G8MNY  > TECH     29.11.21 08:00l 340 Lines 16937 Bytes #193 (0) @ WW
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Subj: FM Stereo Radio Principles
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Sent: 211129/0813Z @:GB7CIP.#32.GBR.EURO #:54365 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                       (Updated Dec 19)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Being involved with 30 short term radio stations in UK (RSLs) since 1994 & now
a perminant station, here is part of a talk I do on the technical side, of it.

AUDIO
There are 3 main parameters to Audio quality.

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

 │Sub Sonic│<- - - - -Young Human ear response - - - - - - ->│Ultra Sonic >
 │I            BASS                MID              TREBLE
 │N    <- - - - - - - - - - -Disk H i F i- - - - - - - - - - - >
 │F       < - - - - - - - - -C.D. H i F i- - - - - - - - - - >
 │R          <- - - - - - - Broadcast HiFi - - - - - - - - >
 │A             < - - - - - Original Cassette- - - - - ->
 │               <- - - - - ─ A.M. Radio - - - - - ->          
 │ F E E L I N G              < - - COMMS - - ->                B A T S
 └───┬────┬──┬───┬────┬───────┬───┬────┬───────┬───┬────┬──┬─┬──┬>Frequency
 DC 10   20  30  50  100     300 500   1k      3   5   10 15 20 30kHz

 CDs are the best source most people are familiar with, its frequency range is
 limited by the 44kHz sampling rate & the requirement to filter off all the
 frequencies higher than 22kHz to stop aliasing mixes caused by the sampling
 rate.

 Broadcast FM is limited to 15kHz to stop problems with the stereo pilot
 used. But 15kHz represent a good compromise for the upper limit which is why
 it was chosen.

 Broadcast AM (EU) uses 9kHz chanel spacing, so in theory 4.5kHz should be the
 upper limit, but in practice 6kHz is the limit (-40dB @ 9kHz) to make it sound
 a bit better.

 Comms Audio is the smallest bandwidth that can easily be understood, but with
 "no treble" there is confusion over "A K J, F S, B C D E G P T, M N" sounds!

 I have not included Digital Broadcast, as the quality is quite variable, from
 near CD quality, right down to phone call quality, dependent on the data rate
 assigned for a particular programme/ch.

 SIGNAL TO NOISE RATIO
 This is the measure of unwanted noises below the wanted sound..
 e.g. Hiss & Hum, or windage/engine noise, Neighbours/street noises etc.

  0dB┤ Noisy Conversation
  10 ┤ Poor Comms, NORMAL CAR
  20 ┤ Fair Comms
  30 ┤ VERY QUIET CAR                        AM RADIO
  40 ┤ Typical Cassette Tape, Living rooms
  50 ┤ Reel-Reel tape, Dolby Cassette
  60 ┤ New Vinyl Record                      FM RADIO/TV SOUND
  70 ┤ Mini disk (unmasked noise)
  80 ┤ Dat tape.                             DAB
  90 ┤ Perfect Digital CD, apparent Minidisk
 100 ┤
 110 ┤
 120 ┤ Ear Threshold Signal : Silence to Pain ratio

 With Digital sources there is also "Quantization Noise/Distortion", which is
 due to the sample step size, & is a set No of dBs below any sound level for
 logarithmic level sampling.

 HARMONIC DISTORTION
 This is 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 mathematical design
 feature. Note the ear generates these too!

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

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

FM EMPHASIS
The FM Radio system suffers noise level that rises with frequency. To mask this
the treble is lifted on Tx & cut on Rx, this substantially reduces the hiss, &
top end harmonic distortion, but at the loss of treble peak loudness!
e.g. with a time constant of 50uS (75uS USA) 15kHz is lifted by 14dB (17dB USA)
that is 14dB less peak loudness at 15kHz, or only 20% of max level after
de-emphasis.

+14dB_│ TX Response    _        │ Rx Response         │Overall Response│
      │              ,/│    0dB_│  ________       0dB_│  ____________  │_100%
      │            ,/           │ /  Flat  `\         │ /       `.   | │
  0dB_│  ________./ 6dB/O       │       6dB/O`\       │/          `. │ │`.Level
      │ /Flat       Lift   14dB_│         Cut  `\     │             `. │
      └──┬──────┬──────┬─       └──┬─────┬──────┬─    └──┬─────┬─────┬─┴ 0%
        30Hz   1kHz   15kHz      30Hz   1kHz   15kHz    30Hz  1kHz  15kHz

15kHz WALL FILTER
As the treble is lifted & there is a requirement the audio does not interfere
with the 19kHz stereo pilot tone, & higher frequencies of the stereo system, so
a matched pair of sharp audio cut off filters are needed. The filter time delay
MUST be the same, as the stereo image positioning you hear is all about treble
timings.

Other audio tailoring may reduce the subsonic audio as well.

  0dB_│           _____________________________________________
 -3dB-│        .-'                                             '-.
      │      /'                                                   Ţ
      │   ,/'                                                     Ů 
-70dB_│ /'                                                         V'-.__
      └┬────────┬────────┬────────┬────────┬────────┬────────┬──┬──┬─────
       10      30       100      300      1k       3k       10k15k 19k Hz

LIMITING
As FM must not be over deviated to keep the bandwidth down, a limiter is used,
this is unlike a simple clipper used on comms Tx that lets the signal distort.

Broadcast limiters have fast attack to cope with the spikiest peak, & with
several decay time constants, that mask the limiter's breathing effects.
Complex limiters may also treat the treble separately with separate faster time
constants, as the treble content will be a more prominent part of the pre-
emphasised signal. Any peaks then left not properly gain limited (hardly any)
then get hard clipped.

To maintain the stereo image both left & right gains must be tracked together!

The result is a signal that has it's ˝ peak value accurately limited, but
sounds perfect! With a good limiter you should not be able to tell the
difference between live studio feed & off air with limiting of around 12-20dB.

The peak values result in the actual FM deviation, which ensures the correct
overall Tx bandwidth.

      │  │ Peaky pulse
     +│  ││                          The result is a signal that has
      │_.│ │    /'\                  it's ˝ peak value accurately
     0┼─────┼──┴──┼──────.           limited, but sounds perfect!
      │           '|,^| │            With a good limiter you should
     -│              U\/             not be able to tell the
                                     difference between a live
          Multiple CR                studio feed & off air even with
   0dB│-.  Recovery        .         high limiting levels of around
      │  │  ..-'\/\  _  _.'          12 - 24dB used for AM.
GAIN  │  │|'       `' \|
 -20dB│  ' Fast attack
      └───┬────┬────┬────┬───>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 DC being added to the signal.

A scope X-Y plot of Stereo (Pilot tone filtered off)
┌─────────┐  ┌─────────┐  ┌─────────┐  ┌─────────┐  ┌─────────┐  ┌─────────┐
│         │  │    ┬    │  │ ˙       │  │       ˙ │  │       ┐ │  │         │
│         │  │    │    │  │  \      │  │      /  │  │      /  │  │  ▒▒▒▒▒  │
│ ├─────┤ │  │    │    │  │    \    │  │    /    │  │    /    │  │  ▒▒▒▒▒  │
│         │  │    │    │  │      \  │  │  /      │  │  /      │  │  ▒▒▒▒▒  │
│         │  │    ┴    │  │       ˙ │  │ ˙       │  │ └       │  │         │
└─────────┘  └─────────┘  └─────────┘  └─────────┘  └─────────┘  └─────────┘
         Limited           Anti-phase    Normal    Hard Limited  Stereo fills
 Left only    Right only   (behind U)     Mono          Mono      ˝75kHz Box

If you set up a scope (or PC) to display broadcast audio you can see the
stations that run hard limiting, as they end up with a tightly defined box
filled all the time. This is not that they have clipped audio with distortion,
but quickly acting AGCs do that do not generate AF harmonics. The result is
very LOUD audio, each audio frequency can still have 40dB or so dynamic range,
but the overall modulation is 100% nearly all the time (each millisecond). Such
stations can be a strain on the hearing to listen to!

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

       No Phase error @ 20Hz          +110kHz_│   ._ With Poor LF Phase
+100%_│ _  __________  _ _ _ _ _         140%_│_ _│ `-._ _ _ _ _ _ _ _│ _
+75kHz│   │          │                   100% │   │     `-._          │
      │   │          │                        │   │         │         │
      │   │          │                        │   │         │         │
      │   │          │                        │-._│         │        _│
-100%_│___│ _ _ _ _ _│__________│ _     -100%_│_ _ _ _ _ _ _│    _.-'_ _
-75kHz│                                 -140%_│             │_.-'
        Audio with large 20Hz          -110kHz│             '
      content limited to just fit                   -3dB @ 20Hz
      inside this 20Hz envelope.        Tx Peaks now seriously over modulated

The same problem occurs in an AM Tx, where the poor LF phase response on high
level Modulation Transformers cause unexpected hard clipping on certain AF
envelope waveforms. (not the AF's LF content!)

In practice an FM Tx will have another hard limiter (clipper) to protect it
from accidental over modulation.

STEREO MULTIPLEX
The system used for all stereo radio broadcasts is known as the Zenith-GE Pilot
Tone System (so-called after the names of the two companies who devised it).
It has been designed to be fully compatible with Mono FM radio Rx & without too
much increase in bandwidth.

The MUX signal can be made with a 38kHz Double Side Band exciter fed with a LR
difference signal, & added to the mono signal. But modern linear switching
electronics, & good filters means the simpler methord is now used. That is to
take a sample of the left & right channels every 38kHz. So a switch samples the
left or right channels @76kHz, to keep the Rx's switch in step a locked 19kHz
sinewave pilot tone is sent at -20dB (10%) below peak level. The phase of the
pilot tone is critical to good channel seperation.

          76kHz
Left ──o  Switch
        \,_______________________┌─────┐    ┌────────┐    ┌─────┐
Right──o  /|\                    │ Add ├──>─┤ 53kHz  ├─>──┤ Add ├──> to FM Tx
           │   ┌──┐   ┌───────┐  └──┬──┘    │Low Pass│    └──┬──┘
  76kHz >──┴───┤÷4├───┤ 19kHz ├─────┤-20dB  │ Filter │       │-30dB
               └──┘   │Filter │     │       └────────┘  ┌────┴────┐
                      └───────┘     └─────────────────>─┤RDS 57kHz│
                                                    PLL │   UNIT  │
                                                        └─────────┘

  _ Left        Right          MUX    _
 / \        ───┐   ┌─────      ─ ─ ─/─ ┐\        ┌ ─ ─ ─ ─
Ů   Ţ          │   │              Ů    │  Ţ      │
│   │          │   │            │      │    │    │      │
    Ů   Ţ      │   │                   │      Ů  │    Ţ
     \_/       └───┘                   └ ─ ─ ─ ─\┘_ /
                                    Output after switch
                                   (Expanded for diagram)

The low pass filter @ 53kHz used, must have low phase shift (group delay) so
that the timings of the stereo samples are not affected. In some designs
digital tricks are used to null out the 2 & 3rd order harmonics of the
switching process (76kHz & 114kHz), so a less aggressive "lower distorting" low
pass filter can be used.

RDS (Radio Data System, similar to ARI system in Germany)
This is data phase modulated on to a 57kHz carrier that is phase locked to the
19kHz, so it actually reduces the overall modulation & is added at 2-3% to the
MUX output. The data is QPSK @ 1187.5Hz (76kHz/64) which only occupies about
2kHz bandwidth (seen as 2 data Carriers). It contains many features, not many
supported by all stations, the common ones are...

  PS = Station Service Name
  PI = Tx ID code, up to 256 stations in a network
  AF = Alternative Frequency list (radio searches this on weak signal)
  CT = Time
  TA & TP = Traffic Flags, allows radio to change AF source/level.
  PTY= Programme Type 16 types, e.g. Jazz, News, Pop. etc.
  RT = Radio Text, e.g. current song title

MUX BASEBAND SPECTRUM
                      __         __                               __
  0dB_│ _____...---'''  │   P   │  ```---..._____   _____...---'''  │
      ││                │   I   │                │ │                │
      ││                │   L   │   Difference   │ │   Difference   │
      ││      L+R       │   O   │     Signal     │ │     Signal     │
-20dB_││      MONO      │   T   │    DSB L-R     │ │    DSB L-R     │  RDS
-30dB_││                │   │   │                │ │                │
      └┼────────────────┼───┼───┼────────────────┴┬┴────────────────┼───đ──
      30Hz             15  19  23kHz            38kHz              53  57kHz

To achieve good channel separation linear frequency & phase response is needed
between the Tx coder & the Rx decoder. Reduced levels or phase shift @ 38kHz
make the channel separation poor (tending to MONO), & increased HF gain widens
the channel separation.

The increase in baseband bandwidth from 15kHz for mono to 53kHz for stereo
causes about 20dB loss in overall signal to noise ratio of the FM system. As it
adds in, not just the noise from 3x the bandwidth (9dB), but the very poor
signal to noise, of the higher frequency stereo difference signal that you Rx
on FM systems. The resultant "noise" in the stereo image appears as noise from
behind you....

   LEFT       RIGHT
         o^o
        [   ]
         \_/

        NOISE

TX SPECTRUM
The Bessel Functions shows the FM sideband harmonic levels, for any particular
modulation index. For mono the modulation index is peak Dev/Mod 75kHz/15kHz = 5
but this analysis is less useful for real very complex signals.

This is where Carson's rule for minimum bandwidth is needed, can give clearer
indication.
             Bandwidth = 2x Peak Dev + 2x Highest Mod Freq

This gives the bandwidth of sidebands needed for NO distortion. But it does not
take into account, that the levels of the highest modulation frequency are only
3% (-30dB) of the peak deviation, with resultantly weak sidebands.

             Deviation
     Mod   <-2x 75kHz ->  Mod           0dB_│         _         │_100%
    <57.5>┌────────────┐<57.5>              │        / \        │
    ______│            │______        -20dB-│       │   │       │─10%
   │                          │            _│     /'     `\     │_
   │                          │       -40dB │  ./'         `\.  │ 1%
───┼─────────────┼────────────┼───         ─┴─┴────┬─────┬────┴─┴─
   < - - - - - 265kHz - - - - >       -60dB         100kHz        0.1%
    Theoretical Full Bandwidth              More Typical bandwidth
  of RDS Stereo Broadcast signal         as seen on spectrum analyser
       for ZERO distortion.                 under heavy modulation.

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 or
directional coupler (Circulator/isolator) in the Tx feed can protect the Tx
from these signals.

\│/             TYPICAL  TX  SITE  LINE  UP
 │    ┌──────────┐ ┌──┐ ┌──┐ ┌─────┐ ┌──────┐ ┌──────────────┐
 └─><─┤CIRCULATOR├─┤PA├─┤FM├─┤ RDS ├─┤STEREO├─┤PRE EMPHASISED├─o-o─<L STUDIO
 Rx+Tx│OR FILTER │ │  │ │Tx│ │CODER│ │CODER ├─┤STEREO LIMITER├─o-o─<R FEED
Signal└──────────┘ └┬─┘ └─┬┘ └──┬──┘ └──┬───┘ └────┬─────────┘ │ │
test point         \│/   \│/   Data    Mono       \│/         Backup
                  Power  Mod   flags             Levels       Source

Also see my buls on "FM Deviation Calibration", "RF Directional Coupler",
"AM Broadcast principles" "1W @ 531kHz MW system" & "Marconi 2331A Distortion
Factor".


Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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