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Tuesday, August 19, 2008

Vacuum Packed

Compress audio files without losing quality? You can, if you measure them the right way.

My last two blog tutorials discussed neural masking, and how an mp3 or AAC can be good enough for broadcast or film sound when you do it right. (If you followed the link to my website, you even got proof.) But sometimes, even AAC’s tiny losses can be too much: you might be sending elements that will be processed or compressed more, or be saving an archive. While most non-audio files can be successfully squeezed with Winzip or Stuffit, those processes behave strangely with audio.

Zip-like algorithms look for repeating patterns in a file, and create shortcuts for the ones it finds; when you open a Zipped file, it replaces the shortcuts with the original patterns.

That’s fine for a document or spreadsheet. But audio files seldom have that kind of repetition. Even if a musical pattern or spoken phrase occurs twice, the audio sample data can be different because of tiny differences in performance or background noise.

Want proof? I created four sixty-second stereo files: a sinewave, a sweep, pink noise, and a typical voice/music mix. All were recorded at 16 bits 48 kHz, the usual standard for DV sound.

Sinewave:     A steady tone at a single frequency. I used 1 kHz at -20 dBFS, the lineup sound that’s usually played with color bars.

Sweep:     A pure tone with a constantly changing frequency. In this case, it went from 20 Hz to 20 kHz at -20 dBFS over 30 seconds, then repeated.

Pink Noise:      A random “shhh” with equal energy in each octave, used for acoustic testing. This one was at -20 dBFS, and since it was generated in a computer with pseudo-random numbers, the pattern does repeat after a while.

All four files were 11.2 MB before compression. Zipping them gave me these results:

If the numbers (shown in Kilobytes) make your head spin, relax. I’ve boiled them down to a nice-looking graphic, later in this article.

The sinewave was easily analyzed by the Zip process - measure one pure wave at a specific frequency, you’ve measured them all - and shrank to less than 2% of its original size. The sweep and pink noise were harder for Zip to deal with, but it found enough patterns to save about 25%. But who saves compressed test signals? The typical mix hardly shrank at all when Zipped; that wasn’t worth processing at all.

There is a better way to squeeze audio files, and still recover the sound with no loss. Turn the page to find out.

Audio • Distribution • Post Production •

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Thanks a ton for this, and the last two articles. (They are the first of your that I have found.) Tremendously helpful and a great balance of science and simple terms.

Apple Lossless.... here I come.

Posted by Travis  on  08/20  at  05:34 AM

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Glad you liked it, Travis.  Spread the word.

There are other articles with a scientific bent in my Inside Track blog. They take some effort to write and illustrate, and I’m basically lazy, so they’re balanced with articles that are based on humor or on observations about our industry. But always with some kind of audio tutorial or tip. It’s what I do.

You might also like some of the tutorials on my own site, dplay.com.

Posted by Jay Rose  on  08/20  at  07:05 AM

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