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home | newest check | boards | help index | log | ps | userlogin | send sysop | slog | status forward | bcm news | users | version | remove cookieG8MNY > TECH 16.05.24 18:15l 228 Lines 10754 Bytes #10 (0) @ WW BID : 7802_GB7CIP Subj: AM Broadcast Radio Principles Path: JH4XSY<IW0QNL<SR1BSZ<EA2RCF<I0OJJ<GB7CIP Sent: 240516/0846Z @:GB7CIP.#32.GBR.EURO #:7802 [Caterham Surrey GBR] $:7802_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! Sub Sonic< - - - - - - - Human ear response - - - - - - - - >Ultra Sonic> I BASS MID TREBLE N < - - - - - - - - - - - - Disk H i F i - - - - - - - - - - - - - > F < - - - - - - - - - - C.D. H i F i - - - - - - - - - - - - - > R < - - - - - - - - FM Broadcast HiFi - - - - - - - - - > A < - - - - - - - * Mpeg/D.A.B. Radio - - - >...........> <- - - - - - - A.M. Radio - - - - - - - - > F E E L I N G < - - - COMMS - - - -> >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 -20 Cheap AM Radio at high Volume, Comms Audio 5 -26 Film Optical Sound (density type) 3 -30 Cassette & Reel Tapes Low bit DAB 1 -40 Quality Valve Amps AM BROADCAST .5 -46 High Quality Disk FM BROADCAST .3 -50 Most Loudspeakers?? .1 -60 Most AF Amps High Bit DAB, NICAM 0.05 -66 Good modern AF Amps 0.03 -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, also 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_ TX Response Rx Response Overall Response ,| 0dB_ ________ 0dB_ __________ 0dB_ ________,/ | -2dB_/ Flat `\ -2dB_/ `. \ / Flat | `\ `. Lift | Cut `\ -40dB -40dB -40dB 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_ _______________________________ 100%Mod / `\ -40dB_ \.___ 1% Mod 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. Peaky pulse + The result is a signal that has _. /'\ it's peak value accurately 0. limited, but sounds perfect! '|,^| - U\/ With a good limiter you should not be able to tell the Multiple CR difference between a live 0dB-. Recovery . studio feed & off air even with ..-'\/\ _ _.' high limiting levels of around GAIN |' `' \| 12 - 24dB used for AM. -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 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%_ ._ HARD CLIP Poor LF Phase 200%_ _ __________ _ _ _ _ _ __ _ `-._ _ _ _ _ _ _ _ _ 200% `-._ 100% 100% -._ _ 0%____ _ _ _ _ ___________ _ 0%__ _ _ _ _ _ _ _.-'_ _ -40%_ _.-'HARD Audio with a 20Hz content ' 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 As each sideband can only peak to -6dBc with 100% modulation depth, subsequent distortion -6dBc_ sideband products are with respect to that. e.g. 1% distortion harmonic is @ -46dBc. -46dBc_ Dist 1% 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 ,/ Peak sides ,Ŀ - Clip/compress /' crest ,/' No <' Carrier 2x -=: Carrier Carrier`\ Carrier Carrier `\. `\ __ not 0% ` 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 45 45 3dB Leading Lagging lagging \ /leading less Phase Phase sideband \/ sideband modulation 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
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