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| Fig. 1 Car Audio |  |
Home Audio |
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previous issues, we have explained DSP technology, which can reproduce
sound in the limited space inside a car, and create the same ambience
as if we were listening in a concert hall. We would like to continue the
discussion with an explanation of position offsetting, another digital
technology that enables us to enjoy higher quality music in a car. Three factors that deteriorate sound quality High-end car audio equipment offering design and quality comparable to home audio has been introduced to the market recently. However, these products often do not perform to their full capability when actually mounted in cars, despite impressive performance in listening room trials. This is entirely attributable to the hostile sound reproduction environment in automobiles. The space within a car has several acoustic disadvantages compared to an ordinary listening room: (i) The shape, volume, and materials of the interior compartment of a car generate a sound frequency peak dip. (ii) The many restrictions governing the speaker-fixing position demand the adoption of multi-channel (mainly 3-way) systems to reproduce broad-band sound. Full-range speakers cannot be used. (iii) The sensation of stereo sound deteriorates when speakers are tuned to accommodate the nearest listener. Quality suffers due to the different distances between listeners and the speaker. (Fig. 1) Specialty stores have improved their installation techniques over the years, but this hasn't fundamentally eliminated sound quality deterioration, since the physical restrictions of automobiles still have not been overcome. Beyond installation skill, addressing and eliminating this deterioration requires a parametric equalizer, a channel divider function enabling cross-point adjustment of speakers with different bands, and a time alignment function that incorporates DSP technology. |
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Doing
the impossible with digital technology Parametric Equalizer In our earlier discussion on equalizers, we explained the workings of the parametric equalizer. With a parametric, abnormalities in frequency properties, which vary from car to car, can be offset and adjusted for all bands. Channel Divider To maximize the potential of a 3-way speaker, the channel divider function removes the frequency bands which are hard for each of the speakers to reproduce, and at a micro level crosses over the bands assigned to the respective speakers. By smoothly connecting the different bands, the channel divider enables sound reproduction like that of a full-range speaker, and because the fine tuning is accomplished with digital technology, it is effective over a prolonged period of time, with no deterioration. Time Alignment The last and perhaps most difficult challenge is sound position fixing. This involves eliminating the lag in time it takes for sound to reach the listener due to the positioning of fixed speakers. In the most common speaker layout, a tweeter is placed in the position nearest the listener's ears, while a mid-range is established off to one side, at the passengers' feet, and a sub-woofer is positioned in the trunk room. In this arrangement, a deviation arises from the different distances at which the speakers are fixed from the listener. Since the respective sounds reach the listener at different times, it seems that sound which should be arriving from in front is actually coming from behind. Time alignment technology offsets this deviation and fixes the ideal sound phase in the front. To achieve the alignment, DSP is used to adjust the timing of sound coming from each sound source, at 1/10,000 second increments. Digital technology has made these once difficult if not impossible adjustments simple, bringing control of the entire process to the fingertips. Thus, it helps maintain the properties of reproduced sound, and assures listening-room sound quality, even in the unfavorable acoustic environment of a car. Thanks to digital technology, a new frontier for audio equipment and sound enjoyment is opening up before us.
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