Ok, I just did some tests on my MKI and the results were not what I expected, and I think they illustrate some interesting things.
For a test signal, I used Reaper’s built in tone generator, fed through an oscilloscope plugin, and then out a pair of channels on a Black Lion Audio modded Digi002.
To calibrate, I fed the outputs back in to a pair of line inputs (operating at +4db) and made sure that the direct loop was as close to unity as I could get it. In the end, I settled on the test tone at -2dbfs (the hottest I could get it without clipping the inputs) and added .399db of makeup gain on the input to correct for a bit of signal loss (basically setting it by eye so the oscilloscope on the test tone channel pre-output visually matched the oscilloscope on the input).
After that, I patched the test tone output to inputs A and B on my Octatrack, and the main outputs back into Reaper. I started a new project, set it up to record A and B into record buffer 1 and simultaneously play back record buffer 1 through the main outs. I turned timestretch off, but otherwise started with the OT defaults.
With a -2dbfs signal on the inputs, the output was hard clipped at just above -12db:
In the OT audio editor, the recording was digitally clipping at 0dbfs:
Next, I turned the mixer level for in AB to -32 to see if that kept the signal from clipping (which would mean that the level controls in the mixer were actually adjusting the input gain in the analog domain, pre ADC). I didn’t expect to see any change in the clipping, and there wasn’t any, which means that boosting and cutting the input level in the mixer is happening in the digital domain, between the ADC and the record buffer. The signal level had been lowered digitally, but the clipping was identical:
Next, I returned the mixer settings to their defaults and reduced the level of the tone generator until the output from the OT was as close to clipping as it could get without actually clipping. I ended up with the test tone set to -8dbfs. With the OT’s gainstaging at default settings, the output was approximately 4db lower than the input:
The input LEDs were still pinned in the red, but the audio in the record buffer wasn’t clipped:
Next, I moved on to the AMP page, and this is where I got my most surprising result. With the test tone still at -8dbfs and the mixer level for he inputs at 0, increasing the level on the AMP page to +1 or +2 started to clip the signal right away. By the time it was up to +12, the output was clipped heavily, and was still peaking at just over -12db:
As far as I can deduce from this, the OT’s signal path is not floating point, which means it’s possible to clip it internally! I have no idea (and no clue how it would be possible to test) what the internal signal path is like, whether it’s all fixed point or whether individual processes are done in floating point and then dithered back to 24 bit, but I think it’s pretty safe to say that at least some parts of the internal signal path are fixed point and can be clipped. The takeaway here is that increasing the volume on the amp page (or, presumably, with effects) can digitally clip - it’s not like a modern DAW where everything is floating point and you have essentially unlimited internal headroom. This is a big deal in terms of how you approach gainstaging.
Next, to see if the track level control could also clip the signal path, I returned all of the track parameters to their defaults again, and then turned up the track level. What I ended up with was just shy of unity gain when the track level was all the way up to 127, which suggests that track level is cut-only. However, because the record buffer’s Gain attribute was set to the default of +12 for all tests, track level at 127 presumably corresponds to 12db below unity gain).
Default OT settings with test tone at -8dbfs and playback track level at 127:
Next, I went to the settings menu an adjusted the gate threshold to the highest setting that would open on both channels with the tone generator still at -8dbfs. The highest I could get it was -4.5, although input A wuld still pass with it set at -3.3 (so either my interface’s outputs aren’t matched as well as I’d like or the OT’s inputs aren’t). At any rate, the gate settings are pretty coarse but in this test, a gate threshold of -4.5 roughly corresponded to an input level of -8dbfs, which more or less corresponds to just below clipping the input. Obviously this is pretty far from lab conditions but since you’d never want to drive the inputs hot enough to reach 0dbfs anyway, it’s still accurate enough to be instructive, if only because it shows that the gate threshold doesn’t necessarily correspond to a decibel value (or if it does, it’s calibrated quite differently than the line outs on my interface).
Next, I adjusted the tone generator’s level and watched the input LEDs, to find out where the transition from orange to red happens. First, what I noticed is that there are 5 distinct colors: green, yellow, light orange, dark orange, and red - if you gradually increase the input level you can easily see where the LED flips over from one color to the next - it’s not a gradual transition. The other thing I noticed is that the threshold between one color and the next is pretty inconsistent. I did a bunch of slow level changing and what I found was that if you start in the red and decrease the level, the transition from red to orange is much lower than if you start low and increase the level until you go into the red. In my tests, if you start low and push the input level up at the source, it will hit the red somwhere between roughly -16db and -13db, but if you start on full and lower the input level it willl go from red to orange somewhere between roughly -18db and -22db! Sufce it to say, the LEDs are a rough guide at best, and in either direction the level where they were pinned in the red was at least 4-5db below clipping.
Just to see, I set the test tone to -20db and adjusted the OT’s gate as before, to the highest threshold that still passed both channels. What I got was a threshold of 15.7. From this and the previous gate experiment, I’m speculating that the gate numbers are approximately input level in +4. Since my interface is (nominally, at least) operating at a +4 line level, perhaps the OT’s inputs are calibrated to a nonstandard +0db line level, which would be a reasonable way for Elektron to assure that they worked acceptably with both +4 and -10 equipment.
Finally, I returned the OT to its default gainstaging once more, returned my test tone to -8dbfs (which according to my tests corresponded to just below 0dbfs in the record buffer) and increased the master level in the OT mixer until I achieved unity gain, which happened at +12.
Soo, that’s a whole lot of information that may or may not be useful, but my main takeaways are:
- A -8dbfs signal at +4 line level corresponds to 0dbfs (without clipping) at the OT’s ADC. Where the OT’s -12db pad falls in this equation I don’t know.
- A gate threshold of -4.5 corresponds to just about the hottest signal that you can safely record into the OT without clipping
- It is possible to digitally clip the OT’s internal signal path using the AMP level, and therefore AMP level should be used for cutting more than boosting and is the first thing to check if a signal is clipping but the inputs still have headroom to spare.
- From the OT default settings, increasing a track’s level to 127 should result in unity gain.
- From the default OT settings, increasing the master to +12 should also result in unity gain.
- Track level appears to be cut-only. I was unable to clip a signal that was hitting 0dbfs in the track recorder by increasing the track level.
- Don’t pay much attention to the LEDs and definitely don’t worry about keeping them in the green! Yellow to light orange is probably the sweet spot but even in the red there’s headroom to spare.
Another test I didn’t think of would be to set up something similar (a test tone on the OT inputs that was peaking at or just below gate threshold -4.5, recorded into a track buffer and played back from a flex machine), but put an analog pad of some kind (basically a resistor or pot, something that couldn’t distort) between the OT outputs and the audio interface inputs so that you could turn the OT mixer’s main level up all the way and see if it was possible to clip the OT’s DAC by running the mixer too hot. I assume it would clip but you never know.
Anyway, if anyone is up to try to replicate some of this on their setup I think it would be really interesting to see if the results were the same (and it would help eliminate the variables introduced by my interface maybe not being that accurately calibrated). Likewise, if anyone notices any glaring mistakes please point them out! Also, in some of the images the title on the return signal oscilloscope window might not match the test conditions I described for that image, but that’s because I forgot to rename it for a couple of them. Likewise the filenames don’t always match the test conditions exactly, but were close enough for me to keep track of what was what while I wrote this post.
Hope this helps! It definitely made me rethink my approach to OT gainstaging a lot.