diff --git a/README.md b/README.md
index 0785ccd..bd1c837 100644
--- a/README.md
+++ b/README.md
@@ -6,6 +6,15 @@ Pi-FM-RDS
 
 This program generates an FM modulation, with RDS (Radio Data System) data generated in real time. It can include monophonic or stereophonic audio.
 
+This version modulates the PLL instead of the clock divider for superior signal purity. The harmonics are unaffected, so the [legal warning](#warning-and-disclaimer) still applies.
+
+![](doc/spectrum.png)
+
+TODO list
+watchdog for PLL settings (the GPU changes them sometimes, like brownout)
+( force_turbo=1 may help prevent this)
+measure PLL loop filter response
+
 It is based on the FM transmitter created by [Oliver Mattos and Oskar Weigl](http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter), and later adapted to using DMA by [Richard Hirst](https://github.com/richardghirst). Christophe Jacquet adapted it and added the RDS data generator and modulator. The transmitter uses the Raspberry Pi's PWM generator to produce VHF signals.
 
 It is compatible with both the Raspberry Pi 1 (the original one) and the Raspberry Pi 2 and 3.
diff --git a/doc/spectrum.png b/doc/spectrum.png
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