John, as far as I can see, if the original of that lecture was in German, it has not been posted for download on the Neumann Web site. I would certainly like to read it if it's available by other means.
After some further study I think that I may understand the diagram a little better, though. I've attached a copy of it here with two points labeled 1 and 2 for discussion.
At point 1, there is a slight downward jog representing ca. 1 dB loss of signal-to-noise due to the inherent noise of the microphone preamp. This seems to be showing a theoretical minimum of added noise here, such as might occur with a very good preamp that is optimally designed and optimally set for the given recording situation. Also, this would seem to be most consistent with unweighted noise measurement, since the noise of condenser microphones and the noise of mike preamps typically have rather different spectra, and almost any kind of weighted measurement will reveal this.
What seems to happen then is that an "alternative line" is shown, which plunges downward some 13 further dB below the 1 dB minimum loss. This was the most mysterious feature of the graph for me. It appears to be due to some kind of additional gain in the preamp which was needed to drive the A/D converter that is to follow. This might occur if a consumer preamp was used to drive a professional A/D converter, for example.
In other words it seems to represent a completely different kind of thing all of a sudden--instead of "this is a minimum loss which will always occur in a setup with an external analog preamp and A/D converter," it is "this is a possible loss which might occur in a less-than-optimal setup." It certainly could happen, but it might not--and some other loss of signal-to-noise ratio (either lesser or greater) is also possible, one assumes.
Now to me that seems true enough; mismatched components and needless losses are factors to reckon with in the real world. But if that's really what the "plunge" is about, we shouldn't mistake it for a necessary, inherent loss that will always be part of the process, such as the 1 dB loss indicated earlier.
Similarly there is a 1 dB downward jog from the preamp output to the A/D input (which I circled and labeled "2"), symbolizing the minimum loss of s/n which will inevitably occur, because a dB now and then is simply the price you and I must pay for living in the real world. But then again, beneath it there's another plunge downward--this time of some 9 dB for no particular reason, I think, except to illustrate what could happen with equipment that was sub-par as to its performance or its compatibility with the other equipment being used. There certainly is no logical relationship between the marking "ADC with headroom 14 dB" and a 9 dB drop in level; that's simply got to be a mistake of some kind, or else an estimate of "what could go wrong."
If it is a mistake, then most likely I think that the 13 dB (below 1 dB) drop shown in the preamp is due to the desire to leave 14 dB of headroom in the A/D converter--which would simply mean that 14 dB of its range is being thrown away completely. That isn't the right way to record anything, but some people do it, I guess.
But if we make an optimal live recording, and the sound pressure levels happen not to be enough to force a microphone to put out its highest possible signal levels, we haven't lost any signal-to-noise advantage that we could otherwise have had. Numerically, abstractly, sure--we weren't using the full 130 dB dynamic range of the microphone. But that's nothing to cry about. If the dynamic range in the room was 100 dB and our recording system functioned with, say, 110 dB of dynamic range, we've captured the recording just as well (audibly) as we could possibly have done with a system having 120 or 130 or even 200 dB of dynamic range.
What matters is whether we can fit the dynamic range of the microphone's signals into the rest of our recording apparatus, whatever it may be. Got the loudest part without distortion? Good. Got the softest part without any audible added noise? Good. Get them both, and we've got all there is. I don't see why I am supposed to feel unhappy because the microphones were capable of 130 dB range but my recording only preserved 110 (if in that performance in that room, only 110 dB existed to preserve).
There is a similar tizzy which some anti-digital people get into, which is the notion that in a 16-bit system, for example, any signals that are below (say) -48 dB are represented by only 8 bits, and signals 12 dB lower than that are represented by only 6 bits, and so on. Eventually you get to a level at which only one bit is being toggled on and off, which they imagine will produce square waves; it's all a sort of panic induced by the way they imagine things to work, as opposed to how they really do work.
So again I refer you to the specifications of these Neumann digital microphones, which compared to their analog counterparts resemble the minimum losses shown in Mr. Peus' diagram (ca. 1 dB at the preamp and another in the A/D converter). And again I say that Neumann did a great job of keeping the loss of signal-to-noise down to such small amounts. But your initial statement that there is an improvement of 30 dB in signal-to-noise ratio (or indeed any improvement at all) is simply unfounded, I'm sorry to say. A "digital microphone" can, at best, preserve but can never exceed the dynamic range of its analog components.
If you do contact Mr. Peus please give him my very best regards; I don't know him at all well but we are on an AES standards committee together, and he seems to be a very nice and very honorable human being.
--best regards to you, too!
Topic: KM 184 D- Story? (Read 8154 times)