DIGITAL MIXING CLARIFICATION
Dec 1, 2003 12:00 PM, COMPILED BY SARAH BENZULY
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This Month in Mix
Mix wishes to clarify the following statements found in the Dangerous Music review (October 2003, p. 118).
We stated: “Digital mixing and summing inside your DAW to stereo outputs requires that you do not overload the internal digital mixing bus by lowering all the mixer faders.” According to Digidesign's documentation, the Pro Tools mix bus has 48 dB of internal headroom available above 0 dBFS. It is possible for Pro Tools to sum 128 tracks of coherent, full-level signals, with all faders set to maximum gain (+6 dB) and the internal mix bus won't overload. In this case, to get a clean output, the user would simply install and set a master fader to just below -48 dB, and the result is a clean, full 24-bit output.
Next, we stated: “When any track fader's level is internally reduced, its digital resolution is also reduced.” Digidesign states that the mixer inside Pro Tools is 48 bits wide, with 24 bits (144dB dynamic range) reserved for the normal range of signals. Another 8 bits (48 dB) of headroom is reserved for signal levels above 0 dBFS, and a final 16 bits (96 dB of dynamic range) reserved for the lower-level signals that result from attenuated channels. This makes the total dynamic range 288 dB. It is possible for a single channel fader to be pulled down to somewhere near -90 dB and still be contributing a full 24-bit signal to the mix bus.
Finally, we stated: “Spreading out a mix over many stems and direct outputs lets you maintain hotter digital levels for higher resolution, resulting in a better-sounding mix with increased depth, image width and headroom, and less distortion.” In reality, the 2-Bus' distortion spec is listed as 0.005% with the noise floor at -80 dBu (no weighting specified). The 192's DAC distortion is 0.00056% and the noise output is -94 dBu (-118 dB “A” weighted down from +24 dBu), which means that passing the 192's output through the Dangerous 2-Bus increases the distortion nearly 10x while adding 14 dB to the noise floor.
Digidesign has addressed these and other issues of digital mixing in a white paper series. Visit www.digidesign.com.
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