Added reference for oversampling and noise shaping DAC

applications/lofar2/model/pfb_os/dsp_study_erko.txt

@@ -43,10 +43,11 @@
 # * [WOLFSOUND] https://thewolfsound.com/
 # * [NOISESHAPING]
 #   . "Digital Signal Processing Oversampled Analog to Digital Conversion with
-#      Noise Shaping", D. Richard Brown III
+#      Noise Shaping", D.R. Brown --> ADC
+#   . "Noise shaping", Markus Nentwig, December 9, 2012 --> DAC
+#      https://www.dsprelated.com/showarticle/184.php
 #   . "Realisering van Digitale Signaalbewerkende Systemen, Toepassingen",
-#      5N290, TUE, P.C.M. Sommen, slide 19, 20,
-#
+#      5N290, TUE, P.C.M. Sommen, --> DAC slide 19, 20,
 
 1) Linear Time Invariant (LTI) system [LYONS 1.6]
 
@@ -932,7 +933,7 @@ Noise shaping
   frequencies. An LPF filters the higher frequencies and thus increases the
   effective number of bits of the low pass output, by about 1 bit for first
   order feedback.
-- ADC: [NOISESHAPING Brown]
+- ADC: [NOISESHAPING]
   For an ADC noise shaping requires feedback in the analogue domain, because
   after quantization the error information is only available when comparing
   the digitized input with the analogue input. Noise shaping of digital input
@@ -945,7 +946,7 @@ Noise shaping
   feedback signal jumps between too high or too low, which thus contains the
   more detailed accuracy on the analogue input signal level that leads to the
   extra bits for oversampling.
-- DAC: [NOISESHAPING TUE]
+- DAC: [NOISESHAPING]
   For a DAC noise shaping uses feedback in the digital domain. The round
   off LSbits of the LPF output are fed back to the input, so that the noise
   power is shaped towards higher frequencies. This noise power at higher