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First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
https://forums.stereotool.com/viewtopic.php?t=7092		 Page 1 of 2
Author:  sebdvvv [ Sun Oct 18, 2015 1:41 pm ]
Post subject:  First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
Hello everybody,

I'm not sure to post at the right place but i'll go anyway...

Very recently i changed my FM Transmission equipment, i have gone for a hand made 300W Italian trasmitter found on ebay. Very nice construction, very nice transmission but now everything is in MONO MPX, I'm still learning about it all so be indulgent :)

I'm doing radio from a Windows XP SP1 station + Winamp + Roland DuoCapture.
Everything has always worked fine with the previous FM transmitter (Cheap 15 W +Stereo built-in unit).

Now the Duo-Capture Line Out (Left) is directly connected to MPX Input.


I have a simple license of Stereo Tool, i'm currently looking to upgrade for FM Standard but i'd like to be sure about the origin of the issue i'm encountering...

I'm facing a problem when the bass is playing in the music... There is like a high frequency noise over the bass on some loud music. Please check this sound sample to get a better idea :

http://vrfm.free.fr/noise_bass_fm_monom ... mitter.mp3

This is a recording straight from good quality FM receiver.

Pay attention to the noise when the bass plays...


To fix this, I already tried :

- Reducing the MPX Volume....the noise is reducing too BUT the station is now low on volume.
- Filtering the 20Hz-60Hz frequencies of the music BUT i don't have bass anymore.


When i bypass Stereo Tool, the noise is less present but i loose all the dynamics processed by ST....
I also played with the clipping and bit of FM settings (not so easy due to my license limitations) but the noise is always there ! I'm afraid it is the quality of my MPX transmitter...


Can someone please help me understand what's happening here ? I never faced this kind of noise when 20Hz-60Hz are playing... It sounds like a sample rate issue maybe ? I don't really know...


Thank you for this community and this great tool !
Author:  Modulator [ Sun Oct 18, 2015 4:50 pm ]
Post subject:  Re: First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
It seems that the Duo Capture is NOT 192kHz samplerate capable (only 44.1 or 48kHz), am I correct? This means that you are not able to produce the necessary signal, meaning that everything over 24kHz (at 48kHz samplerate) is either filtered out, or maybe in this case upper frequencies (38kHz stereo difference signal, maybe there was no RDS since the ringing wasn't constant?) were mirroring or something so they became audible?

So, unless you upgrade your audio interface to be 192kHz samplerate capable (and ASIO for your comfort), you won't be able to produce MPX signal that consists stereo information and RDS, thus you have mono signal even though your pilot tone will get through and receivers will display an Stereo indicator.
Author:  hvz [ Sun Oct 18, 2015 11:20 pm ]
Post subject:  Re: First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
You have a loud tone at among other 12400 Hz. If you're running the sound card at 192 kHz this shouldn't be happening. If you're running it at 44/48 it could though. Please check 2 things:
1. Do you hear it if you listen directly to the sound card output?
2. Turn all the MPX stuff (stereo encoder, rds encoder) in Stereo Tool off if they are on (they won't work anyway unless you use 192 kHz output). Do you still hear it on FM?

A Realtek @ 192 kHz is not much worse than a real sound card, I would definitely use it (I even used it on trade shows). Main disadvantages are: Less good stereo-to-noise ratio, more latency. Also you need to calibrate it to not get overshoots on bass sounds.
Author:  sebdvvv [ Mon Oct 19, 2015 3:12 pm ]
Post subject:  Re: First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
Realtek HD produces of course a great difference, unfortunately there is some slight noise remaining.

I tried to play with some of the transmitter and BS412 limiter.
It has a great effect on reducing the hotness of signal without loosing much levels...nice work on that one.

I have a MPX Volume at -4 db so it softness the whole thing too... is it something regular or do i have to try not to decrease this volume ?

I have also noticed that other processing like multiband were not reacting anymore, does the FM Transmitter override all other processes ?


Here is a second test on laptop with Realtek HD Audio at 192 khz :


test1

test2

test3


On those one we can hear that the noise is coming when bass fade in and that it acts like a micro delayed sound or something, in the high freq. of course... damn weird noise!!


Could it be related to mpx unit or antenna ?
Author:  Bojcha [ Mon Oct 19, 2015 3:58 pm ]
Post subject:  Re: First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
Sample rates all wrong. It's not transmitter or ST. It's your playout, your files, your soundcard wrong settings. Common thing for Wndows7
Author:  sebdvvv [ Mon Oct 19, 2015 10:10 pm ]
Post subject:  Re: First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
What do you mean the files and the playout ?

Today i tried a new setup with another PC that has a SoundMax device setup to output 192 khz too...

I tried both with ST standalone and DSP in winamp....results are the same, still this noise collapsing the bass frequencies... Volume doesn't matter, even very low, the noise is still here...

Bojcha : it's also doing the noise thing with Vinyl and Audio CD (played through ST with FM...)

I'm afraid it has something to do with mpx unit
Author:  hvz [ Tue Oct 20, 2015 1:35 am ]
Post subject:  Re: First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
Please respond to this:
Quote:
1. Do you hear it if you listen directly to the sound card output?
2. Turn all the MPX stuff (stereo encoder, rds encoder) in Stereo Tool off if they are on (they won't work anyway unless you use 192 kHz output). Do you still hear it on FM? Alternatively, play something not via Stereo Tool.
Author:  sebdvvv [ Tue Oct 20, 2015 3:10 am ]
Post subject:  Re: First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
No i don't hear the noise directly on the soundcard output. Issue is happening only through FM.
Author:  Bojcha [ Tue Oct 20, 2015 10:50 am ]
Post subject:  Re: First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
Well acording to sample, it's classic samplerate mismatch.
I see now that you said same happens on WindowsXP, wich is strange. Usually problems are on windows7. (scratching)
Author:  hvz [ Tue Oct 20, 2015 1:26 pm ]
Post subject:  Re: First Mono MPX Transmitter : Noise Problem when 20-60 hz plays
Quote:
No i don't hear the noise directly on the soundcard output. Issue is happening only through FM.
And the other part? What if you don't play through Stereo Tool?
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