Maximum input/output volume

#1

Hi there :smile:

I've had my Axo for a couple of months now, and won't hesitate to say that it's the greatest achievement in the world of synthesis since the G2! I do, however, have a nagging question that I couldn't find anything about on the forums.

Actually it's two separate, but related, questions:

  1. Is the maximum output volume -18dBFS? When I connect an oscillator module directly to the output module, I can't seem to get it any louder than this - if I gain the output of the oscillator, the waveform clips.

  2. Is the maximum input volume -8dBFS? If I make the input (eg. a sine wave from Ableton) any louder than this, the waveform clips - or rather, folds!

I would like to know if it's possible to get 0dBFS output - the signal-to-noise ratio would be better, and it would be more convenient to use the Axo with live PA.

I'd also like to know if I can somehow give it 0dBFS input - it would give me better resolution, and it would be more convenient not having to check whether the signal I send it goes over -8dBFS.

Peace :sunny:

Updated or Pro-grade version of Axoloti?
#2

The input signal can be configured with the audio/inconfig object. You can set gain and boost.

Output config is only available for the headphones.

#3

These objects do not seem to bear on the issue...

At 0db headphones volume, I get the same volume as I normally would from the standard output. At +6dB I get clipping.

Boosting and/or gaining the input results in clipping. At -12dB input gain, I can make the input signal approx. 0.5dBFS louder before it clips :smile:

#4

What your reference for dBFS? Axoloti's output connected to a soundcard input? In that case your -18dBFS depends on the input sensitivity of your soundcard. Axoloti's output is around 2Vpp for 0dBFS (where 0dBFS is axoloti digital computation full scale). Boosting the gain of the converter will only add clipping indeed, as the outputs are not able to deliver much more than 2Vpp.
2Vpp is typical for consumer audio devices, pro audio devices can be much hotter.

The idea in the object set is to promote attenuation rather than amplification. Attenuation is always safe to do, when amplifying one needs to be careful about clipping.

Again the input will clip at over around 2Vpp, reducing the input amplifier gain to -12dB will not prevent clipping hot signals, as they clip when entering the input amplifier already.
These limits come from using a single 3.3V supply for the converter, keeping the design simple and compact.
A passive attenuator (just some resistors) could be used, with a little soldering this can be added on the Axoloti Core board. I can make a sketch if you want.
The reason such an attenuator is not part of the design is that it would degrade the performance when boosting weak signals with the preamp inside the ADC.

I'll verify the folding you describe, I believe it should just clip.

#5

Ah, I see - I had no idea about this. I'm using a Motu Ultralite mk3, which according to this post seems to have much wider peak-to-peak. I guess I just assumed 0dBFS was pretty universal when it comes to 'line level' devices!

For sure - I just wanted to make sure I was using all the available headroom.

Thanks for the suggestion, but with my limited soldering skills this would not be worth it for me personally - I'd rather fire up my preamp then :smile:

Thanks again for making things clear. It's not really a problem, since my primary motivation for this post was a pretty rare use case. I was using the Axo as a 'live mastering chain' at a live gig, so we had our main output directly from the Axo. When we connected it to the club's DJ mixer, our signal level couldn't get near that of the CDJ's and turntables playing, so we had to ask the soundman for a dedicated channel on the main mixer, with extra grain. Next time, I'll simply bring some extra gain myself to be safe! And for use in the studio, I'll just use preamps.