When speaking to a portable radio transmitter the sounds usually gets distorted. However in radio stations that are dedicated to play music the sound is significantly better. First of all, why? And second how can I make it better? The system I use can transmit from 30Mhz to 87.975Mhz with 30Khz bandwidth (military system) and I want to play music over it (I connected the audio input to my phone). I know it's a radio problem because when I use wire the sound is better.

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    $\begingroup$ This isn't strictly ham radio. Having said that, Wikipedia says, "AM radio transmitters can transmit audio frequencies up to 15 kHz (now limited to 10 kHz in the US due to FCC rules designed to reduce interference), but most receivers are only capable of reproducing frequencies up to 5 kHz or less." So it might be your receiver rather than your transmitter. $\endgroup$ – user3486184 Oct 12 '17 at 21:53
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    $\begingroup$ @user3486184 It's not strictly ham radio, yes. But it is about the technology of radio, which is acceptable here. $\endgroup$ – Mike Waters Oct 12 '17 at 22:06
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    $\begingroup$ your military system will certainly be audio-processing-wise be optimized for speech clarity, not "perfectly linear reproduction of the audio", and certainly not "nice sounding music". That is, mathematically, not even possible with FM in 30 kHz. So, wrong tool for the job – honestly, just do FM with an SDR and you can tweak all your transmitter's properties till you're satisfied. $\endgroup$ – Marcus Müller Oct 14 '17 at 11:36

Bandwidth: Broadcast FM stations are spaced 200 KHz apart, with the intention of allowing this kind of signal structure without interference:

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The FM broadcast bandwidth of just more than 50 KHz is quite sufficient for near-CD quality stereo.

Normal broadcast FM uses multiplexing so the main channel includes L+R - both channels - what we call Mono. Then there is a subcarrier that sends L-R. The receiver detects the pilot signal (subcarrier) and lights up the "stereo" light, so the receiver knows to use a demultiplexer to separate the L and R signals which it then sends to your amplifier and speakers.

Sound quality: FM broadcast stations always use audio processors that are specifically tuned to make it sound better than any handheld radio's microphone circuitry will be able to duplicate.

enter image description here

(This one is roughly $10,000)

They also incorporate pre-emphasis so the roll-off of higher frequencies in the receiver IF bandpass filters are not as audible.

Dolby sound operates on a similar principle - to counteract artifacts introduced by the entire receiving chain including the speakers.

It takes a lot of work to properly tune the audio processor at a broadcast station.

Thus there is no comparison to a ham signal with 5 KHz deviation and no audio processing, with a small microphone. It can be easily heard.

The military system you describe operates with 30 KHz deviation. The receivers probably also have a 30 KHz bandpass filter, which means theoretically it should be able to reproduce up to 15 KHz audio frequencies.

But no filter is has perfect "cliff" edges, so there will be some kind of gradual roll-off at the higher audio frequencies.

One thing you can do with it for music, short of adding an audio processor, is to provide some sort of pre-emphasis.

This can be done with a regular equalizer on the input.

In broadcasting, to really tune it up well you would want to put a pink noise source (maybe an app) into the transmitter, then watch on a receiver with an audio spectrum analyzer. Adjust the transmitter's audio-input equalizer until you get a flat response end-to-end.

  • $\begingroup$ Thank you for the answer. Why would deviding the deviation by 2 give me the spectrum for audio? $\endgroup$ – Adam Katav Oct 15 '17 at 8:57
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    $\begingroup$ An FM signal with no modulation is just a simple carrier. It is a continuous wave. Then when you begin to add modulation the audio waves cause it to move in frequency to follow the waves. Imagine how a sine wave has a zero point and goes up and down from that. So a 15 KHz audio modulation signal will deviate the RF output signal by +/- 15 KHz, giving a total of 30KHz bandwidth. Ham radio's 5 KHz bandwidth will support up to about 2.5 KHz of audio frequency. Ma Bell designed the telephone system to carry 300-3000 KHz for voice, so our NFM is similar but not quite as good as a telephone. $\endgroup$ – SDsolar Oct 15 '17 at 15:12
  • $\begingroup$ But if you have 30KHz bandwidth then why won't the audio wave which starts from 0Hz deviate the carrier by 0Hz and 30KHz by 30KHz? Why do the audio wave use +/- of its frequency? $\endgroup$ – Adam Katav Oct 15 '17 at 15:17
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    $\begingroup$ Think of a sine wave. If you put a frequency in into an oscilloscope it will go up and down across the zero-volt-crossings. This is also why your light bulbs flicker at 120 Hz when given 60 Hz power - because they go dark at the zero-crossings. It uses 120 Hz of bandwidth in that sense. That is what happens in FM, except instead of voltage variations you have frequency variations. In other words, 30 KHz bandwidth means +/- 15 KHz of deviation $\endgroup$ – SDsolar Oct 15 '17 at 15:36

The two audio channels have been designed for two different purposes and thus the added distortions are very likely not the same.

Music channels are designed to make mediocre music sound good and captivating to radio advertisement consumers. Those audio channels commonly use boosting of some frequencies via sound processors to maximize that effect, and thus produce the illusion that the sound is "better".

Military/aviation comm channels are designed so that the voice commands can be understood with minimal error under typical noisy conditions, thus other audio frequencies are boosted or filtered in order to render a lower error rate in receiving the voice messages (which isn't necessarily produced by flat/low-distortion/hi-fi audio quality).


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