Um problema complicado, nomeadamente para quem se inicia na árdua tarefa de ripar a colecção de CDs, é decidir a taxa de compressão a usar na conversão para MP3.
Seguindo uma certa tradição, baseada essencialmente no tamanho dos ficheiros resultantes, costumava achar que 128 Kbps é um bom compromisso, com uma boa qualidade de som. No entanto, não tinha maneira de garantir isto.
Finalmente encontrei uma opinião bem fundamentada, que explica estas coisas em termos simples e compreensíveis e até dá uma boa receita:
MP3 Compressing Issues
"I've always found that trying to discern a decent sound recording from an outstanding one is like tasting wine. I can tell the difference between a nice Cabernet and a bottle of Trader Joe's Two-Buck Chuck, but I'm not as sure when I'm trying to differentiate between a Sonoma Valley Pinot Noir and an Oregon Pinot Noir. But much like when I taste wine, if I have someone pointing out the differences in audio recordings and I know what I'm listening for, I'll put my ears up there with anyone's.
If digital music sounds good to you, isn't that enough? Using portable digital audio players and earbuds, most people can't tell the difference among music files encoded at moderately different bit rates. But what happens when you start piping music through an expensive stereo system?
"If you listen to CDs over crappy speakers, then listen to MP3s over crappy speakers, you're not going to be able to tell the difference," says R. Luke DuBois, an instructor at New York University's Interactive Telecommunications Program. "But once you move up to a prosumer-quality amp and speakers, you'll start to hear a difference at low bit rates."
If sound quality is critical to you, you can encode music using a "lossless" codec, which means it should sound identical to the original source. There are myriad lossless encoders around, including FLAC (Free Lossless Audio Codec) and Monkey's Audio. Windows Media Player 9 also offers a lossless WMA codec. However, using a lossless codec will result in huge files, which defeats one of the purposes of digitally encoding in the first place--saving drive space. But even though drive space is far less critical an issue when hard drives cost less than $1 a gigabyte, you can run out of space faster than you can say Itunes if you get too carried away. Coldplay's "The Scientist" takes up a mere 4.8MB when encoded using WMA at a data rate of 128 kilobits per second; with WMA Lossless, you'll need a staggering 31.5MB of space for the same song.
As for which compression format best preserves fidelity, audio experts tend to be split. Despite its prevalence--especially in file-sharing environments--MP3 is not considered the best format from a fidelity perspective. In our last lab test of audio formats, we found that, at the lowest data rate (64 kbps), listeners could easily tell the difference between a compressed MP3 file and uncompressed audio, but they were much less able to distinguish a 64-kbps WMA file from the original. However, at bit rates above 128 kbps, almost no one could differentiate compressed audio from the original, and they certainly couldn't distinguish differences among various compression formats.
However, those tests were conducted using headphones--high-quality headphones, to be sure. But many audiophiles feel that differences in fidelity are best discerned by using loudspeakers. In that type of setting, many listeners say they prefer WMA and AAC over MP3, saying that those two formats provide richer sound, particularly at the extremes of the listening range. Like WMA, the AAC format is popular with the recording industry because it contains copy protection, and experts say it provides quality that's comparable if not superior to WMA, especially at higher bit rates. Craig Eggers, Director of CE Technology Marketing at Dolby Labs, creator of the AAC format, agrees that compressed music can "lose some warmth, and can drop some subtle things at the high and low ends," but says that "the technologies are getting better and better, and a lot of average listeners can't tell the difference above certain bit rates."
Since popular music is already compressed, you're less likely to lose a lot of quality by using a lower bit rate, says DuBois. But if you plan on ripping a lot of classical and jazz recordings, you'll likely want to pick a higher bit rate to ensure that you don't drop any of the original recording--especially at the lower end--and you should use sound-leveling settings (if your ripping and/or playback software offers them) to minimize the boosts in volume most compression techniques typically produce.
As a general rule, for pop music either 64-kbps WMA or 128-kbps AAC or MP3 should satisfy the vast majority of listeners; for classical and jazz, you may want to step up to 192 kbps. Some codecs, including WMA 9 and MP3, allow you to select another option--Variable Bit Rate (VBR)--which adjusts the bit rate based on the needs of the music you're encoding. Though the AAC codec does allow for VBR encoding, Apple's popular ITunes software supports it only for ripping MP3s, not AACs.
Since this all comes down to personal preference, here's mine: I use WMA 9 with a variable bit rate of 85 kbps to 145 kbps, although I should admit that I rarely rip classical recordings. And in case you're curious, I'm also a fan of 2000 Bordeaux and 2001 German Rieslings."
-- Josh Taylor
Retirado de PCWorld.com - Digital Audio Hi-Fi
Seguindo uma certa tradição, baseada essencialmente no tamanho dos ficheiros resultantes, costumava achar que 128 Kbps é um bom compromisso, com uma boa qualidade de som. No entanto, não tinha maneira de garantir isto.
Finalmente encontrei uma opinião bem fundamentada, que explica estas coisas em termos simples e compreensíveis e até dá uma boa receita:
MP3 Compressing Issues
"I've always found that trying to discern a decent sound recording from an outstanding one is like tasting wine. I can tell the difference between a nice Cabernet and a bottle of Trader Joe's Two-Buck Chuck, but I'm not as sure when I'm trying to differentiate between a Sonoma Valley Pinot Noir and an Oregon Pinot Noir. But much like when I taste wine, if I have someone pointing out the differences in audio recordings and I know what I'm listening for, I'll put my ears up there with anyone's.
If digital music sounds good to you, isn't that enough? Using portable digital audio players and earbuds, most people can't tell the difference among music files encoded at moderately different bit rates. But what happens when you start piping music through an expensive stereo system?
"If you listen to CDs over crappy speakers, then listen to MP3s over crappy speakers, you're not going to be able to tell the difference," says R. Luke DuBois, an instructor at New York University's Interactive Telecommunications Program. "But once you move up to a prosumer-quality amp and speakers, you'll start to hear a difference at low bit rates."
If sound quality is critical to you, you can encode music using a "lossless" codec, which means it should sound identical to the original source. There are myriad lossless encoders around, including FLAC (Free Lossless Audio Codec) and Monkey's Audio. Windows Media Player 9 also offers a lossless WMA codec. However, using a lossless codec will result in huge files, which defeats one of the purposes of digitally encoding in the first place--saving drive space. But even though drive space is far less critical an issue when hard drives cost less than $1 a gigabyte, you can run out of space faster than you can say Itunes if you get too carried away. Coldplay's "The Scientist" takes up a mere 4.8MB when encoded using WMA at a data rate of 128 kilobits per second; with WMA Lossless, you'll need a staggering 31.5MB of space for the same song.
As for which compression format best preserves fidelity, audio experts tend to be split. Despite its prevalence--especially in file-sharing environments--MP3 is not considered the best format from a fidelity perspective. In our last lab test of audio formats, we found that, at the lowest data rate (64 kbps), listeners could easily tell the difference between a compressed MP3 file and uncompressed audio, but they were much less able to distinguish a 64-kbps WMA file from the original. However, at bit rates above 128 kbps, almost no one could differentiate compressed audio from the original, and they certainly couldn't distinguish differences among various compression formats.
However, those tests were conducted using headphones--high-quality headphones, to be sure. But many audiophiles feel that differences in fidelity are best discerned by using loudspeakers. In that type of setting, many listeners say they prefer WMA and AAC over MP3, saying that those two formats provide richer sound, particularly at the extremes of the listening range. Like WMA, the AAC format is popular with the recording industry because it contains copy protection, and experts say it provides quality that's comparable if not superior to WMA, especially at higher bit rates. Craig Eggers, Director of CE Technology Marketing at Dolby Labs, creator of the AAC format, agrees that compressed music can "lose some warmth, and can drop some subtle things at the high and low ends," but says that "the technologies are getting better and better, and a lot of average listeners can't tell the difference above certain bit rates."
Since popular music is already compressed, you're less likely to lose a lot of quality by using a lower bit rate, says DuBois. But if you plan on ripping a lot of classical and jazz recordings, you'll likely want to pick a higher bit rate to ensure that you don't drop any of the original recording--especially at the lower end--and you should use sound-leveling settings (if your ripping and/or playback software offers them) to minimize the boosts in volume most compression techniques typically produce.
As a general rule, for pop music either 64-kbps WMA or 128-kbps AAC or MP3 should satisfy the vast majority of listeners; for classical and jazz, you may want to step up to 192 kbps. Some codecs, including WMA 9 and MP3, allow you to select another option--Variable Bit Rate (VBR)--which adjusts the bit rate based on the needs of the music you're encoding. Though the AAC codec does allow for VBR encoding, Apple's popular ITunes software supports it only for ripping MP3s, not AACs.
Since this all comes down to personal preference, here's mine: I use WMA 9 with a variable bit rate of 85 kbps to 145 kbps, although I should admit that I rarely rip classical recordings. And in case you're curious, I'm also a fan of 2000 Bordeaux and 2001 German Rieslings."
-- Josh Taylor
Retirado de PCWorld.com - Digital Audio Hi-Fi
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