Modifying discriminator coil for narrow band FM. - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Radio Wrangler · 9th Oct 2019, 5:50 am

Quote:

Originally Posted by Synchrodyne

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.

David

Synchrodyne · 9th Oct 2019, 7:46 am

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.

Cheers,

Biggles · 10th Oct 2019, 7:17 pm

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.
Alan.

9th Oct 2019, 7:46 am	#22
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	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. Cheers,

10th Oct 2019, 7:17 pm	#23
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	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. Alan.