That said, summing units are essentially built around electronic transformers whereas digital summing which works much differently. This is why you can red the shit out of an individual track post-fader in pro tools and not necessarily clip your master...the amount of headroom "inside" the pro tools mix bus is so astronomically large you essentially can't clip PT in the box, only on the master fader/output stage where it must conform to -18dBfs being analog zero in order to be handled properly by D/A converters and not go rambo on your monitors. Sorry for being so nerdy, but an old professor of mine was one of a handful of Pro Tools experts acknowledged by AVID and he never once hesitated to overshare on all of the things he had to stay current on.
What transformers are in the Dangerous 2-Bus and 2-Bus LT?
Your old professor recommends you red the shit out of an individual track [sic]?
The master fader must conform to -18dBFS being analog zero? Did you CAL your IO? What about audio post in the US? What about Mastering facilities?
I think you will find that if you don't red the shit out of your tracks, your ITB summing is pretty darn good.
AFAIK analog summing circuits are dependant on transformers to some degree.
Let me be a little clearer on the second part. Of course my professor did not reccommend we red the shit out of individual tracks. He merely pointed out from a technical standpoint that the Pro Tools "virtual" mix bus essentially cannot clip internally, only at the master fader level (possibly only PT10 or later). To elaborate further, it was taught to me by more than one source that -18 dBFS is a digital equivalent of "zero" when thinking about digial audio in the context of analog signal. That is why for most plug-ins, processors, and internal summing, operate ideally at that signal level. This is also why there are two numerical gauges on either side of the meter of a given channel in the PT mix window. The one on the right side, you'll notice, is "0" at the top of the meter and moves down from there. That is dBFS, which as I understand is a term to quantify the fact that while analog equipment can have varying degrees of headroom, digital audio cannot pass above that zero point; it can only flatline and square out from there. Due to this principle, a digital audio signal that passes above -18 dBFS can be considered from a logical standpoint to be passing the point at which in the analog realm we would consider 0 dB (the area above which we consider "headroom.") Since plug-ins are nothing but hardware emulations of real circuitry (at least the good ones), the software side of the plug-in needs a way to conform it's analog principles to the digital audio realm. If it used 0 dBFS as it's zero point, it wouldn't function properly due to the lack of headroom from the digital audio, so instead that zero point is typically acknowledged to be -18 dBFS though it is sometimes considered to be -20 or -16. This allows the RMS of a signal to maintain it's position in the proverbial sweet spot while still having room for peaks to operate in that do not introduce distortion.
To bring it all full circle and calrify some of my admittedly bad wording, the virtual mix bus inside of Pro Tools can actually compensate for the limitations introduced by the concept of "0 dBFS" being a brick wall as long as it is a track being fed to a master fader. Since a master fader is, from a software standpoint, assumed to be the point before digital audio is converted back to analog, if it passes that point of 0 dBFS it will distort and clip due to the AD/DA's inability to account for any digital audio higher than 0 dBFS. In the case of the D-Box, the output of my converters/interface/whatever should feed it a signal where -18 dBFS ITB would be hitting the D-Box at it's approximated "analog zero" assuming the line levels are matching. It's a standard that allows interplay between the two realms. Imagine a world where everything that came out of your digital interface would hit your analog equipment where it's loudest point was 0 dB peak level in the analog realm. None of us would be hitting our equipment at favorable voltages without further amplification.
I'm sure that is why in audio post, as you rightly mentioned, has reference levels that typically correlate to -20 or -18 dBFS. I do believe the FDA standard is 83 dB at -18 dBFS. It is setting a value for the zero point as much as it's providing a standard for calibration. It's why it can be much easier to make things "louder" in the analog realm than the digital realm. By virtue of this principle digital audio is limited to +18 dB of headroom, whereas much mastering equipment can far surpass that due to the more organic nature of electricity. It also backs up your point that ITB summing operates best when your master fader is spitting out a signal that averages out to -18 dBFS. As I'm sure you know, dBFS as a unit is not a unit of volume but a unit of reference and only has a value based on some degree of correspondence.
Note that I'm only explaining things as I've come to understand them, and am more than welcoming to any questions, debates or disgareements. It's how I/we learn best. :)
Perhaps you may want to refresh and clarify what your old professor taught you.
AFAIK analog summing circuits are dependant on transformers to some degree.
You may want to change that to ISTR.....
Summing boxes are not dependent on transformers at all; but they are on resistors. The answer to what transformers are in the Dangerous 2-Bus and LT is: the null set. The 2-Bus Plus, however, does use transformers on the output.
I am not sure where to begin about your muddled commentary on levels, although, I do like the idea that the Federal Drug Administration has a recommended Sound Pressure Level dosage for human consumption! I suspect you are beginning to have a feel for what is going on vis a vis analogue and digital, but the grasp is not there yet. You know some of the numbers, but you don't quite know what they mean.
A relationship has to be established between digital and analogue by calibration of the Digital to Analogue Convertor. Most manufacturers setup the DAC for you, and most set it for music calibration. Not all DAC's have accesible trim pots for calibration, but many of the better ones do. First, you have to decide what your claibration levels will be which will be dependent on the kind of work you do.
For music, it can be -16, or -18 or -14. Mastering is anywhere from -14 to -8 depending on how the Engineer wants to work. For post, in the US it's -20 in Europe it is largely -18. To do this, you need a 1 kHz Sine Wave Generator (either the plugin or a hardware box) and a multimeter.
Make sure you use mono channels, not stereo ones. Set your -1kHz sine to, say, -18 then pull out your multimeter to measure at the output. Turn the trim pots until the meter reads 1.228VRMS. Do this for each channel on the DAC. Now -18dBFS = 1.228VRMS = +4 dBU = 0 VU. I suspect that your "analog zero" is intended to mean 0 VU. Every number in engineering has to relate to something. Analog Zero could mean 0 dBU, but I suspect you had a vague grasp of nominal VU levels. Now you have an established relationship between the analogue and digital realms.
I don't have time to get into the "FDA" 83 dB standard. .... I suspect you heard about this SPL number somewhere, or read it in Bob Katz's book, and just sort of threw it in....
but.... Theatrical Film calibration in the US is -20 dBFS 1k SINE = 1.228VRMS = +4 dBU = 0 VU. -20dBFS RMS PINK = 0 VU = 85 SPL across the front screen channels, 82 SPL for EACH of the surrounds and, well, the SUB is +10 in-band above the screen channels when measured by an RTA (which will usually be 91-92 SPL on an SLM).
I also don't have time to get into digital summing and 48 bit fixed point, or 48 floating or 32 bit floating and how at the summing bus it will ultimately be 24 (or 16) bit fixed to pass through the (now calibrated) DAC analogue outs, but, if you read and study some good literature on how digital works, some more lights may come on.
I should add, it was Paul Frindle in the epic PSW thread that taught many of us about gain staging and real head-room in digital plugins and summing and how important it is to keep the levels down on your digital channels. After I started doing that, I sold my Dangerous 2-Bus.