Vimes

The piece I posted I collected some time back on Digg or something like that. Clearly, whoever posted it there had cut it down. Thanks for the URL for the extended version. Ii don't necessarily agree with the "engineer" (?) I just thought his points were worthy of interest. I like iPods as nice little toys and use iTunes (now playing, a 128kbps copy of the Beiderbecke/Trumbauer set on Mosaic.) Given the surface noise in the originals, 128 doesn't seem too much of a problem. BUT the MP3s are playing via an M Audio Firewire Audiophile which, to my ears, does improve the sound. Is that because this device is a pretty good digital to audio converter? The Audiophile, incidentally, is linked to an old Kenwood stack which samples at 44.1kHz using ;large front and smaller rear speakers (KEF and Kenwood respectively). Sorry if I've unintentionally changed the topic...

I thought this might be of interest - I saved it a year or two back, but the original URL is long gone. Nevertheless, its worth a glance! iPods by a sound engineer My wife got herself an iPod music player. I was volunteered to fill it with her music from her CDs. You can get one like her 4GB white Nano here or here. iTunes is Apple's free software used to copy all your CDs into an iPod. It also allows you to play and manage your entire CD library from your computer, and it sounds great for serious listening. I had never bothered with these little players or compressed digital audio since I presumed the sound quality was awful. WRONG! The little things sound great for serious music listening, presuming you click a couple of the right buttons in iTunes' preferences. I'll explain these in this article. I was prejudiced after decades of designing my own recording equipment and working in and around the music and recording industries. Not only have I understood all the math behind digital since the 1970s, I even worked with one of the original designers of the MPEG audio standard and delivered a paper to the audio engineering industry's standards-setting body, the AES, in the 1990s. I've been a member of the AES also since the 1970s. Back a few years ago compressed digital audio wasn't that great and even at 384kbs you were making excuses. Today the AAC coder in iTunes sounds great at 128kbs, and that's for serious listening on high-end audiophile systems with your eyes closed and your full attention on the music. Just plug in a set of professional headphones that cost more than the iPod, or plug the one ounce Nano, which is smaller than a business card and 1/4" thick, into a 400 pound classical recording studio monitoring system and you'll have the same epiphany I did. Let me tell you how to do it. Some odd people prefer listening to the equipment and not the music. We call those people audiophiles as opposed to music lovers. Audiophiles share the same prejudice I just overcame against compressed digital audio. For them iTunes also allows recording music without any data loss and also at 96kHz sampling rates. I'll get to that later. WHAT BOXES TO CHECK Error Correction I'm amazed that I caught Apple in a technical faux-pas. If you use iTunes' default importing options you will probably get an occasional click in your sound. That's because under PREFERENCES > ADVANCED > IMPORTING Apple left the "Use Error Correction" box unchecked. This means when any inevitable invisible speck of dust covers a couple of bits on your CD that you may get a click or short dropouts. It drove me nuts at first thinking I had a dirty connection on playback. I never thought Apple could make such an obvious mistake. CDs always need error correction. Without error correction you'll always have problems. Every CD player and every CD-ROM reader always applies error correction without you ever having to ask. CDs don't work without it. CHECK THE "USE ERROR CORRECTION" BOX under PREFERENCES > ADVANCED > IMPORTING. Once I did I've never had a problem. Apple cautions that checking this may take more time to import. So what? It takes no longer from what I can see, and without checking it you'll wind up having to re-import everything as I did. If it does take any longer it's because it's saving music that otherwise would be useless. Encoder Preference After finding Apple's error correction blunder I decided to spend the day experimenting on myself to hear what I could hear for encoder quality. I used my reference Beer DT990 headphones, since I can hear more through them than my Quad ESL63 electrostatics or B&W 801 studio monitors. I compared results using a stack of our favorite commercial CDs. I was struck with how great this all sounds today. Even the distortions introduced by crummy digital audio compression are nicer than the distortions in previous consumer distribution media like LP vinyl records and cassette tapes. The great thing about CDs is that they offer consumers the same quality we took for granted from master tapes, but never heard outside a studio until CDs came along. Even master tapes are gone today. Today music is recorded uncompressed on hard drives. Back to the story: At the lower rates the audible distortions are swishy phasing effects, especially audible on choral backgrounds during percussive events. I knew that. These are the same weird sounds one hears over cell phones. At the bit rates used for music these only happen if you really listen for them. The most surprising and annoying defect is alteration to the stereo image. I first heard this with a true stereo (two-mike) choral recording. The natural reverberation decayed normally, and depending on the data rate would eventually decay to mono! At higher rates it's fine and at 96kbs it sounded weird when I first heard it. There's an obvious explanation for this. Data compression is all about eliminating redundancy. The more uniform the sound the easier it is to compress. Music compression takes a lot of advantage that most music is similar in both channels. Natural stereo reverberation becomes completely random with zero correlation between channels. Therefore it's much tougher to compress a reverb tail than the program sometimes, which is why the lower rate encoders gave up and summed it to mono at low levels. The biggest audible defects at lower rates today aren't obvious things to which you can point a new listener, like clicks or pings. The degradation is a loss of stereo space and image. This becomes obvious on headphones. You'd probably never notice on speakers, and never if you're doing something else while playing music. 128kbs AAC iTunes 6 uses 128 kbs AAC as default. This is fine. I really have to use my imagination to make myself think I hear any difference between audio coded that way and the original CD. I could almost hear the ends of reverb tails in one classical recording sum to mono and that was about the only defect. This default is fine, especially if you have an old iPod which can't handle the variable bit rates which sound better below. With a newer iPod I use: 128 kbs Variable Bit Rate (VBR) AAC : My Choice For only about a 5% penalty in file size I use variable bit rate encoding for better quality. This lets the coder use more bits when it has to. I set this under PREFERENCES > ADVANCED > IMPORTING > Import Using & Setting > Custom, and then check "Use Variable Bit Rate Encoding (VBR)." Apple has this pretty well hidden. I leave the rest at default of 128kbs, auto and auto. VBR sounds better for the same file size. As far as I can see the only reason Apple doesn't default to this is for compatibility with old iPods. Having a new iPod Nano, no problem! I couldn't hear any defects. 128kbs VBR AAC sounds the same as my CDs. Any defects I heard were accurate reproductions of flaws in the original CDs. 96kbs AAC 96kbs AAC sounds fine for normal use while not paying rapt attention to your music. It sounds worse than the others listening carefully. It got a little bit swimmy, phasey or flangey if you compared it to the original, and the ends of classical reverb tails would sum to mono if you were paying close attention. I'd not use this unless you're stuck on an old iPod which can't handle VBR. 96kbs VBR 96kbs VBR sounds better than regular 96kbs. It's not much bigger, maybe 5%, in file size. I can hear a difference between regular and VBR at 96kbs. I can't hear a difference between 128kbs standard and 128kbs VBR. The reason I chose 128kbs VBR over 128kbs standard is because I can hear the difference at 96kbs. This way I've got some extra, although inaudible, quality improvement over 128kbs standard with almost no file size penalty. Since 96kbs VBR is about the same size as regular 96kbs I'd suggest 96kbs VBR if you're really trying to cram in as much music as possible. I wouldn't notice anything wrong if I was doing something else while listening, which is how most people listen. 160kbs VBR I also tried this. It makes bigger files and I couldn't hear any improvement over 128kbs VBR. 128kbs VBR sounded identical to my CDs. WHAT IS AAC? AAC is one of many ways to compress audio files. MP3 is another. There are many people with PhDs who spend careers developing and comparing them. Many thanks to these people whose work has given us transparent audio quality at 128kbs. I didn't go off and try to compare all these systems. It took a day just to compare a few bit rates of AAC. My research was trying to find an optimum setting for getting my wife's CDs into her iPod with the best possible quality at the smallest file size. I'm a music lover who prefers listening to music over comparing coding schemes. Encoders get better every year. That's why 128kbs sounds better today than 384kbs used to sound. MP3, actually MPEG-2 layer 3, was OK several years ago and is has established itself as a popular format for illegally copied music. AAC works better today. Better means better sound for the same file size, or smaller files for the same quality. You have to ask yourself if it makes sense to re-encode all your stuff every couple of years for better sound and smaller files. Today I'd skip MP3. I'm not going to explain AAC itself, sorry. I already explained all this in a paper I delivered some years ago. You can find this and info on all the other compression systems elsewhere in the Internet. HOW ABOUT THOSE AUDIOPHILES? For them there is lossless coding which preserves each and every bit of audio data. They would select Apple Lossless Coder under PREFERENCES > ADVANCED > IMPORTING > Import Using & Setting > Custom. This lossless coder makes big files, about five times as big as 128kbs VBR. I find 128kbs VBR transparent. If I was really worried about things I can't hear I'd use a higher rate AAC VBR setting, like 160kbs or 256kbs. The beauty of iPod and iTunes is that you have the flexibility to use any of these schemes, and that it sounds great if you leave everything alone so long as you remember to check "Use Error Correction" as I warned at the top. Audio has come a long way. Again.