Stereo Tool

Hi,

Just for fun I have made a comparison measurement of the actual spectrum with ITU-R SM.1268 (Stokkemask) enabled and disabled, using an RF spectrum analyzer.
Now I'm actually curious why below depicted irregularities appear in the spectrum when Stokkemask is disabled? No matter what kind of audio input is applied (music, pink noise, pink noise L/R out-of-phase) and regardless of any audio processing enabled/disabled in ST, after some time running the spectrum analyzer in max-hold, it ends up in these shapes at the same position.
The analyzer was set as per ITU-R SM.1268-5, RBW/VBW 10 kHz, span 340 kHz, ST 1 ms/kHz.
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Edit: corrected mask

The mask was designed such that you're always inside it if you transmit mono audio, no pilot, stereo or RDS. So basically, only audio below 16 kHz, at 75 kHz modulation.

Adding pilot/stereo/RDS will cause overshoots. This is why many stations in countries where Stokkemask is mandatory modulate at 71 kHz instead of 75, if they use a processor that doesn't enforce Stokkemask compliance.

Understood, thanks!