Meeting Topic: High Resolution Audio
Moderator Name: Conrad Cooke
Speaker Name: Keith O Johnson - Reference Recordings
Meeting Location: Aoole, Cupertino, CA, USA
AES San Francisco's March meeting was titled Science and Art to Make Great Recordings and Playback-The High Resolution Experience. Keith Johnson provided something like a play book on how to make to sound recordings with a sense of realism.
Mr. Johnson has a distinguished career in audio engineering, and is known as "the Professor."
The meeting attracted seventy people, and was held at Apple Computer, in California's Silicon Valley.
Mr. Johnson began by giving an overview of the entire audio chain, from concert halls to microphones, electronics, processes and loudspeakers, and finally the human ear and its interaction with the brain.
How is high resolution sound achieved? Among many factors, the human ear is sensitive to time response.
High-resolution sound is hard to verify using instantaneous measurements with an FFT (fast fourier transform), and usually can't be reproduced with waveform reconstruction.
Some of the factors that make up high-resolution sound are not audible, but the combination of several things that are inaudible can result in low-resolution sound.
Listener expectations and frame of mind play key roles in sound perception.
Sound nirvana is a state of psychological absorption from a sound experience. It is difficult to create, easily destroyed, and hard to re-create.
There are positive feedback loops within the ear. When one set of cells is stimulated, another set is sensitized. There is also processing in the nerves between the ears and the brain.
Microphones do not have uniform polar response with respect to frequency, creating off axis coloration.
The inverse square law is affected by the size of the sound source. The atonal sounds created by plucking a guitar string don't carry as far as the tonal sounds from the broad surface of the guitar body. A small sound source creates a smaller radius of sound waves, with greater attenuation with respect to distance than a large radius. As with many instruments, close miking a guitar creates a different sonic impression than backing away.
When using multiple mikes for a symphony recording, leaving all mikes open creates a mish-mash. The sound engineer must know the musical score, and open only those mikes that are needed.
Accent mikes are often necessary to prevent certain instruments from being lost in the mix. Stereo accent miking is an effective tool for creating realism and high resolution, if the two mike signals are not combined into mono, causing comb filtering.
Mr. Johnson closed by playing some award-winning recordings, and a remarkable example of how higher levels of reverberation can seem to slow the tempo of an orchestral piece.
by Paul Howard
