there is not enough bandwidth or dynamic range to really use pre-
emphasis & de-emphasis, as there is with FM Radio, also unlike FM the noise
floor is quite flat & noise does not need masking so much.

However most Rx do cut the treble in their narrow IFs & AF detectors & some
compensation of the odd dB or so of treble lift is sometimes applied below the
sharp 7kHz cut off at the Tx.

+2dB_³ TX Response            ³ Rx Response            ³ Overall Response
     ³           ,|       0dB_³ ________           0dB_³ __________
 0dB_³ ________,/ |      -2dB_³/   Flat `\        -2dB_³/       `. \
     ³/ Flat      |           ³           `\           ³          `.³
     ³        Lift |          ³        Cut  `\         ³            ³
-40dBÅÄÂÄÄÄÄÂÄÄÄÄÂÄÅ     -40dBÅÄÂÄÄÄÄÂÄÄÄÄÂÄÂ     -40dBÅÄÂÄÄÄÄÂÄÄÄÄÂÄÂÄ 0%
       50   1k  5k 9k Hz        50   1k  5k 9k Hz        50   1k  5k 9k Hz

9kHz WALL FILTER
There is a requirement that the Tx sidebands do not interference to adjacent AM
channels, so higher AF frequencies that would cause Tx sidebands that would
interfere with adjacent channel carrier MUST BE removed to > -40dB level.

   0dB_³  _______________________________
100%Mod³ /                               `\
       ³                                   ³
 -40dB_³                                    \.___
 1% ModÀÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄ
         50   100  200  500   1k   2k   5k   9k  20k Hz

To give this level of filtering something like a 2 stage M derived filter is
needed to give the sharp cut off starting @ 7kHz.

LIMITING
As AM must not be over 100% modulated, a limiter is used, this is not like a
simple clipper used on comms Tx that lets the signal distort.

Broadcast limiters have electronic gain controls with fast attack to cope with
the spikiest peak, but the gain recovery uses several decay time constants to
mask the limiter's "breathing effects".

Complex limiters may be multi band & treat the treble separately with separate
faster time constants, as the treble content will be a more prominent on some
types of programme content.

       ³  ³ Peaky pulse
      +³  ³³                            The result is a signal that has
       ³_.³ ³    /'\                    it's ñ peak value accurately
      0ÅÄÄÄÄÄÅÄÄÁÄÄÅÄÄÄÄÄÄ.             limited, but sounds perfect!
       ³           '|,^| ³
      -³              U\/               With a good limiter you should
                                        not be able to tell the
           Multiple CR                  difference between a live
    0dB³-.  Recovery        .           studio feed & off air even with
       ³  ³  ..-'\/\  _  _.'            high limiting levels of around
GAIN   ³  ³|'       `' \|               12 - 24dB used for AM.
  -20dB³  ' Fast attack
       ÀÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄ>Time
           2    4    8   10s

As the limiting process must have fast attack times to handle all the peaks
one half cycle will Rx a different compression factor to the other half cycle
this results in some low frequencies down to near DC being added to the signal!

BAD LF RESPONSE AFTER LIMITER
It is also important that there is no phase distortion between the limiter &
the Tx modulation process, over all the frequencies to be transmitted. If there
is then the carefully peak limited signal can actually get larger....

Carrier     No Phase error @ 20Hz       240%_³   ._  HARD CLIP   Poor LF Phase
200%_³ _  __________  _ _ _ _ _             _³_ _³ `-._ _ _ _ _ _ _ _³ _
     ³   ³          ³                   200% ³   ³     `-._          ³
     ³   ³          ³                        ³   ³         ³         ³
100% ´   ³          ³                   100% ´   ³         ³         ³
     ³   ³          ³                        ³-._³         ³        _³
  0%_³___³ _ _ _ _ _³__________³ _        0%_³_ _ _ _ _ _ _³    _.-'_ _
     ³                                  -40%_³             ³_.-'HARD
       Audio with a 20Hz content             ³             '    CLIP
          limited to just fit                      -3dB @ 20Hz
         inside this envelope.         Tx Peaks now seriously over modulated

In AM Tx, poor LF phase response on high level Modulation Transformers often
cause unexpected hard clipping on certain waveforms!

Poor LF phase causes over modulation on some programme material containing deep
LF. This is one reason why loud LF is often cut on input of broadcast limiters.
But the cause is the "AF envelope shape" & not the "AF frequency content"!

TX LINEARITY
The AM sidebands should be symmetrical & contain no significant levels of
sideband harmonics. This is all about modulation linearity.

Methods of checking linearity use pure sine wave modulation source an either
an AF distortion analyser with a perfect Rx or a spectrum analyser.

   0dBc _
carrier ³          ³         As each sideband can only peak to -6dBc with
        ³          ³         100% modulation depth, subsequent distortion
  -6dBc_³          ³         sideband products are with respect to that.
        ³       ³  ³  ³      e.g. 1% distortion harmonic is @ -46dBc.
        ³       ³  ³  ³
 -46dBc_³       ³  ³  ³
 Dist 1%ÀÄÁÄÄÁÄÄÅÄÄÅÄÄÅÄÄÁÄÄÁ Spectrum
         -3 -2 -1 Fc +1 +2 +3

While this is sort of testing is OK into a dummy load for actual performance
figures, it does not reflect the modulation process into a typical high Q
aerial system.

One old & easy way to see the linearity while programme is being Tx is to use a
scope in XY mode, with the applied modulation on the X axis & RF on the Y axis.

 IDEAL 100% MOD                      TYPICAL TX
   straight              __            curvy              Max peak output
    sides  ,/³           Peak          sides     ,úÄ¿   - Clip/compress
          /' ³ ÄÄ        crest                ,/'   ³ ÄÄ
    No  <'   ³ Carrier    2x                -=:     ³  Carrier
  Carrier`\  ³ ÄÄ       Carrier      Carrier  `\.   ³ ÄÄ
           `\³           __           not 0%     `úÄÙ

UNWANTED PHASE MODULATION
Due to Hi Q tuned circuits in the PA & especially the aerial itself, there is
significant risk of the higher audio frequency sidebands being phase shifted.
This means there vectors will no longer add up to produce 100% carrier
modulation in the Rx. This effect looks like reduce bandwidth which it is.
It normally shows up shows up as an effect of kicking SWR to syllabance showing
the sidebands are seeing the bad aerial match.

SWR  2:1  1:1  2:1
           Fc                         Fc
           ³                 45ø      ³     45ø               ³  3dB
 Leading   ³   Lagging     lagging \  ³  /leading             ³  less
 Phase³    ³    ³ Phase    sideband  \³/  sideband            ³  modulation
      ÁÄÄÄÄÁÄÄÄÄÁ
        Spectrum                   Vectors             Resultant Modulation

But it can be in earlier stages as well, reducing the treble modulation level
even further.

TX RF Harmonics & Mixes
These should all be > -60dBc, so added filters are normal. On multiple Tx sites
there is a risk of PA mixing, where RF from a nearby Tx can be Rx at the Tx PA
at enough strength to cause a Mix. A narrow resonant channel filter (aerial
tuning L & C are normally enough in most designs!) in the Tx feed can protect
the Tx from these or specific suckout may be needed.


Also see my buls on "1W @ 531kHz MW station system".


Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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