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 Do MP3 encoders sound different? Pt. 3 Read Pt.1   Read Pt.2

 Will Ryu


Testing with real music: "Dirty Blue"

Next, I fed the encoders real music. The test file was a four second clip from "Dirty Blue" by Adam Makowicz with Phil Woods, a jazz instrumental which includes drums, bass, and saxophone. 

Since the encoders are nearly identical in the low frequency range, the graph above shows only the information above 10 kHz. At a 128kbs bit rate, the Blade and LAME encoders politely bow out at 16 kHz. The FhG encoder strives mightily all the way out to 20 kHz, but this results in obvious errors in the power spectra. The Xing encoder has the oddest behavior - - most of the high frequency information past 16 kHz is gone except for two spikes of energy around 18 kHz. Curious indeed.

 

Blade meets "Dirty Blue"


Blade meets "Dirty Blue"

As I increased the bit rate the power spectra of the encoded files better matched the power spectra of the test signal in all cases except that of the Xing encoder. The power spectra of the Blade encoder is shown above for the "Dirty Blue" test file encoded at five bit rates (128, 160, 192, 256, and 320 kbs). For 160 kbs and above, this graph is also representative of the behavior of the LAME and FhG encoders. As you can see, the response extends out to 20 kHz for bit rates of 160 kbs and higher. In addition, while the error in the spectra is obvious at 160 kbs, it gets much smaller at higher bit rates. The graph below shows the same test for the Xing encoder.

 

Xing and "Dirty Blue"


Xing & "Dirty Blue"

The Xing encoder never quite makes it out to 20 kHz, even at 320 kbs. Why does it cut off the music file and not the pink noise? At these frequencies, note the difference in the power between the pink noise and the music test file. The power of the music file past 16 kHz is about two orders of magnitude lower than the pink noise. Perhaps the Xing encoder considers this "below threshold" or "masked" and has changed the frequency spectrum accordingly. Can we hear this high frequency hash as well as we can see it in the power spectrum? I'll address that question in the listening tests.

 

Accuracy and Mean Standard Error (MSE)

Before we do the listening tests, we have to address the question of accuracy. Now you wouldnt expect the decoded MP3 waveforms to match the test waveform bit for bit since MP3 encoding is a perceptually based compression scheme. The point of the encoder is to sound the same and not necessarily to measure the same, but arguably an encoder which most accurately reproduces the original waveform should also sound the best. 

The above graph shows the MSE (arbitrary units) between the original waveform and the encoded signal at various bit rates. Note that the Fraunhofer encoder has the lowest MSE at 128 kbs by a large margin. FhG continues to have the least error up until a bit rate of 256 kbs, at which point all the encoders essentially have the same MSE. There's a diminishing rate of return as the bit rate increases past 192 kbs -- improvements continue with higher bit rates but relative gains become smaller and smaller.

 

Listening tests

Numbers can only tell you so much. Trying to choose an MP3 encoder by measuring its bandwidth and waveform MSE is like trying to choose a car by only clocking its 0-60 mph acceleration time and counting the number of cup holders. We've still got to talk feel. So how do these encoders sound? For this test I used the following audio tracks:

Op.59 no.3 "Razumosvsky" Beethoven Quartet from Key to the Quartets, performed by Emerson String Quartet

String instruments are harmonically rich and display a wide array of tonalities and dynamics. In addition to their distinctive sound, aggressive bowing brings out sharp transients with complex overtones which might make for difficult encoding.

"Setting Sun," from Dig Your Own Hole, Chemical Brothers

This song is a wall of sound from top to bottom. In the upper frequencies there are plenty of percussive tracks, frequency sweeps, and some constant high pitched tones. In addition, there's a deep, gated bass drum that solidly cuts through the music. Plenty here to trip up an encoder.

"Tears in Heaven," from Eric Clapton Unplugged, Eric Clapton

This recording has great "presence," in the sense that it gives you a feeling of being right there during the recording. On my stereo the imaging is wide and deep, with well defined space between instruments. Also, there are plenty of audio cues to listen for when doing critical comparisons. At the beginning of the track you can hear someones foot tapping with short but distinctive reverberations coming from the wooden stage. The guitars produce sharp and clean attack transients. Clapton sings with backup singers, and vocal tracks can be revealing. Finally, there's a high pitched bell with a long and clear decay.

 Copyright 2000 arstechnica.com

Next: Results & observations

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