Spectrum linear mode
Added the option to view the spectrum in linear mode (squared magnitude). Please check the documentation for details: https://github.com/f4exb/sdrangel/tree/master/sdrgui#4-spectrum-display-control
Other changes
- Scope: fixed power overlay display
- Channel analyzer: fixed scope vertical scale display and extended amp range
- DSD demod: Serial DV: use audio compressor at the end of audio filter chain