Meeting Topic: Acoustic and Psychoacoustic Issues in Room Correction
Speaker Name: James (JJ) Johnston* and Serge Smirnov - Microsoft Corporation (*now with Neural Audio)
Other business or activities at the meeting: Proposal of a AES PNW live sound workshop fro a fee; report from NAMM
Meeting Location: Microsoft, Redmond WA
The PNW Section's January meeting took place at Microsoft Studios in Redmond, WA. It featured James (JJ) Johnston and Serge Smirnov of Microsoft Corporation speaking on "Acoustic and Psychoacoustic Issues in Room Correction." There were about 82 attendees (23 were AES members) in the standing-room only crowd.
James (JJ) Johnston began the presentation with some basic acoustic and psychoacoustic issues regarding room correction. He described the acoustic characteristics of a room, then some of the problems. If you want to correct the room-speaker combination for good imaging and timbre, exactly what do you correct and how? Long-term or short-term frequency response, or some combination? What about interchannel matching?
The precedence effect, where the brain discounts arrival of signals after the first, helps. The first arrivals also are the primary cue for binaural localization, with the uncorrelated late arrivals sensed as "envelopment."
The upshot is to smoothly equalize the direct arrivals at high frequencies, remove the worse peaks in the lows, and not touch deep low frequency dips. Broad dips can be equalized. Do not correct overall phase and magnitude with the processing or you will cause unpredictable pre-echo. You need to equalize gain and delay from each speaker just for the first arrival, which locks in the spatial cues. Frequency response correction for the first arrival should be applied - exact for better speakers and relative (i.e. between channels) for sets of poor speakers. Correct the worst problems with the speakers or room, and leave other problems alone. You should also try to cancel some of the first reflection at low frequencies.
One of Microsoft's goals here is essentially to help make mediocre speakers sound good in a real room with minimal overhead and cost.
After a break, PNW Vice Chair Steve Turnidge gave a brief report on the recent NAMM (National Association of Music Merchandisers) convention.
A prize drawing was conducted, with several donated prizes awarded.
Serge Smirnov then described how this room correction procedure is implemented in the Windows Vista operating system.
Probe (test) signals are synthesized which have the same magnitude at all frequencies, and with continuous phase across frequency. The probe signals are sent in a specific sequence over each audio channel within a few seconds in order to do the analysis.
The captured signal from the microphone uses a Hilbert envelope to figure delay. Gain and frequency response probes start with a narrowband probe, then several wideband probes for redundancy (discarding obvious bad responses), ending with a final narrow probe. Gain is computed from the 800-2,000 Hz average of power spectrum coefficients. For frequency response analysis, the room is measured first. Then the captured probe signal is analyzed and processed to determine separate FIR (finite impulse response) filters for high and low frequencies, and follow design "rules" for smoothness and bandwidth limiting. Detecting and canceling some of the first reflection is computed from the Hilbert (analytic) envelope. A rendering engine applies the corrections with low demand on the computer.
There was a discussion of Time Delay Spectrometry (TDS), of which this may be considered a variant.
Report by Gary Louie, AES PNW Secretary