Yes, you are right it is a compression over compression. I compared my recordings with Youtube encoded video but my ears couldn't detect the difference. So I think it is a good example for seeing the major differences in sound.
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Hello everyone, in this video you can see the differences between various VoIP codecs. I also put the frequency analysis of each codec, so you can see the differences clearly.
Here are the codecs in the video : G.711 alaw, G.711 ulaw, G.722, G.723, G.729, G.729a, G.729ab, G.729b, AMR 8KHz, AMR-WB 16KHz, GSM 8KHz, iLBC 8 KHz, iSAC, OPUS 48KHz, OPUS 24KHz, OPUS 16KHz, OPUS 8KHz.
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