Integrator's Reference Manual for Polycom HDX ... - 1 PC Network Inc

Integrator’s Reference Manual for Polycom HDX SystemsAudio OutputFor conference communication we do not really care about producing thethundering roar of jet aircraft engines, or other sounds reproduced on TV or inthe movies. We are interested in reproducing the human voice. The tone,intonation, pitch and level of people speaking from the far-end should soundas much as possible like the sound they would make if they were speaking inthe room. Given what has been covered in other sections of this book [Basics ofAudio and Visual Systems Design], we will touch base here on a couple of simple,basic elements of the speaker technology we deploy in the conference room.These basics fall into three subcategories: direction, power andrange/frequency response.DirectionAs human beings, we feel most comfortable when the voice we hear appearsto come from the same direction as the image of the person speaking. Thismeans that reliance on ceiling speakers alone is not an ideal practice when thesystem is used for videoconferencing. In many small and medium-sizedsystems, front-firing speakers alone can provide proper direction andadequate coverage. Larger rooms (greater than 12’x15’) probably need bothfront-firing and side or top-fill speakers in order to maintain proper coverageat nominal power levels.In planning systems for larger rooms, we need to take advantage of the HAASeffect. Basically stated, this is the human brain’s interpretation of sounddirection when the same sound arrives at the ear from two or more directionswithin a certain time period. We attribute the direction of the sound to thedirection from which the sound is first perceived, even if it is mixed with thatsame sound arriving from a completely different direction, as long as the two(or more) instances of the sound are within about 30ms of one another. Sincesound travels faster electronically than it travels through the open air we mayneed to add audio delay to the side firing or ceiling speaker arrays in order tokeep the primary perceived point source as the front of room/front-firingspeakers.PowerPower is a function of loudspeaker efficiency and total available systempower. Most speakers operate in a power range that is broader than the rangein which they operate without distortion. For the purpose of conferencecommunication, we are interested in sound that has little or no distortion.Sound that is reproduced accurately (with no distortion) will most accuratelyrepresent the voice of the people from the far-end (our primary goal). Accuratereproduction also will aid the echo-cancellation circuitry in the system,minimizing the amount of echo that the system sends back to the people at thefar-end, and thereby increasing perceived ease of intelligibility andunderstanding. Remember that any distortions present in the playback audiosystem—whether harmonic, amplitude (gain compression) or temporal (timeA - 10