Albert wrote on Thu, 23 December 2004 15:38 |
This thread is sort of an offshoot of the thread about digital attenuation. After reading about the issues regarding simple attentuation of a signal in the digital domain, I'm curious about what must be an even more involved process, that of mixing in the digital domain.
With mixing, not only do you have attenuation and gain, but you have things like fx plugins, sometimes sending and returning to external analog or digital processing boxes, and finally, very importantly, digital summing of all signals that have been through the previous processes.
The math involved in doing attenuation properly is staggering, based on what I read in the other thread. So what is really happening to a group of digital tracks in a DAW that must go through attentuation and gain, plugins, and summing? Is there anything left of the original track, meaning the original ones and zero's? I'm particularily interested in how the numbers work when summing digital tracks.
I apologize if this topic has already been covered here. Thank you in advance.
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This thread is sort of an offshoot of the thread about digital attenuation. After reading about the issues regarding simple attentuation of a signal in the digital domain, I'm curious about what must be an even more involved process, that of mixing in the digital domain.
With mixing, not only do you have attenuation and gain, but you have things like fx plugins, sometimes sending and returning to external analog or digital processing boxes, and finally, very importantly, digital summing of all signals that have been through the previous processes.
The math involved in doing attenuation properly is staggering, based on what I read in the other thread. So what is really happening to a group of digital tracks in a DAW that must go through attentuation and gain, plugins, and summing? Is there anything left of the original track, meaning the original ones and zero's? I'm particularily interested in how the numbers work when summing digital tracks.
I apologize if this topic has already been covered here. Thank you in advance.
This is a big question. I’ll try to make it reasonably short:
The math for gain and attenuation is not all that difficult. If you know where the noise floor is for a given track, than if you increase the gain by Xdb, the noise will rise by X dB. For attenuation it is a bit more difficult but not too bad…
In all cases, from a purist standpoint, the best you can do is to record each track with the music peaks as close to full scale as possible, but never to the point of clipping. That way, you utilize digital audio in the best possible way. That way you do not need to amplify signals (and with it amplify their noise floor). If you need to attenuate, you will end up about where you would be recording further down from the full scale, no real loss. A weak signal in digital audio is simply closer to the noise floor. Often you such signals have somewhat “masked” by a stronger signal within the mix.
Mixing digital is not so bad. Say you add 2 identical 1KHz tones (IN PHASE), each at say -1dBFS (dB from full scale) Say each has a -90dBFS noise. The added outcome is doubling of the signal. Double means +6dB so you now have +5dB signal. What happened to the noise? Unlike the signals – an addition of 2 identical waves, the noise of each channel is different. Say it is random, than the total noise will go up by only 3dB, to -87dBFS. Of course a +5dBFS is over full scale. Say we attenuate it by 6dB. The signal is now again at -1dBFS, the noise went down to -93dBFS. This is an improvement.
This is not a typical case. How often do you add 2 identical signals? It is safer and more realistic to assume that 2 signals (at the same overall levels) are very un related to each (low correlation) and than the addition will not double, but increase the overall level by some amount, such as 3dB, just like the noise. So an addition and rescaling will not alter the signal to noise ratio. It is better than “attenuation only” because you not just attenuating, you are also adding a signal.
It does get a bit more complicated when the signals are to be mixed at unequal relative strength, but not impossible to comprehend. The best way to be safe is to have a reasonably low noise on the original track and a wide enough mix bus (bits).
Analog attenuation is cleaner than digital one. Analog gain is cleaner than a digital one. But analog mix bus (adding of signals) also has its own peculiar set of problems…
Regards
Dan Lavry