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G8MNY > TECH 19.09.06 21:47l 196 Lines 8762 Bytes #999 (0) @ WW
BID : 46474_GB7CIP
Read: DL1LCA GUEST OE7FMI
Subj: AM Broadcast Radio Principles
Path: DB0FHN<DB0RGB<DB0MRW<DB0SON<DB0SIF<DB0EA<DB0RES<DK0WUE<CE8FGC<ON0BEL<
GB7CIP
Sent: 060914/1657Z @:GB7CIP.#32.GBR.EU #:46474 [Caterham] $:46474_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Sep 06)
(8 Bit ASCII Graphics use code page 437 or 850)
(This is a modified bul from my Stereo Principles one)
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 < - - - - 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
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
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 parameters are joined together in a Signal In Noise
And Distortion (SINAD) rating for measured RF signal level of a Rx.
EMPHASIS
There is not enough bandwidth or dynamic range to really use pre-emphasis &
de-emphasis as there is with FM Radio, & unlike FM the noise floor is quite
flat. 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
before 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%
50Hz 1kHz 5kHz 9KHz 50Hz 1kHz 5kHz 9kHz 50Hz 1kHz 5kHz 9KHz
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 the -40dB level.
0dB_³ ______________________________________________
100%Mod³/ `\
³ ³
-40dB_³ \.__
1% ModÀÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄ
50Hz 100Hz 200Hz 500Hz 1kHz 2kHz 5kHz 9kHz
To get this step filter something like a 2 stage M derived filter is used with
a 9KHz AF phasing notch as well.
LIMITING
As AM must not be over 100% modulated, 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.
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.
³ ³ Peaky pulse
+³ ³³
³_.³ ³ /'\
0ÅÄÄÄÄÄÅÄÄÁÄÄÅÄÄÄÄÄÄ.
³ '|,^| ³
-³ U\/
Multiple CR
0dB³-. Recovery .
³ ³ ..-'\/\ _ _.'
GAIN ³ ³|' `' \|
-20dB³ ' Fast attack
ÀÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄ>Time
2 4 8 10s
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....
Carrier No Phase error @ 20Hz 280%_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 large 20Hz ³ ' CLIP
content limited to just fit -3dB @ 20Hz
inside this 20Hz 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.
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 eg. 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 etc.
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 O/P
sides ,/³ Peak sides ,Ä¿ - Clip/compress
/' ³ÄÄ crest ,/' ³ÄÄ
No <' ³ Carrier 2x -=: ³ Carrier
Carrier`\ ³ÄÄ Carrier Carrier `\ ³ÄÄ
`\³ __ not 0% `úÄÙ
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 mist designs!) in the Tx feed can protect
the Tx from these or specific suckout may be needed.
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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