- March 6, 2016 at 8:09 pm #1059081
I got the code from here
if you do “try this at home” (or anywhere else) – be well aware that although it is impressive the radio signal generated by this method (using some very clever maths and DSP that I hardly understand) is very rough and full of harmonics – it makes use of a “feature” of PWM signals that can often be a nuisance (like LED displays that are pretty but make unwanted noise on audio and radio equipment…)
Transmitting these can upset a lot of other important radio communications on other frequencies, including aviation and emergency services comms and even some mobile telephones. That can get you in trouble with your country’s Communications Ministry!
In many nations (even where mini FM transmitters are permitted below a certain power) they also generally do not approve of RDS data being transmitted as well in case it confuses or distracts people driving motor vehicles.
In my country (UK) even licensed broadcasters must only transmit static channel IDs (unlike elsewhere in mainland Europe) – there is more leeway with regard to RT (radiotext) that does not automatically appear on most older receivers.
I used the very shortest length of wire that would give a reliable signal to receivers a few metres away and placed the RPi low down in an area where the signal would be blocked by various other metal items connected solidly to earth; and even then only ran this for a few minutes as a demo of how RDS works (otherwise I just use a legal mini FM TX without RDS)April 1, 2016 at 6:33 pm #1279365
I gave this a try – it works pretty well. The harmonic crud from the R Pi is horrid – the spectrum looks like a hedgehog on my analyser. Fortunately, the required signal is reasonably far away from the nearest spurious outputs, so by putting the R Pi in a screened box, using a critically tuned filter connected to the “aerial” pin and then using an MAR6 amplifier pill with a further critically tuned filter on the end, I was able to get about 350mW of clean signal. The worst spurs were -65dB ref carrier (which means that they’re in the µW range and won’t get much beyond the box! The whole thing is now housed in a neat metal box, and it makes a great little “house” transmitter for home use. The trickiest party of the whole deal is the screening and the supply filtering.
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