75us de-emphasis for Broadcast AM

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75us de-emphasis for Broadcast AM

Post by KM4JOJ » Fri Mar 31, 2017 5:13 pm

Back in the mid 80's the National Radio Systems Committee (NRSC) created some voluntary standards for commercial AM stations in order to address quality issues they faced at the time. One of these improvements was a pre-emphasis curve that is essentially based on the 75us curve used for FM pre-emphasis. At the start, all of this was strictly voluntary; however in 1990, the FCC made NRSC compliance a requirement. As a result, a large number of stations are employing the NRSC pre-emphasis to their signals.

SDRUno currently does not take in to account of this pre-emphasis; as a result the broadcast audio demodulated from a US AM transmitter sounds wrong. It is exceptionally shrill and has the wrong EQ...regardless of IF width. While reducing one sideband will somewhat help; this is still not a practical solution.

As I bought my SDRPlay primarily for listening to AM stations at full width; this becomes an issue. While my previous SDR program had a plugin that provided this de-emphasis; the SDRPlay does not function with that program. SDRUno does not currently provide this; leaving me with no choice but to either listen to badly EQ'd AM radio, which I tend not to do because it gets hard on the ears.

As 75us de-emphasis is already available for FM; it would be nice to see it available as a filter for AM as well. While the actual NRSC curve isn't an exact copy of the FM 75us; it is "close enough" that one would have a difficult time knowing. In my own tests with the 75us curve; the results were very acceptable.

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Re: 75us de-emphasis for Broadcast AM

Post by g1hbe » Fri Mar 31, 2017 11:00 pm

Have you tried the 'soft filter' setting? It gives a less 'rectangular' IF filter shape and rounds off the shoulders of the slope, rather like an analogue filter would do. I use this and I find it much kinder on the ear than the default setting.

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