| |
G8MNY > TECH 18.08.10 06:45l 332 Lines 16560 Bytes #999 (0) @ WW
BID : 60085_GB7CIP
Read: GUEST
Subj: FM Stereo Radio Principles
Path: IZ3LSV<IK2XDE<DB0RES<DK0WUE<IK6ZDE<VE3UIL<N9PMO<N4JOA<GB7CIP
Sent: 100817/2330Z @:GB7CIP.#32.GBR.EU #:60085 [Caterham] $:60085_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Mar 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Being involved with 28 short term radio stations in UK (RSLs) over the last 12
years, here is part of a talk I do on it on the technical side.
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!
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 <- - - - - - - Broadcast HiFi - - - - - - - - >
³A < - - - - - Original Cassette- - - - - ->
³ <- - - - - Ä A.M. Radio - - - - - ->
³ F E E L I N G < - - COMMS - - ->
ÀÄÄÄÂÄÄÄÄÂÄÄÂÄÄÄÂÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÂÄÄÄÄÂÄÄÂÄÂÄÄÂ>Frequency
DC 10 20 30 50 100 300 1k 3 5 10 15 20 30kHz
The C.D. is 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 ch spacing so in theory 4.5kHz should be the
upper limit, but in practice 6kHz is the limit (9kHz -40dB) 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 F & S, B P E G D, M N letters!
I have not included Digital Broadcast as the quality is quite variable from
near CD quality down to phone call quality dependent on the data rate
assigned for that particular programme etc.
COMMON 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.
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, TV NICAM
90 ´ Perfect Digital CD, apparent Minidisk
100 ´
110 ´
120 ´ Ear Threshold Signal : Pain
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 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
3 Å -30 Cassette & Reel Tapes
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, DAB, NICAM
0.05Å -66 Good modern AF Amps.
0.03Å -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 cost of the treble dynamic range!
e.g. with a time constant of 50uS (75uS USA) 15kHz is lifted by 14dB, that is
14dB less dynamic range at 15kHz, or only a 20% level after de-emphasis.
+14dB_³ TX Response _ ³ Rx Response ³Overall Response³
³ ,/³ 0dB_³ ________ 0dB_³ ____________ ³_100%
³ ,/ ³ / Flat `\ ³ / `. | ³
0dB_³ ________./ 6dB/O ³ 6dB/O`\ ³/ `. ³ ³`.Level
³ /Flat Lift 14dB_³ Cut `\ ³ `. ³
ÀÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄ ÀÄÄÂÄÄÄÄÄÂÄÄÄÄÄÄÂÄ ÀÄÄÂÄÄÄÄÄÂÄÄÄÄÄÂÄÁ 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 time delay
through these filters 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_³ ______________________________________________
-3dB-³ /~ ~\
³ /' ³
³ ,/' ³
-70dB_³ /' V'-.__
ÀÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÂÄÄÂÄÄÄÄÄ
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, & several
decay time constants are used to 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.
To maintain the stereo image both left & right gains must be tracked together!
The result is a signal that has it's ñ peak value accurately limited, but
sounds perfect! With a good limiter you should not be able to tell the
difference between a 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 bandwidth.
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 With a good limiter you should
-³ U\/ not be able to tell the
difference between a live
Multiple CR studio feed & off air even with
0dB³-. Recovery . high limiting levels of around
³ ³ ..-'\/\ _ _.' 12 - 24dB used for AM.
GAIN ³ ³|' `' \|
-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 low frequencies down to DC being added to the signal.
A scope X-Y plot of Stereo (Pilot tones filtered off)
ÚÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄ¿
³ ³ ³ _ ³ ³ ³ ³ ³ ³ ³ ³ ³
³ ³ ³ ³ ³ ³ / ³ ³ \ ³ ³ / & ³ ³ ±±±±± ³
³ ÃÄÄÄÄÄ´ ³ ³ ³ ³ ³ / ³ ³ \ ³ ³ // )) ³ ³ ±±±±± ³
³ ³ ³ ³ ³ ³ / ³ ³ \ ³ ³ \\=// ³ ³ ±±±±± ³
³ ³ ³ ~ ³ ³ ³ ³ ³ ³ ³ ³ ³
ÀÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÙ
Limited Mono Anti-phase 2D Stereo Hard Limited
Left only Right @ 45ø (behind U) Pattern Stereo fills
ñ75kHz 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 the 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!
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_³ ._ With Poor LF Phase
+100%_³ _ __________ _ _ _ _ _ 140%_³_ _³ `-._ _ _ _ _ _ _ _³ _
+75kHz³ ³ ³ 100% ³ ³ `-._ ³
³ ³ ³ ³ ³ ³ ³
³ ³ ³ ³ ³ ³ ³
³ ³ ³ ³-._³ ³ _³
-100%_³___³ _ _ _ _ _³__________³ _ -100%_³_ _ _ _ _ _ _³ _.-'_ _
-75kHz³ -140%_³ ³_.-'
Audio with large 20Hz -110kHz³ '
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
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.
In practice an FM Tx may 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.
To make stereo radio signal a sample of the left & right channels is
transmitted every 38kHz. A switch samples the left or right channels @ 76kHz,
to keep the Rx in step a locked 19kHz pilot tone is sent at -20dB (10%) below
peak level. Here is the simplest way of making this signal..
76kHz
Left ÄÄo Switch
\,_______________________ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿
RightÄÄo /|\ ³ Add ÃÄÄ>Ä´ 53kHz ÃÄ>ÄÄ´ Add ÃÄÄ> to Tx
³ ÚÄÄ¿ ÚÄÄÄÄÄÄÄ¿ ÀÄÄÂÄÄÙ ³Low Pass³ ÀÄÄÂÄÄÙ
76kHz >ÄÄÁÄÄÄ´ö4ÃÄÄÄ´ 19kHz ÃÄÄÄÄÄ´-20dB ³ Filter ³ ³-30dB
ÀÄÄÙ ³Filter ³ ³ ÀÄÄÄÄÄÄÄÄÙ ÚÄÄÄÄÁÄÄÄÄ¿
ÀÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>Ä´RDS 57kHz³
PLL ³ UNIT ³
ÀÄÄÄÄÄÄÄÄÄÙ
Left_ Right MUX _
/' `\ ÄÄÄÄ¿ ÚÄÄÄÄÄ - -/-'- ¿` \ Ú - - - - - -
³ ³ ³ ³ ³ ³ ³ ³
³ - - - ³ - - - ³ - ³ - - ³ ³ ³ ³³ ³
³ ³ ³ ³ ³ ³³ ³
\,_,/ ÀÄÄÄÄÄÙ À Ä Ä Ä Ù \ , _ , /
Output after switch
(Expanded for diagram)
The low pass filter @ 53kHz used must have now phase shift (group delay) so
that the timings of the stereo samples are no affected. In some designs digital
tricks are used to null out the 2 & 3rd order harmonics of the switching
process (76kHz & 114kHz) so that a less aggressive 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 that it reduces the modulation & added at 2-3% to the MUX output.
The data is actually QPSK @ 1187.5Hz (76kHz/64) which only occupies about 1kHz
bandwidth. It contains many features, not many supported by stations, the
common ones are..
PS = Station Service Name
PI = Tx ID code, up to 256 station 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.
PTY= Programme Type 16 types, e.g. Jazz, News, Pop. etc.
RT = Radio Text, e.g. current song title
MUX SPECTRUM
Baseband spectrum
__ __ __
0dB_³ _____...---''' ³ P ³ ```---..._____ _____...---''' ³
³³ ³ I ³ ³ ³ ³
³³ ³ L ³ Difference ³ ³ Difference ³
³³ L+R ³ O ³ Signal ³ ³ Signal ³
-20dB_³³ MONO ³ T ³ DSB L-R ³ ³ DSB L-R ³ RDS
-30dB_³³ ³ ³ ³ ³ ³ ³
ÀÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÅÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÂÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÐÄÄ
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 a 20dB loss in signal to noise on an FM system, as it adds in, not
just the noise from 3x the bandwidth (9dB), but the very poor signal to noise,
of the high 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 very complex signals.
This is where Carson's rule for minimum bandwidth 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_³ _ ³_100%
<57.5>ÚÄÄÄÄÄÄÄÄÄÄÄÄ¿<57.5> ³ /' `\ ³
______³ ³______ -20dB-³ ³ ³ ³Ä10%
³ ³ _³ /' `\ ³_
³ ³ -40dB ³ /' `\ ³ 1%
ÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄ ÄÁÄÁÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÁÄÁÄ
< - - - - - 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.
\³/ TYPICAL TX SITE LINE UP
³ ÚÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄ¿ ÚÄÄ¿ ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
ÀÄ><Ä´CIRCULATORÃÄ´PAÃÄ´FMÃÄ´ RDS ÃÄ´STEREOÃÄ´PRE EMPHASISEDÃÄo-oÄ<L STUDIO
Rx+Tx³OR FILTER ³ ³ ³ ³Tx³ ³CODER³ ³CODER ÃÄ´STEREO LIMITERÃÄo-oÄ<R FEED
SignalÀÄÄÄÄÄÄÄÄÄÄÙ ÀÂÄÙ ÀÄÂÙ ÀÄÄÂÄÄÙ ÀÄÄÂÄÄÄÙ ÀÄÄÄÄÂÄÄÄÄÄÄÄÄÄÙ ³ ³
\³/ \³/ Data Mono \³/ Backup
Power Mod flags Levels Source
Also see my buls on "FM Deviation Calibration", "AM Broadcast principles" &
"1W @ 531kHz MW system".
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
