| |
G8MNY > TECH 18.12.17 11:40l 228 Lines 10750 Bytes #999 (0) @ WW
BID : 2698_GB7CIP
Read: GUEST
Subj: AM Broadcast Radio Principles
Path: IW8PGT<IZ3LSV<IR2UBX<SR1BSZ<GB7CIP
Sent: 171218/1003Z @:GB7CIP.#32.GBR.EURO #:2698 [Caterham Surrey GBR] $:2698_GB
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, & 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
Read previous mail | Read next mail
| |
