Do Higher MP3 Bit Rates Pay Off?

willsmith

In a tidal shift akin to the Sexual Revolution, the heyday of Napster (that’s 1.0) markedly changed Americans’ perceptions of intellectual property rights and fair use. Some folks considered the music-swapping orgy a shameful, lawless epidemic of hedonism that would inexorably lead to social and economic chaos, while others considered it a thrilling, lawless epidemic of hedonism that liberated all of us from the musty stench of an increasingly bland and commercial culture. But everyone understood one thing: MP3s were here to stay.

Back then, most MP3s were encoded at constant bit rate (CBR) 128Kb/s, striking a balance between acceptable audio quality and file sizes that were small enough for easy trading over a dialup connection. But what was born out of necessity endures today, as most of the music available on rogue peer-to-peer networks is still compressed at this rate. It’s been called “near CD” quality, but we know better—it isn’t even in the same Zip code as CD audio.

But at what point do higher bit rates stop paying off and simply take up too much space? 160Kb/s? 192Kb/s? And can a hardcore audiophile really tell the difference between a 320Kb/s track and an uncompressed one? What about a normal music listener?

These are the questions we wanted answered when we set out bear traps around the office and came back later that afternoon to retrieve the snared employees. From the bunch, we selected four representing a range of musical tastes and quality demands. We handed them headphones, pressed the play button, and got some surprising results.

How We Tested

Each lucky participant was asked to bring in a CD with a track that he or she has listened to for years and knows so intimately that a single missing hi-hat tap would stand out like a sudden blast from a tuba. We ripped each track using iTunes at three quality levels: 160Kb/s, 320Kb/s, and uncompressed WAV. The compressed files were ripped using variable bit rate (VBR) encoding, meaning that a 160Kb/s VBR track allows the bit rate to rise and fall depending on the complexity of the music while maintaining the selected bit rate as the minimum bit rate for the track.

In a quiet room with mood lighting and kitschy Scandinavian furnishings, the participants put on a pair of Sennheiser HD 580 headphones that were attached to our test PC’s Creative X-Fi soundcard. The participants listened to not only the three versions of their own track, but also the three tracks from each of the other participants, for a total of 12 tracks in all. Each participant was allowed to listen to each track as long as he or she could stand it, and was allowed to repeat portions of the track and do A/B testing with the other tracks.

How Good is Your Encoder?

The thing that most often fools newbs is not knowing what kind of sounds are evidence of bad encoding or heavy-handed compression, but there are a couple tracks you can throw at your software to see how well it’s doing. Of course, you should begin with uncompressed files, so you can’t download these test tracks from an online service. If you don’t want to pony up for the CDs, you might want to check your local library.

The first is “Neon Reprise” from Metropol by Lunatic Calm. The song contains a whooshing upward sweep that audio compressors have a very difficult time with. Turn off Joint Stereo and keep upping the bit rate if you hear any warbling (like a cassette tape that’s been mangled).

The second comes from the legendary 1965 RCA Victor Opera Series recording of Verdi’s La Traviata. In Act II, Track 10, position 3:16, Anna Moffo belts out a bone-liquefying high note. Focus your attention not on her voice, but on the instrumentation behind it. If there’s any fuzz or rumbling in the background, again, dial up the bit rate.

The Testers

The Curmudgeon
It’s not the music itself—it’s the idea of audio compression that intrigues the curmudgeon. Can an audio file compressed one-third to one-tenth of its original size really sound as good? A cynic with an open mind and bat-like hearing, the curmudgeon reserves judgment—for now.

The Hipster
Electro. Drum ‘n’ bass. Post-trance
trip-hop downtempo crossover. The hipster’s onto a genre 10 minutes after it arrives, and can instantly ID a quarter-second sample as Sly and the Family Stone. But can this ear for detail discern a few frequencies that get left behind by audio compression?

The Audiophile
Proudly, defiantly discriminating, our audiophile takes a dim view of MP3’s “lossy” audio compression. And don’t even get the audiophile started on the pitiful quality of tracks peddled by downloading services like iTunes or Napster To Go. Can an expertly compressed track fool even those golden ears?

The Pluralist
You’re just as likely to find the pluralist kicking back barefoot on the futon listening to Miles Davis’ Spanish Quay as you would at a Pixies reunion show covered in beer and sweat. But at what point does even this most laid back of listeners balk at bad audio?

The Hipster


The Hipster was frantic. She listened to tracks over and over again, ferreted out loud sections and asked them to be repeated and compared with the same section in another version of the track. She leaned forward several times, pressing the cups of the headphones to the sides of her head. She removed the headphones, took a breath, and put them back on. It was all for naught—“this is hard” she said, “this is really hard.”

In the end, the Hipster confessed that she resorted to guessing in most cases, relying on a hunch based on instinct rather than any identifiable portion of the tracks. Her instincts proved to be accurate when listening to her own track; she correctly identified the bit rates of all three versions. As for the rest, guessing didn’t pay off so well—none of the other participants’ tracks were correctly identified, and she had to settle for a humbling three out of 12 correct, the lowest of the group.

In her frustration, however, she managed to deliver one zinger. While listening to a choral track provided by another participant, she told Maximum PC: “After listening to this song, I don’t feel so bad about my track.”

Score Card
Pluralist's Track
Curmudgeon's Track
Hipster's Track
Audiophile's Track
Uncompressed
fail
fail
pass
fail
320kbit/sec VBR
fail
fail
pass
fail
160kbit/sec VBR
fail
fail
pass
fail

The Pluralist


The Pluralist appeared to be having too much of a good time, smiling, frisking through tracks, and tapping her fingers on the table in time with the music. She was fearless about volume level and didn’t hesitate to ask us to crank it up while scrutinizing certain portions of the tracks. But she found the exercise to be anything but a breeze. “I’m pretty sure about this,” she told us, but quickly added, “sort of.” And then: “Wait, can I hear them again?”

Despite this last-minute waffling, she was very confident about identifying the quality levels of her own track, as well she should have been: She correctly identified all three. With songs she was not familiar with, or with tracks that made her burst out laughing (see the last sentence of the Hipster’s report), she identified only two of the 160Kb/s tracks and a single 320Kb/s track. The final result—six out of 12 correct—doesn’t seem particularly impressive until you consider that this was the highest score of the entire group!

Score Card
Pluralist's Track
Curmudgeon's Track
Hipster's Track
Audiophile's Track
Uncompressed
pass fail
fail
fail
320kbit/sec VBR
pass
pass
fail
fail
160kbit/sec VBR
pass
fail
pass
pass

The Curmudgeon


Although ostensibly objective and cool-headed, the curmudgeon was none too happy once the listening tests began. He was initially grumpy, and as the minutes flew by, he became increasingly restless and even hostile at times. He wanted to know, for example, how the test administrator knew that the files were done correctly if only the staff member who labeled the tracks knew which was which.

Nonetheless, he marched forward, and was particularly demanding of A/B testing, comparing portions of his own track with numerous repeats. Apparently, the diligence didn’t pay off: The curmudgeon was unable to correctly identify the quality level of any of the three versions of his own track! In an eerie coincidence, however, he dissed the Hipster’s track much as she dismissed his (“I thought Ani DiFranco got all her anger out already....”), yet correctly identified the quality levels of the three versions of that track. He was also able to identify a 320Kb/s version and an uncompressed version of the other track sets, but that’s all, for a modest total of five out of 12 correct.

When the test results were revealed, the curmudgeon threw a hissy fit, questioning our methodology before sinking into an oppressive quiet behind his keyboard.

Score Card
Pluralist's Track
Curmudgeon's Track
Hipster's Track
Audiophile's Track
Uncompressed
pass
fail
pass
fail
320kbit/sec VBR
fail
fail
pass
pass
160kbit/sec VBR
fail
fail
pass
fail

The Audiophile


We all thought it would be a piece of cake, for that matter. With the finest consumer-level soundcard, ace headphones, and tracks we knew by heart, who would have thought that identifying a compressed audio track could be so difficult? No one was more surprised than the audiophile. Like all the other participants, there was a lot of leaning forward, as if getting closer to the PC might expose some hidden flaw in the audio stream. Near the end of the tests, the Audiophile remarked, “I specifically chose this song because I thought it would be easy [to identify the compressed version]. It wasn’t.”
That’s pretty much the story told by his results—he was able to identify the uncompressed version of his own track after considerable A/B comparisons with the compressed versions, but mixed up the 320Kb/s and 160Kb/s versions. He was also able to correctly identify the quality levels of another participant’s track, and the 320Kb/s version of yet another’s track, but had to settle on a tie with the Curmudgeon, at five out of 12.

Score Card
Pluralist's Track
Curmudgeon's Track
Hipster's Track
Audiophile's Track
Uncompressed
pass fail
fail pass
320kbit/sec VBR
pass
fail
pass
fail
160kbit/sec VBR
pass
fail
fail
fail

The Upshot

With the possible exception of the USB Key that survived a washing and drying cycle, no other Maximum PC Challenge has ever surprised us as much as this one. It’s downright humiliating, in fact, that in many cases, we were unable to tell the difference between an uncompressed track and one encoded at 160Kb/s, the bit rate most of us considered the absolute minimum acceptable for even portable players.

Some follow-up testing confirmed our suspicions: variable bit rate encoding makes a tremendous difference in the audio quality results, certainly enough to justify—many times over—the slight file size increase. Capping the bit rate at 160Kb/s in MP3 files can be pretty harsh on a track, but allowing the bit rate to wander upwards during more complex passages—as variable bit rate encoding does—and throttle down during quieter sections captures an astonishing amount of complexity while keeping file sizes down to an impressive minimum.

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