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G8MNY > TECH 03.08.05 10:20l 127 Lines 5130 Bytes #999 (0) @ WW
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Subj: FM Stereo Radio Principles 1/2
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Sent: 050803/0847Z @:GB7CIP.#32.GBR.EU #:56775 [Caterham] $:56775_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Dec 04)
Being involved with 25 short term radio stations in UK (RSLs) over the last 10
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 responce - - - - - - - ->³Ultra Sonic >
³I BASS MID TREBLE
³N < - - - - - - - - - - - Disk H i F i - - - - - - - - - - - >
³F < - - - - - - - - - C.D. H i F i - - - - - - - - - - ->
³R < - - - - - - - - Broadcast HiFi - - - - - - - ->
³A < - - - - Ä - A.M. Radio - - - - - - ->
³ F E E L I N G <- - - COMMS - - ->
ÀÄÄÄÂÄÄÂÄÄÄÂÄÄÂÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÂÄÄÄÄÂÄÄÄÂÄÂÄÄÂ>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 15Hz 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 (7kHz -40dB) to make it sound
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!
COMMON SIGNAL TO NOISE RATIOS
This is the measure of unwanted noises below the wanted sound..
eg. 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 Noise/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 paramaters are joined togther in a Signal In Noise
And Distortin (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!
eg. with a time constant of 50uS (75 USA) 15kHz is lifted by 14dB, that
is 14dB less dynamic range at 15KHz or 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 15KH 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 frequiencies the the stereo system, 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 ________
³ /' `\
³/ ³ _
-70dB_³ V' `\.__
ÀÄÄÄÂÄÄÄÄÄÄÄÂÄÂÄÄÄÄÄÄÄÄ
30Hz 15 19kHz
In part 2
LIMITING
BAD LF RESPONSE AFTER LIMITER
STEREO MULTIPLEX
RDS Data
MUX SPECTRUM
TX SPECTRUM
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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