OTHER TECH

The slopes of the high-pass and low-pass (subwoofer-output) filters are determined from the slanting portions of the response graph curves. Knowing whether the rolloff is steep or gradual is of less practical value than knowing the crossover frequency, but these numbers may prove useful in ultra-critical applications. In theory - and when it comes to playing low frequencies with real speakers in real rooms, theoretical considerations must be taken with mountains of salt - an ideal crossover network will have unequal high-pass and low-pass slopes. That's because the main speakers will naturally roll off at the low end of their frequency range, usually at 12 dB per octave below their -3-dB point, and this rate must be added to the high-pass filter's rolloff rate, which is often 12 dB per octave also. The result in this case (24 dB per octave) should equal the low-pass rolloff rate at the subwoofer output.

Besides the speaker responses at the crossover frequency, the relative phases of the main-channel and subwoofer outputs have to match for the blend to be as smooth as possible. All these conditions are fulfilled only when the subwoofer is down 6 dB at the crossover frequency and rolling off at 24 dB per octave, and the high-pass filters and main-channel speakers are both down 3 dB at the crossover point and rolling off at 12 dB per octave below that (this is the situation, minus a speaker-response curve, shown in Figure 2). These are also, by no accident, the basic requirements for the bass-management filtering system used in THX-certified products. Lately we've seen such behavior in non-THX-certified receivers too - a welcome development.

If a bass-management system has to be used at all - and it must be with most subwoofer/satellite setups - then it should operate the same way with all inputs, analog or digital, stereo or multichannel, in order to avoid changes in bass balance when you change the program source. We now perform some quick checks to see whether this is the case.

Unfortunately, the bass management in most (if not all) receivers currently on the market does not act at all on their multichannel analog inputs. As a result, depending on your speaker setup, a source heard through those inputs (say, a DVD-Audio disc) could end up sounding very different from one whose audio is connected digitally (say, a DVD-Video disc). In addition, in some receivers the bass-management system turns itself off when you switch from multichannel Dolby Digital or DTS playback to stereo playback from a digital source, as would happen if you played a CD in your DVD player right after watching a multichannel movie. Some models also provide no bass management for their analog stereo inputs unless one of the surround sound modes is activated, like Dolby Pro Logic or a DSP (digital signal processing) ambience mode. When a receiver has such quirks or deficiencies, we mention it either in the main text or in the tech notes accompanying our lab data.

In any case, even with a properly operating bass-management system in a receiver, you still need to properly set up the receiver and speaker system, preferably using a good test disc and a sound-level meter. Proper setup can compensate for many of the bass-management anomalies that might turn up in our lab tests.