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AAC Put To The Test
technology is sexy writes "Following the increasing popularity of AAC in online music stores and the growing amount of implementations in software and hardware, the format is now being put to the test. How well does Apple's implementation fare against Ahead Nero, Sorenson or the Open Source FAAC at the popular bitrate of 128kbps? Find out for yourself and help by submitting the
results. You can find instructions on how to participate here. The best AAC codec gets to face MP3, MP3Pro, Vorbis, MusePack and WMA in the next test. Previous test results at 64kbps can be found here."
I note that in the 64 Kbps test, they used the AAC-LC encoder from QuickTime 6.0. This was a pretty darn lousy one, lacking any ability to specify a sample rate at a given data rate, and had poor quality. The current version of QuickTime 6.3 (for Windows and MacOS X), has a much improved, more flexible AAC-LC encoder, so if they did that test today AAC would likely rank higher.
If using the Apple encoder, encode in "Better" mode with 16-bit source, and in "Best" mode with source that's more than 16-bits per sample (and hence isn't a CD rip). Support for mastering from 24-bit when running in "Best" is one of the reasons why the AAC-LC files as part of iTunes sound so good.
My video compression blog
First, read this:
.zip files.
http://www.ff123.net/abchr/abchr.html
This describes the program and testing methodology used here, which, btw, is based on widely accepted perceptual testing conventions. And yes, by the scientific community. These are the same techniques used by the scientists that do the research and development on these formats. Please note the references at the bottom of the page.
1. Wrong, the MP4 files are already encoded and created for the user, stored in the
2. Wrong, the Hidden Reference (ABC/*HR*, please read the page at the first link), ensures that if the user honestly cannot tell the difference but thinks that one exists (placebo), and rates the original lower than one of the encoded versions, that their results are discarded.
3. This is where the statistics come in. With enough listeners, the "noise" gets weeded out of relevant results. Most past tests using this methodology have been shown to provide highly relevant and fairly uniform results when all the data is factored together.
An open call to the masses is the only way to measure the perception of the masses, and if the test is performed properly (which it is in this case), then it *is* scientific.
Next time, please read up a little more on what is happening before jumping to all sorts of incorrect conclusions.
This is what the cdparanoia faq has to say about ripping...
I can play audio CDs perfectly; why is reading the CD into a file so difficult and prone to errors? It's just the same thing.
Unfortunately, it isn't that easy. The audio CD is not a random access format. It can only be played from some starting point in sequence until it is done, like a vinyl LP. Unlike a data CD, there are no synchronization or positioning headers in the audio data (a CD, audio or data, uses 2352 byte sectors. In a data CD, 304 bytes of each sector is used for header, sync and error correction. An audio CD uses all 2352 bytes for data). The audio CD *does* have a continuous fragmented subchannel, but this is only good for seeking +/-1 second (or 75 sectors or ~176kB) of the desired area, as per the SCSI spec.
When the CD is being played as audio, it is not only moving at 1x, the drive is keeping the media data rate (the spin speed) exactly locked to playback speed. Pick up a portable CD player while it's playing and rotate it 90 degrees. Chances are it will skip; you disturbed this delicate balance. In addition, a player is never distracted from what it's doing... it has nothing else taking up its time. Now add a non-realtime, (relatively) high-latency, multitasking kernel into the mess; it's like picking up the player and constantly shaking it.
CDROM drives generally assume that any sort of DAE will be linear and throw a readahead buffer at the task. However, the OS is reading the data as broken up, seperated read requests. The drive is doing readahead buffering and attempting to store additional data as it comes in off media while it waits for the OS to get around to reading previous blocks. Seeing as how, at 36x, data is coming in at 6.2MB/second, and each read is only 13 sectors or ~30k (due to DMA restrictions), one has to get off 208 read requests a second, minimum without any interruption, to avoid skipping. A single swap to disc or flush of filesystem cache by the OS will generally result in loss of streaming, assuming the drive is working flawlessly. Oh, and virtually no PC on earth has that kind of I/O throughput; a Sun Enterprise server might, but a PC does not. Most don't come within a factor of five, assuming perfect realtime behavior.
To keep piling on the difficulties, faster drives are often prone to vibration and alignment problems; some are total fiascos. They lose streaming *constantly* even without being interrupted. Philips determined 15 years ago that the CD could only be spun up to 50-60x until the physical CD (made of polycarbonate) would deform from centripetal force badly enough to become unreadable. Today's players are pushing physics to the limit. Few do so terribly reliably.
Note that CD 'playback speed' is an excellent example of advertisers making numbers lie for them. A 36x cdrom is generally not spinning at 36x a normal drive's speed. As a 1x drive is adjusting velocity depending on the access's distance from the hub, a 36x drive is probably using a constant angular velocity across the whole surface such that it gets 36x max at the edge. Thus it's actually spinning slower, assuming the '36x' isn't a complete lie, as it is on some drives.
Because audio discs have no headers in the data to assist in picking up where things got lost, most drives will just guess.
This doesn't even *begin* to get into stupid firmware bugs. Even Plextors have occasionally had DAE bugs (although in every case, Plextor has fixed the bug *and* replaced/repaired drives for free). Cheaper drives are often complete basket cases.
Rant Update (for those in the know):
Several folks, through personal mail and on Usenet, have pointed out that audio discs do place absolute positioning information for (at least) nine out of every ten sectors into the Q subchannel, and that my original stateme
Do you even lift?
These aren't the 'roids you're looking for.