On 8/2/2014 6:27 AM, Lostgallifreyan wrote:
Jeff wrote in :

The point that Jerry is missing is that the in-car devices that link an
MP3 player, and the like, to a car radio are being received by just
that, a car radio; so if the deviation were as low as 5 or even 15kHz
then the level of audio coming out of the radio would so so low as to be
unusable.

The deviation of the transmitters must match the bandwidth of the
receiver being used, ie the normal FM broadcast band deviation of 150kHz
of more.

I am not sure what the FCC power limit for such devices is, but in the
UK is it is 50nW erp in the FM broadcast band, yes 50 nanowatts. I
expect that the devices sold in the US are similar output power.

Jerry does not seem to take on board that this is an international
group, not a group of ex-US cable technicians who slovenly misuse dBm to
mean dBmV, when the rest of the world uses dBm correctly to mean dB
relative to a milliwatt.

Jeff


Thanks, that looks clear, and fits with other things I read, especially the
dBm bit and the 50 nW.

One thought: Does the bandwidth match have to be exact? Or is there a case
for some reduction on the transmitter to allow more of them to make
potentially less local competition between each other when lots are used? Is
there some compromise that doesn't afflict the sound too much, while reducing
both the occupancy of the spectrum and the need for power for some given
reach of distance?

(I don't intent using one, but they do interest me strangely...)


In an FM transmitter, the amount of deviation is proportional to the
amplitude of the modulating signal. That is, the louder the audio, the
greater the deviation.

Transmitter bandwidth cannot be greater than receiver bandwidth without
loss of signal and distortion. Transmitter deviation can be less than
receiver bandwidth; this only results in lower audio. This is why
commercial FM receivers typically have a bandwidth of 170-180 kHz, when
deviation is only 75 kHz. It makes things less critical.

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Jerry, AI0K

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