add references web links

README.md

@@ -388,15 +388,21 @@ Limitations
 
 ---
 
-References
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+References (freely available)
+------------------------------
 
-- ISO/IEC 11172-3:1993 - Coding of moving pictures and associated audio for
- digital storage media at up to about 1.5 Mbit/s, Part 3: Audio (MPEG-1
- Audio Layer III, "MP3").
-- ISO/IEC 13818-7:2006 - Generic coding of moving pictures and associated
- audio information, Part 7: Advanced Audio Coding (AAC).
-- Zwicker, E. & Fastl, H. - *Psychoacoustics: Facts and Models*, Springer,
- 1999 (Fletcher–Munson equal-loudness contours).
-- Lerch, A. - *An Introduction to Audio Content Analysis*, Wiley, 2012
- (spectral features for audio forensics).
+- Nyquist–Shannon sampling theorem — Wikipedia
+  https://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem
+- Equal-loudness contour (Fletcher–Munson curves) — Wikipedia
+  https://en.wikipedia.org/wiki/Equal-loudness_contour
+- LAME MP3 encoder psychoacoustic model — Hydrogenaudio Knowledge Base
+  https://wiki.hydrogenaudio.org/index.php?title=LAME
+- MP3 lowpass filter cutoffs by bitrate — Hydrogenaudio forum discussion
+  https://hydrogenaud.io/index.php/topic,71398.0.html
+- GPSYCHO — open-source psychoacoustic model (used by LAME)
+  https://lame.sourceforge.io/gpsycho.php
+- LAME source code (lowpass filter implementation) — GitHub
+  https://github.com/lameproject/lame
+- "Audio Authentication Using Spectral Analysis" — University of Michigan
+  open-access thesis, 2025
+  https://doi.org/10.7302/28306