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"Post amplifier" level slider (Processing) doesn't work for FM-output
https://www.forums.stereotool.com/viewtopic.php?t=34144		 Page 1 of 1
Author:  Pethunia [ Mon Jun 17, 2024 10:47 am ]
Post subject:  "Post amplifier" level slider (Processing) doesn't work for FM-output
The "post amplifier" level slider (Processing) is not working for FM-output. If one wants to have a permanent small audio level output reduction on FM (eg 70 kHz instead of 75 kHz deviation, which most Dutch public FM-stations seem to do), the only option now is to adjust the composite output level, but that reduces pilot and RDS-levels as well, wich will then need to be compensated individually to get them back on their standard levels of 9% and 4,5% respectively.
Author:  hvz [ Mon Jun 17, 2024 1:48 pm ]
Post subject:  Re: "Post amplifier" level slider (Processing) doesn't work for FM-output
That is on purpose. The Post Amplifier is used for HD output, to give headroom to codecs etc.

If you want to lower the peak level without affecting RDS and pilot, you'll need to raise the pilot and RDS levels by the same amount that you lower the modulation.

By the way, the reason that most Dutch stations modulate around 70 kHz is Stokkemask (ITU-R SM.1268). Stereo Tool has a Stokkemask filter built in that you can enable (if you're using the composite clipper), which means that you don't need to lower the modulation to comply.
Author:  Pethunia [ Mon Jun 17, 2024 2:46 pm ]
Post subject:  Re: "Post amplifier" level slider (Processing) doesn't work for FM-output
Yes, I'm aware about all of the Stokkemask and composite clipper features, I am well into all related settings in ST and the analogue technical backgrounds for that matter :-)
But are you suggesting that these stations don't employ some kind of active Stokkemask-filter and therefore they just lower their entire deviation in order to comply? From what I see looking at their demodulated MPX, they at least clearly employ some kind of composite clipping, that seems to work in the opposite direction of yours (excluding 19k pilot from their MPX makes their peak levels lower, whilst they get higher in ST).
In any case, I appreciate the genius behind your solution (ITU-filter and composite Clipper) quite a lot!
Author:  Bojcha [ Mon Jun 17, 2024 5:28 pm ]
Post subject:  Re: "Post amplifier" level slider (Processing) doesn't work for FM-output
With composite clipper you will not get much modulation more if you set pilot lower. Also not great idea to set pilot low, less then 8%. Default is 9%, wich is good value, you can set it to 8% and you will notice almost nothing changed in demodulated L/R. in my personal testing i actually found 10% pilot is better (safer), meaning some strange receivers need that level to even detect stereo.
Same for RDS, 4.5 is fine, 5% is safer.
Author:  Pethunia [ Mon Jun 17, 2024 9:53 pm ]
Post subject:  Re: "Post amplifier" level slider (Processing) doesn't work for FM-output
Quote:
With composite clipper you will not get much modulation more if you set pilot lower. Also not great idea to set pilot low, less then 8%. Default is 9%, wich is good value, you can set it to 8% and you will notice almost nothing changed in demodulated L/R. in my personal testing i actually found 10% pilot is better (safer), meaning some strange receivers need that level to even detect stereo.
Same for RDS, 4.5 is fine, 5% is safer.
Hi, yes I know, but I was referring to the analysis of the MPX, not to actual pilot/RDS output levels. If you discard the pilot from the demodulated MPX (using Leiff's MPX-tool), then you will see that due to the composite clipper the peak levels will increase with ST, while they decrease with most public Dutch stations that apparently employ some other kind of composite clipping.
As for MPX-output, I prefer to just precisely adhere to the standard (i.e. 9% / 4,5% respectively), but that's only because I'm an obsessed perfectionist, or as they would call it a "pixelneuker" here in analogy to graphics perfectionism ;)
Author:  Bojcha [ Mon Jun 17, 2024 11:44 pm ]
Post subject:  Re: "Post amplifier" level slider (Processing) doesn't work for FM-output
Ah, that only mean they dont use composite clipping. Same effect will be in stereo tool if you disable composite.
Author:  Pethunia [ Tue Jun 18, 2024 11:05 am ]
Post subject:  Re: "Post amplifier" level slider (Processing) doesn't work for FM-output
Quote:
Ah, that only mean they dont use composite clipping. Same effect will be in stereo tool if you disable composite.
Yes, looks like that is the case indeed.
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