Gain staging, unity gain and digital clipping

[amsdenj]

Jason Sadites has a great set of videos explaining the details, myths and facts of gain staging, unity gain and digital clipping: 

 

Not only are these videos extremely informative, but they also demonstrate very useful analytics approaches.

 

Jason's main points are:

1. Getting the most out of Helix is gain staging so that the level and dynamic range of your guitar and playing style fits optimally between the noise floor and digital clipping (primarily caused by the analog to digital converters at the Helix input and output).
2. For linear blocks, levels into the blocks don't matter as long as the result at the output block is what you're looking for
3. For non-linear blocks that would typically follow an amp block (e.g., some delays, compressors, etc.) you can use headroom and threshold to control the amount of desired distortion (if any) that these blocks add.
4. If you don't go crazy on winding up all the levels to the max on every block, you're not likely to need to worry about digital clipping in Helix blocks, except for those where digital clipping is part of the design of the block (e.g., Vintage Digital delay).
5. The optional output level is one that turns the Powercab signal light yellow, but rarely red (which would indicate the possibility of digital clipping in Powercab). This corresponds to approximately -3dB peak headroom and -15dB RMS headroom. You can use these numbers and your DAW meters to set patch levels so they match and are providing the optimum output for Powercab or other FRFR options

That is, for best results, don't be afraid of turning up the level, say in either the amp block channel volume, or the output level to get your patches in this sweet spot. Don't worry about setting all blocks unity gain, and don't worry too much about digital clipping inside blocks.

 

This is great advice. However, I did a brief experiment today that might indicate unity gain may still be a useful starting point. What I found is that a Les Paul, or probably guitars with similar passive double coil humbucker pickups, on the bridge pickup strummed pretty hard into an empty patch pushes the Helix output to just under 0dB peak, about -17dB RMS, -15db LUFS, and is just enough output to flash the Powercab input signal lights into yellow. That means for this guitar at least, there is no unused headroom that you might be leaving on the table, and all the blocks in the signal chain could be unity gain in order to produce the optimal output. Any guitar with hotter pickups would need to use the input pad switch to avoid digital clipping on the Helix guitar input.

 

My Strat with single coil pickups had an output of about -9dB peak, -28db RMS and about the same for LUFS Integrated. That would indicate there is about say 9dB of headroom that you might want to make up somewhere in the signal chain in order to achieve optimal output. The most likely place might be the amp block channel volume and/or the output block level. But as Jason points out, it probably doesn't matter that much where you add the additional level. Keeping most blocks unity gain however would allow you to save them as favorites, or set your own block user defaults, and be able to use the blocks in other patches without worrying about potential unwanted level changes. 

 

So Jason is absolutely right in his analysis of gain staging for optimal signal to noise ratio, not needing unity gain (at least not on all blocks) that leaves headroom on the table, and not worrying too much about digital clipping if you keep the Helix output at a healthy level. However, how much headroom you have depends on the guitar you're using and how hard you typically play. Maybe unity gain in blocks is still a good place to start, especially for higher output guitars. But we shouldn't be afraid of turning up the amp block to get Helix and Powercab into their sweet spot.

 

Thanks Jason for the great videos!

 

 

[codamedia]

Great video.... thanks for sharing. 

Also, as a Canadian I must say.... "love the shirt"!