A free software equalizer for listening to Mix Radio International (and more!)

[ChrisSmolinski]

Mix Radio International transmits wide audio, around 15 kHz or so. But I have found that there is a lot of annoying hiss when setting my receive bandwidth that wide, since there is typically little audio content up there. I ran across this site which shows how to use AU Lab (a free tool for Mac users) as an audio equalizer. It works great for this application, now I have the bandwidth open wide, and have reduced the response on the high end somewhat. I can still hear it, but without the hiss.  Not sure if Mix Radio is using standard RIAA emphasis or not.

https://coderwall.com/p/dn0v-g/create-an-equalizer-for-all-audio-in-mac-os-x

[redhat]

Would it be possible to add NRSC de-emphasis to any of the popular SDR applications?  In the event I get recordings with the bandwidth out to 12.5k (our current transmission bandwidth) I often have to process the recording to cut the highs down and restore flat response.  I believe Mix and Relay Station do similar, plus every AM station in the country.

My WinRadio has a adjustable de-emphasis feature, and I use it religiously when monitoring other stations.

+-RH

[ChrisSmolinski]

Sure, if you can convince the author to do so 

Actually the KiwiSDR software just got such a feature, you can select either 50 or 75 microsecond de-emphasis. I can notice a (good) difference with my KiwiSDR. I only have a 6 kHz maximum bandwidth with my Kiwi, since I have it set for four full featured SDRs, each with a waterfall. There is an option for 20 kHz audio (which is really just +/- 10 kHz), but you lose one of the receivers and only have three total in that case. Personally, I am not sure there are enough wide bandwidth worthy stations to make it a worthwhile compromise. Unless XFM/Mix/Relay Station go 24/7. 

[redhat]

Don't forget Radio Illuminati.  He runs 10 KHz as well.

+-RH

[ChrisSmolinski]

Don't forget Radio Illuminati.  He runs 10 KHz as well.

+-RH

Good point!  Mix was on last night, and I am listening to the SDR recording now, using the equalizer. I am not sure I have the equalizer set to any official standard, I just set it to what "sounds good".

[redhat]

NRSC white paper outlining the response of the modified 75uS cure is here.

https://www.nrscstandards.org/standards-and-guidelines/documents/archive/nrsc-1-a.pdf

+-RH

[ChrisSmolinski]

Thanks, my response was actually pretty close to that, perhaps a little more attenuation on the high end to reduce the hiss.

[JimIO]

RH, what does a 'modified 75uS  NRSC pre-emphasis' circuit look like?
I have searched for it before and now and can't find one. How 'modified' is it compared to FM?

[redhat]

I'll have to dig and see if I can find a good example.  Usually the pre-emphasis is built into the processing HF limiter stage so there is a lot going on and sometimes it isn't just one pole.  The standard defines it as a 75uS pre emphasis network with another pole at 8700 Hz.  In practice, there is not much difference between the responses (4dB more boost at 10 KHz), and the second pole was added to ease transmitter linearity requirements, and also reduce adjacent channel interference.

+-RH

[JimIO]

RH, thanks for pointing out 8700Hz. I only skimmed through that pdf file and missed that.
I did a little more searching found that it is a 'truncated 75 us curve'. I'm not really interested in the dynamic processing part of it.

[Pigmeat]

Don't forget Radio Illuminati.  He runs 10 KHz as well.

+-RH

I ran 10 kHz wide most of my pirate career. With the flavor of Cool Edit I had, you could choose 50 or 75 uS settings. I went with 75 as it was the standard here. My outboard EQ/mixer was the old "WREC" board P.J. sold me. I used the Cool Edit EQ only for touching up the audio when I was doing the final mixdown.

Cool Edit was great for giving you nice tight pro sounding shows. Before that it was the old "turn and half" back of the cassette spool with your little finger and lot of praying you got it right. You could splice it, but if you ever had one give during production, you could be heard cussing for miles. You work hours and days to get halfway close on that final bit or set of songs, only to say "F##* it!" and head out the door with a half-assed show.

It was still a boatload of fun once you got on the air, despite the trouble.

[redhat]

The processing we have now is set for 12.5 KHz, but will allow up to 15KHz if we want.  12-13 KHz seems to be a good compromise between musical content and occupied bandwidth.  10 KHz to me with modern music just sounds too restricted, and 5 KHz is just unacceptable.

These are just my opinions, and are subject to change

+-RH

[ChrisSmolinski]

The processing we have now is set for 12.5 KHz, but will allow up to 15KHz if we want.  12-13 KHz seems to be a good compromise between musical content and occupied bandwidth.  10 KHz to me with modern music just sounds too restricted, and 5 KHz is just unacceptable.

Unfortunately, unless signal strengths are excellent I end up using about a 3 kHz bandwidth. I suspect most others use about the same. Otherwise the resulting audio becomes unbearable to listen to after a few seconds.

[redhat]

I suspect several things are in play here as I noticed similar things when receiving recordings in the early days.  Prior to the SDR era, most guys were listening using communications receivers or ham transceivers with very limited AM bandwidth, and you get used to listening to audio in that mode.  I'm a broadcast engineer and I am used to monitoring signals using high level demodulators with unrestricted audio.  I remember an experience I had when rebuilding a studio for a small AM station in the midwest.  We had finished work for the day and I was sitting in the corner of the control room and the speakers were monitoring the off air wideband inovonics monitor we had installed in the rack room.  I was struck by how good the signal sounded, much better than any AM radio I had previously heard.  In the industry I also sometimes hear engineers lament about how much better AM sounded before the bandwidth limit rules were imposed by NRSC in the late 80's.

Point being I will suffer through a lot of noise and static to hear the wideband audio of a station to the point that it hurts my ears.  Running a little more de-emphasis than usual can greatly assist in this regard.  In the era of Kiwi's, if it sounds like crap in my backyard, I just go hunting elsewhere.

+-RH

P.S. Sorry for hijacking your thread, Chris

[Josh]

A lot of the analog rigs will only pass about 10 to 15Kc, so there's that.