Originally posted by MrHifi
I thought that my FS 1800R's servo was an accelerometer whose output was compared to the output of the amp. The difference emf would be "fed back" to the final amp stages to control the output's linearity. Your description of newer models with "sampling" makes me believe that the servo mechanism has been modified in newer models. Is there a paper describing the operation and topology of my mid 90's 600 watt 18" model? I'd be willing topay for a copy.
Topology paper? You overestimate our documentation department
There are differences in the two servos, but in exection only - not in fundamental design.
The servo in your FS 1800R is an analog servo design. That is, a piezo electric chip strains when the cone moves, generating a tiny signal proportional to the acceleration of the cone. This signal goes through an op amp and is fed to a comparator circuit, which compares it to the input signal, which is also really just a representation of acceleration. The difference between the two signals is amplified and biases the signal to the woofer, essentially pre distorting the signal to reduce distotion. The loop gain (that is, benfit from the servo system) is actually measured in dB, in this case about 30 dB, but we translate it to a numerical figure for ease of understanding, and it amounts to about 3500 cone corrections per second.
The DD servo is digital. Same Piezo chip, same tiny signal, but this time it is run through an a to d converter and a digital representaiton of the signal is fed to an input port on the 2407 DSP chip. Each time through the computer's loop - in our case about 15,800 times per second, the acceleromoter signal is compared to the input signal the same way as in the analog design. The signal sent to the amplifier is again biased to reduce distortion. The comparator circuit in the analog design is now represented by software - an inherently more accurate and flexible way to do things.
There are two key differences between the systems. First, the digital nature of the DD system reduces servo noise. I'll bet that if you stick your ear into your FS 1800R you'll hear a faint white noise - this is the servo loop. The digital system has no such noise at all - it is completely quiet.
The other difference is in loop gain - 3500 versus 15,800 times per second, and we get the benefit of putting in a scale to control the loop gain - this is what we call the theater/music setting.
Other servo systems rely on back-EMF, positional sensors, or put a microphone inside the cabinet, or have other transducers. We've found this way provides the most loop gain and hence, the most benefit.
I hope this helps explain things - and it's free! Of course, you get what you pay for...