What's with the whine on P25 decode with all 396s?

[Pr999]

I've always wondered why my 325 & 996 have a P25 LP Filter setting on them.
Later, I got interested in older scanners, and I became interested in the bcd396xt and the bcd996xt. I also started doing research on just the 396xt and the 396t, and the weirdest thing that stuck out about these scanners is that they have a small 4khz tone in every p25 decoded transmission. This seems to be present only in the 396 line of scanners. I have even purchased a 396xt for myself since I enjoy the scanner a lot, and I can hear a little whine in every p25 transmission.

I also came across stuff about learning to change the threshold levels, but I'm so used to the setting being present on my P2s. This is where we come back to this P25 Low Pass filter setting. So I decided to use it on my new 396xt and it got rid of the noise. I don't know if the scanner is producing it or something else, none of my other scanners have this issue.

So the question is, why does this 4khz whine exist? Why is it only on the 396 series? And what produces it?

I hope I posted this in the right place.

 

[Pr999]

I should've asked, why does this line of scanners have this issue?

 

[Unitrunker2]

I think only Uniden can answer.

Generically speaking, the audio output of IMBE is 8,000 samples per second. Normally, this output is low-pass filtered.

 

[smithken]

I have never heard a whine on my 996T or my 396XT. I have the State of Michigan P25 system programmed into both.

 

[Pr999]

I have never heard a whine on my 996T or my 396XT. I have the State of Michigan P25 system programmed into both.

Do you have the LP filter on?

 

[smithken]

As far as I know I don't have any filters on, I guess I need to check.

 

[Nasby]

I would say because it was developed when George Bush was president. Old / ancient technology like that had all kinds of quirks.