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