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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 9th Oct 2019, 5:50 am   #21
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Modifying discriminator coil for narrow band FM.

Originally Posted by Synchrodyne View Post
My recollection is that the sound volume when tuned to the local NOAA station was not hugely lower than it was for FM broadcasts, which at the time surprised me, given the big difference in maximum deviation.
Back in the day, the FM broadcasts would have been made with some actual dynamic range in their content, while the speech on the weather channel would have been compressed to rather low dynamic range. So your eyes and brain see the specified peak deviations on paper, but your ears hear the difference in the average deviations.

Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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Old 9th Oct 2019, 7:46 am   #22
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
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Default Re: Modifying discriminator coil for narrow band FM.

Indeed. Back in the day (1985 and through the 1990s), 2 of the DFW FM stations, one the PBS outlet and the other the local classical music station, used minimal compression, and I hardly ever listened to other than these when it came to FM. The commercial FM stations were very heavily compressed, and sounded noticeably louder than the other two. So I guess I was comparing NOAA weather radio with what were relatively quiet FM references. PBS had two SCAs, each taking 10%, so with 10% for the pilot tone, maximum signal audio was modulation ±52.5 kHz. And if we assume a 30% average, that’s just ±15.75 kHz.

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Old 10th Oct 2019, 7:17 pm   #23
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
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Default Re: Modifying discriminator coil for narrow band FM.

Well that's given me some encouragement to try a few quick test circuits. As wideband IF strips are typically stagger tuned, it may be possible to tweak them up a bit to obtain a more peaky response more suited to NBFM. As has been said already, any audio levels could be corrected by introducing a bit of gain, if the discriminator noise level isn't too high. If the coils can't be used "as-is" out of the spares box, it's probably a non starter, as there are probably better solutions available, using more suitable components.
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