|
|
Sunday, April 12, 2009
Audio Bit Rate Reduction (MP3, AAC, etc.)
I haven't posted here in a while, but I have been playing with iTunes because I received an iPod Shuffle as a gift a
while back and I finally got serious about putting my CD collection on my hard drive so that I could load up my iPod. Since
I am an audiophile, and I was exposed to the hideous early attempts at audio bit rate reduction back in the 1980s, I was very
suspicious of the compression options.
I scanned the internet and read articles on the various formats and their perceptual differences, including double-blind
tests (so called A-B-X tests) with charts showing the statistical variations between the various encoding algorithms running
at their supported bit rates.
As an aside, I was surprised not to find any "difference files". One powerful way to analyze and understand intuitively
the effect of an audio processing device is to listen to the difference between the original source and the processed version.
It is very simple to implement: Simply invert the phase of one of the signals and add them back together. If the signals are
exactly the same, the sum will be zero -- absolute silence. Any changes in phase, amplitude, distortion, etc., will be clearly
audible. Linear effects can be eliminated by simple attenuation and time-shifting. I might try this and publish the results
here eventually.
Anyway, I decided to copy my CD collection to the hard drive in lossless format, so that I would have reference copies
to work with. However the iPod Shuffle cannot play lossless files. It insists on converting them to 128kbps AAC CBR even though it can play 256kbps AAC VBR. I wanted the higher quality on my iPod. So after careful listening on my AKG K-501s and my old knock-around Sennheiser HD-435s, I discovered that 256kbps AAC VBR was practically indistinguishable from lossless
to my ear. So I decided to store the CDs on my hard drive as AAC 256 VBR.
|
|
Contact Us
Use the form below to send comments about this site.
|
 |
|
|
|
|
|
Welcome!
This site is dedicated to my interest in engineering. I use it as a place to just put stuff "out there" for anyone who
might be interested, as well as a job search tool. This page contains my engineering web log (to your left). Anything that
seems timeless enough, or too involved for a log entry may get it's own page as a full-blown article. And, of course, my resume
is here for anyone who might be considering hiring me to do something. Email me.
About Me
I have worked as a broadcast engineer, electronics designer (analog and digital) and software developer. I especially
enjoy audio engineering, although I've been writing software for the last several years, since that's where the opportunities
are in the Pacific Northwest (this is Microsoft country after all). I'm currently working as a Senior Staff Engineer -- Resume at another large independent software vendor in Washington State.
I majored in mathematics and physics in college. Other interests include music appreciation (baroque, classical and early
romantic, ragtime, and 60's and 70's rock & roll), high fidelity audio electronics, meteorology and of course, computing
hardware and software. I tend to think a lot like this guy.
|
|
|
|
|
