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Will Ryu
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The contestants
Fraunhofer (FhG)
Xing
Blade
LAME The numbers Measurements certainly dont tell the whole story about an encoders audio quality, but they can be helpful in understanding an encoder's sound. Here I give the results of two measurements. First, I measured the bandwidth of each encoder by comparing the power spectra of an encoded test signal. Then, I measured each encoder's accuracy by comparing the encoded signals to the original waveform.
Now here's an interesting result. The test signal was one second of pink noise, a pseudo-random wav file which has equal distribution of energy in each frequency octave (e.g., energy between 1 and 2 Hz is equal to the energy between 10,000 and 20,000). The encoding was done on one channel of the stereo signal at 128 kbs. As you can see in the above graph (x-axis is frequency in Hz and the y-axis is power in arbitrary units), the Blade and LAME encoders start to cut off the signal at 16 kHz. In addition, the Blade encoder produces some ugly spikes past the cut-off region (albeit the peaks are small, note the log axis). The Xing and Fraunhofer encoders almost make it to 20 kHz, with FhG just edging out Xing near the 20 kHz mark. Now why would an encoder throw out everything past 16 kHz? Generally, musical information is mostly located in the lower frequency bands, and, as I mentioned before, our ability to hear high frequencies decreases with age. So, you might be able to get away with throwing out information past 16 kHz. The benefit of doing this is that the encoder can now take its limited number of bits and assign them to the rest of the frequency band. If this is true, then perhaps the Blade and LAME encoders have more accurate power spectra than the Xing and FhG encoders below 16 kHz. To test this I looked at the Mean Standard Error (MSE) between the power spectra of the test signal and the output of the various encoders from 20 Hz to 15500 Hz. The mean standard error is the average of the square of the difference of the signals, or in other words, the smaller the MSE the more similar the signals. I don't have a graph for this one, so I'll just tell you about the result. The Xing (MSE 5.7) and Fraunhofer (MSE 13.5) encoders still had smaller MSEs than the LAME (MSE 17.8) and Blade (MES 65.4) encoders, even in the restricted band. So much for that hypothesis. For higher bit rates the LAME and Blade encoders extended their frequency response out to 20 kHz. Anyway, pink noise results are interesting, but I dont spend much time compressing test signals so I can listen to them on my computer ;) Let's feed in some real music and see what happens. |
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