BEC Posted February 14, 2008 Share Posted February 14, 2008 I happened to run across this trace I made comparing frequency response of various bit-rate MP3s. In each case this was my HP sweep generator feeding my Roland digital recorder. I frist ran the sweep generator directly into the spectrum analyzer to establish what shoud be perfect. Then I recorded the sweep generator output as a WAV and at various MP3 bitrates and then played the recordings into the spectrum analyzer. The levels are all the same, I just ofset the traces a couple of dbs to be able to see each one. Bob Crites Quote Link to comment Share on other sites More sharing options...
jacksonbart Posted February 15, 2008 Share Posted February 15, 2008 Wow that is very interesting, its pretty surprising how much the lower bit rates really do trim off the upper frequencies. Pretty much cooresponds to my real world experiance with lower bit rate MP3s, sounding flat, dull and lifeless. Quote Link to comment Share on other sites More sharing options...
DrWho Posted February 15, 2008 Share Posted February 15, 2008 I wonder if a frequency sweep is really the best test for something like this? The MP3 encoding algorithm is a bit more complicated than just frequency response...for instance, I have mp3 test tones going up to 20kHz no problem... Quote Link to comment Share on other sites More sharing options...
BEC Posted February 15, 2008 Author Share Posted February 15, 2008 I wonder if a frequency sweep is really the best test for something like this? The MP3 encoding algorithm is a bit more complicated than just frequency response...for instance, I have mp3 test tones going up to 20kHz no problem... I think this is partly dependent on how a company writes its code. You can look at the traces above and see that in this case at least (with Roland) they set the upper frequency response of the 128, 160 192 and 224 Kbps all at the same point. About 16 khz. The 320 Kbps is allowed to go a bit higher and the 96 Kbps is chopped off a lot lower. Probably other coding software uses different top end values. Bob Quote Link to comment Share on other sites More sharing options...
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