The subwoofer output voltage and distortion measurements are both worst-case tests in which maximum-level low-frequency signals from each of the main channels as well as the LFE channel are added together and fed to the subwoofer output. The sub outputs of most receivers either grossly overload (meaning distortion is well above 1%) or are limited to some level short of what is needed to preserve full dynamic range. If an overload can be eliminated by adjusting the receiver's subwoofer-output level-trim control, then we give the proper setting. (Not all receivers allow this - some subwoofer outputs always go into clipping with worst-case signals.)
If you set up a receiver we've tested with the subwoofer trim at the level we used or lower, the subwoofer output will not overload provided the main-channel level-trim controls are also all at their 0 or default settings or lower (which is quite often the case). In practice, worst-case signals rarely occur with program material, and the distortion generated by even an overloaded subwoofer output is usually filtered out by the subwoofer's own circuits. In any case, most of the time it'll be masked by the (at this point, very loud) program material from the main channels.
Multichannel-Input Tests Since a receiver's multichannel analog input is, for the time being, the only path through which most of us are going to hear the multichannel music from a DVD-Audio or Super Audio CD player, we've recently instituted a short series of tests for it. They are designed mainly to see if the receiver degrades the incoming high-resolution signals.
The most important number we run here is the A-weighted noise level - which, by a considerable margin, should have the lowest value of any A-weighted receiver noise measurement we publish, -90 dB or lower. With such performance, the background noise level in DVD-Audio or SACD playback will be thoroughly dominated by the noise of the player or the recorded program.
We also include an extended-range frequency-response measurement (up to 96 kHz) for those who think - mistakenly - that flat response far above 20 kHz is audibly important. Unless we find something really peculiar here (which would be pointed out in the notes), you can consider the multichannel-input response measurements irrelevant to a buying decision. However, if and when receiver manufacturers start applying bass-management processing to their multichannel analog inputs, you can be sure that we will add some response measurements of that as well.
Due to the introduction of these multichannel-input tests, and the space they'll take up in our lab boxes, we will have to discontinue a few of the measurements we've run in the past, including channel balance and tone-control response. Not to worry, though: for a variety of reasons, these measurements are not particularly relevant today for making a buying decision, for using the receiver you choose, or for selecting source components and speakers that will perform well with it. The lab results we do publish for receivers represent our best attempt to put the audible performance of current receivers to the test.
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