Meeting held April 28, 2005 at Loud Technologies (aka Mackie Designs).
The PNW Section held its April meeting on gain structure at the Loud Technologies (ne้ Mackie Designs) corporate office training facility in Woodinville, WA. About 65 persons attended. Section Treasurer Dave Franzwa opened the meeting with Section business and notice of elections. The mike was passed around the room and everybody Introduced themselves.
Mark Rogers, PNW committee member and Director of the AV Department at Seattle consulting engineers The Greenbusch Group went through the basic elements of practical analog gain structure and gain balancing in audio systems for best performance.
What do most sound system operators seem to do? Connect all the equipment; set all knobs at 7; set amps at 10 "to get most power"; adjust volume with the console fader; if any "clip" lights blink, fiddle with gains; and on with the show (and noise and distortion)! Unfortunately, audio is almost never plug and play, if you want full output and lowest noise.
Mark explained what gain and gain balance of all parts of a complex sound system are, using a chart to show levels of in/outs of many items in a chain. Basically, set your system so all parts clip at the same time. You must calibrate or test each I/O level. Adjust knobs or add pads or attenuators as needed, while measuring. The most accurate method is with an oscillator and oscilloscope to observe each stage's clipping level.
He noted some common traps: Line level is not really standardized - there is +4/-10 and nothing ever really at those levels, plus varying headroom or output levels anyway; balanced/unbalanced differences; problems from patching equipment manually; funny meters and varying fader positions.
There was a break for refreshments and a drawing for prizes. Door prizes were courtesy of Loud Technologies (Mackie) and Rane Corp.
After the break, Committee member Melissa Rice distributed a meeting questionaire to gather information for future meetings.
Next, James Johnston (J.J.), PNW Committee member and Audio Architect, Codecs Group, Microsoft, covered performance issues of gain control in the digital domain.
He began with the basic properties of PCM signals, and basic problems: clipping, and loss of dynamic range due to a rising noise floor from digital processing. There are many gain issues and problems from digital processing. For example, digital clipping is not like analog clipping, where related harmonics are generated - digital clipping aliases back down to audible frequencies, and are not necessarily related to the signal - which is usually very audibly annoying. He also spoke of noise floor problems, and a little on processing and IIR (infinite impulse response) and FIR (finite impulse response) filter problems.
Mark Rogers returned for some practical considerations for calibrating the power amp in a sound system, and both Mark and JJ handled some Q&A at the meeting's end.
Reported by Gary Louie, PNW Secretary