Low-Pass Filter for scanner feeds

[SlipNutz15]

I notice that when anyone talks on my feeds there is a noticeable PL hum associated with it. I understand that there are low-pass filters that can be put in place to eliminate the hum.

Please verify (or correct me if I'm wrong) that a low-pass filter cuts out any audio below a certain threshold. So the PL in my county is 250.3 so I would want a 300hz filter? My other feed's PLs are 156.7, so I would need a 200hz filter?

I was looking and found these and was verifying they would work:

300hz:
Harrison Labs FMOD Inline Crossover Pair 300 Hz High Pass RCA

200hz:
Harrison Labs FMOD Inline Crossover Pair 200 Hz Low Pass RCA

 

[Voyager]

Actually, you would want the high pass filter.

An aftermarket PL decoder should come with one and you should be able to find one fairly cheap.

 

[SlipNutz15]

Well, I guess I was not paying attention to the links I posted. The 300 is high pass and 200 is low.

So I would want 300 High Pass for the 250.3 PL feeds? and 200 for the 156.7 PL feeds?


And what do I need a PL decoder for?

 

[n5ims]

Well, I guess I was not paying attention to the links I posted. The 300 is high pass and 200 is low.

So I would want 300 High Pass for the 250.3 PL feeds? and 200 for the 156.7 PL feeds?


And what do I need a PL decoder for?

A little terminology refresher for you and others:

Low pass filter - Pass from the specified frequency and below, attenuate frequencies above the specified frequency (hence the "Low" in the name).

High pass filter - Pass from the specified frequency and above, attenuate frequencies below the specified frequency (hence the "High" in the name).

Band pass filter - Pass a specified range of frequencies and attenuate those outside of that range (hence the "band pass" name).

Notch filter - Pass everything except those in a narrow range of frequencies specified as the notch and attenuate those in the notch (hence the "notch" name).

The 300 high pass filter should eliminate both the 250.3 and 156.7 tones. It may actually be a better choice since you'll be covered if they change tones to something above the cutoff of the 200 Hz filter.

 

[Voyager]

Well, I guess I was not paying attention to the links I posted. The 300 is high pass and 200 is low.

So I would want 300 High Pass for the 250.3 PL feeds? and 200 for the 156.7 PL feeds?

And what do I need a PL decoder for?

For stripping PLs (hum), you would want the 300 Hz high pass filter. That's the standard for Land Mobile. It would work on all PL tones.

The PL decoder would have a built-in 300 Hz HPF, and may be a lower cost than the separate HPF - especially if you can get one used. Just make sure the decoder has the 300 Hz HPF. Not that you need it as a decoder - just a source for a low cost HPF.

 

[SlipNutz15]

Thank you both for your replies.

As far as the PL decoder I'm not sure I'd need one. These HPF's I'm looking at are only around $30/pair.

 

[SlipNutz15]

I do have another question. With these filters in place, would it affect how a DPL or NAC sounds? Half of my Juniata County feed is TPL and the other is NAC.

 

[Voyager]

These HPF's I'm looking at are only around $30/pair.

That's not bad as long as they meet commercial specs. I know some cost much more.

 

[SlipNutz15]

I'll post the link again for the 300Hz filters. Let me know what you think. They were mentioned in an archived feed about them from RadioReference form 2009 I think....

Harrison Labs FMOD Inline Crossover Pair 300 Hz High Pass RCA

 

[Voyager]

I'll post the link again for the 300Hz filters. Let me know what you think. They were mentioned in an archived feed about them from RadioReference form 2009 I think....

Harrison Labs FMOD Inline Crossover Pair 300 Hz High Pass RCA

They look pretty good. One thing to keep in mind since you're getting a pair: You can stack them for more effectiveness. IOW, plug one into the other. That will give you twice the attenuation of the lower frequencies in case one is not enough.

 

[Thunderknight]

I do have another question. With these filters in place, would it affect how a DPL or NAC sounds? Half of my Juniata County feed is TPL and the other is NAC.

NAC is part of the P25 header, so there is no separate sound associated with it. It's just part of the data stream.

 

[SlipNutz15]

Voyager - thank you. I'm going to try one first per scanner and see how that helps.

Thunderknight - That's what I thought but I just wanted to verify.

Thank you both!

 

[Voyager]

I do have another question. With these filters in place, would it affect how a DPL or NAC sounds? Half of my Juniata County feed is TPL and the other is NAC.

P25 (NAC) generally doesn't have any High Pass Filters since the audio is digitally encoded, so it may make those signals sound more 'tinny' 9as they too will lose the audio below 300 Hz), but many seem to associate 'tinny' with more crisp audio, so you actually may get better audio reports.

 

[Spitfire8520]

I believe that the RadioFeed software has an option for a software defined notch filter. It is compatible with Broadcastify and might allow for you filter out PL tones like what you're looking for right now. It would be found under the Audio Monitor & Test tab if you have the software. I believe you can set multiple frequencies to filter out, so you can cover what you need in terms of different PL tones.

 

[SlipNutz15]

Ok, I enabled that notch filter, I'm going to try it but I also want a physical filter because I broadcast to multiple audio servers using a separate encoder and also record locally and want clarity across the board.