All electronics create noise, even digital electronics.
All recordings have noise, since all signals in a recording studio must pass through electronics. GMX D-5.1 speakers have tweeters, which in a Klipsch way of thinking, improves the sound quality over the majority of multimedia speakers that don't. The wider bandwidth, in addition to the Klipsch mantra of high sensitivity and high efficiency, forces us to pay special attention to the noise floor of the electronics. If the sensitivity of the speakers goes up by 3dB, the noise in the electronics has to go down 3dB to keep the same noise floor.
Although the GMX D-5.1 decoder is a complex piece, it has a relatively simple signal path, which is typical of other systems in its price category. There is a digital receiver which takes the bitstream (PCM or S/PDIF) from an optical or coaxial digital cable and routes it to the DSP (digital signal processor). There is an ADC (analog to digital converter) that takes the analog input signal and routes it to the DSP. The DSP itself takes the bitstream and decodes it into multiple digital signals, which are then sent to the DAC (digital to analog converter). The ADC and DAC are integrated into a CODEC (coder/decoder), which also does the volume control in the digital domain. The DSP performs bass management duties through the use of digital filters, specifically tailored to the needs of the GMX speakers (no canned “large” and “small” setting here). After the DAC is a low-noise buffer/gain stage which sends the now analog signals to the power amplifier. The GMX uses high quality Crystal 493xx DSPs and 24 bit CODECs, as well as 5532 op amps.
The typical ProMedia system has about 30dB of gain. This allows a wide variety of signal sources to be connected and still produce full system output. If there is any noise in the input signal, it is multiplied by the same gain factor. The difference between a sound card's “line level” output and that of an MP3 player can be as much as 18dB. If the input signal has noise, the output signal will have 30dB more noise at full volume, and there is nothing anyone can do about it.
One of the decisions in a system like the GMX is where to set the gain. As mentioned before there is a wide variation in the level of the analog input signals. The gain has to be high enough so that people using MP3 player are not disappointed, and still allow the higher level inputs of sound cards without causing problems from overdriving the inputs. With the system architecture of the GMX, if you add 6dB of gain you add 6dB of noise, and the analog input is like an “open mic.” There is no way for the DSP to know whether a signal is present or not, so it applies gain to the signal, even when no “signal” is present.
If you connect the digital output of a sound card, you would expect that when there is nothing “playing” that the system would mute in the same way, but that is not the case. The sound card can behave like an “open mic” as well. Look at the sliders on the Windows volume control. The sound card takes all these inputs, mixes them, and puts out a digital bitstream based on the sum off all the inputs. Even with all the sources muted, the soundcard spits out a digital bitstream. When the DSP in the GMX decoder detects a digital bitstream it un-mutes. Usually this isn't much to worry about, but it can add up, and some sound cards are better than others. On the other hand, with the digital inputs, if the DSP does not detect a bitstream, then we have the option of “muting” the outputs of the decoder, which we do. So if you connect the GMX to a DVD player through the optical connector and pause or stop the movie, the GMX recognizes the loss of the digital bitstream and the system mutes.
During times when your not listening to your system, like at night when the hiss might bother you, the mute or power button can be used to silence the speakers.