HS Blog: Oh, the Energy of Time Domain Transients

May 31, 2007

Oh, the Energy of Time Domain Transients

Jon Simpson is The Hanger's resident audio/acoustics/recording geek. So Jon. . .

In today's blog-offering from James Lileks, he comments on an audio phenomenon he frequently encounters when watching old Twilight Zone episodes—something called "pre-echo." Apparently you hear a faint "echo" of some dialogue or a sound before the actual sound occurs.

James links to the following Wikipedia entry that purports to explain the mystery:

Pre-echo is a psychoacoustic phenomenon where an unusually noticeable artifact is heard in a sound recording from the energy of time domain transients smeared backwards in time after processing in the frequency domain due to the Gibbs phenomenon. Because forward temporal masking is so much stronger than backwards temporal masking, the Gibbs phenomenon can give rise to this audible distortion which sounds like an echo which comes before the actual sound.

In an effort to avoid pre-echo artifacts, many sound processing systems use filters where all of the response occurs after the main impulse, rather than linear phase filters. Such filters necessarily introduce phase distortion and temporal smearing, but this additional distortion is less audible because of strong forward masking.

Avoiding pre-echo is a substantial design difficulty in transform domain lossy audio codecs such as MP3, MPEG-4 AAC, and Vorbis.

So. . . Is this for real or some kind of geeky, nonsense-jargon-laden spoof? Are "time domain transients" a real thing and can they really "smear backwards?" Is the "Gibbs Phenomenon" something other than the win-loss record of a former Washington Redskins coach?

And is a "psychoacoustic phenomenon" something that causes sound waves to want to go toward the light?

Posted by David at May 31, 2007 05:09 PM

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May 31, 2007 Oh, the Energy of Time Domain Transients Jon Simpson is The Hanger's resident audio/acoustics/recording geek. So Jon. . . In today's blog-offering from James Lileks, he comments on an audio phenomenon he frequently encounters when watching old Twilight Zone episodes—something called "pre-echo." Apparently you hear a faint "echo" of some dialogue or a sound before the actual sound occurs. James links to the following Wikipedia entry that purports to explain the mystery: Pre-echo is a psychoacoustic phenomenon where an unusually noticeable artifact is heard in a sound recording from the energy of time domain transients smeared backwards in time after processing in the frequency domain due to the Gibbs phenomenon. Because forward temporal masking is so much stronger than backwards temporal masking, the Gibbs phenomenon can give rise to this audible distortion which sounds like an echo which comes before the actual sound. In an effort to avoid pre-echo artifacts, many sound processing systems use filters where all of the response occurs after the main impulse, rather than linear phase filters. Such filters necessarily introduce phase distortion and temporal smearing, but this additional distortion is less audible because of strong forward masking. Avoiding pre-echo is a substantial design difficulty in transform domain lossy audio codecs such as MP3, MPEG-4 AAC, and Vorbis. So. . . Is this for real or some kind of geeky, nonsense-jargon-laden spoof? Are "time domain transients" a real thing and can they really "smear backwards?" Is the "Gibbs Phenomenon" something other than the win-loss record of a former Washington Redskins coach? And is a "psychoacoustic phenomenon" something that causes sound waves to want to go toward the light? Posted by David at May 31, 2007 05:09 PM