Logarithmic vs Linear Setting for Expression Pedal

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soundog
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Joined: Fri Apr 15, 2011 5:21 pm

Logarithmic vs Linear Setting for Expression Pedal

Postby soundog » Fri Jul 15, 2011 8:22 am

Using an expression pedal as a volume pedal is usually smoother and better-sounding if it uses a logarithmic response (human perception of audio volume is logarithmic; we are more sensitive to smaller changes in louder sounds), rather than a linear response. That's why audio volume controls or guitar volume pedals usually use a log pot. With a log response, the control changes less dramatically at the end of its rotation.

For MIDI control other than volume, a linear response may be more suitable (such as changing the rate of a chorus or delay).

For the SoftStep, you may want to experiment with the table settings in your presets being used with an expression pedal. Try it with the setting for "linear", "logarithmic" and "exponential". Logarithmic will normally yield the best response curve for volume, but you may want a linear setting in a separate preset if you are using your pedal for chorus or delay rates. The response will also be effected by whether your pedal contains a log or linear pot, so there are a number of variables. Let your ears be your guide for your final choices.

For the heck of it, I captured the SoftStep's MIDI out using my pedal (modified Ernie Ball VP Jr), and charted the result to compare the response curves for logarithmic (the curve on the left) and linear (the curve on the right) table settings. The X-axis is time; the Y-axis is MIDI value. While I couldn't accurately duplicate the speed at which I pressed the pedal down/up between recordings, the resulting shapes clearly show a log curve on the left (which is what I wanted for volume control), and a linear curve on the right.
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Macciza
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Location: Sydney, Australia

Re: Logarithmic vs Linear Setting for Expression Pedal

Postby Macciza » Sat Jul 16, 2011 7:36 am

Thanks Soundog

I moved this post to 'Tips and Tricks' as it is good info and so it is easier for people to find.
Hopefully we will get editable tables so we can design or own functions too

Cheers
MM
KMI StringPort_1650, SoftStep_VK2 v1.2 & MIDI Expander; Mac_Moore Guitar,
Roland VG 8, 88 & 99, YamahaVL 70m; OSX 10.6.8, iMac i7 2.8G QC; MaxMSP 5.1.8, Ableton Live 8.2.2, M4L, SC3, QC, cSound, etc, etc,
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lphovercraft
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Joined: Thu Dec 29, 2011 1:52 pm

Re: Logarithmic vs Linear Setting for Expression Pedal

Postby lphovercraft » Thu May 24, 2012 9:13 am

Thanks for this - it was actually a huge help for me - but not for the reason that you explained.

:)

To a first approximation I disagree that the pedal response should differ between lin and log output. From my perspective, it should always be linear travel-to-MIDI. The bottom of the pedal should of course always be zero, and the top should always be 127. I'm sure at least on those two points we would agree.

:D

But my contention is that the pedal should always send a value of 64 in the middle of its range. Otherwise you risk (and very often do) lose resolution in values at least half of the travel of the pedal. It's similar to the old analog days (which is fresh in my mind after posthumously feting my first boss Bob Moog on his birthday yesterday) where the 1 Volt-per-octave 'standard' was introduced to make all control signals linear (and in essence putting the onus on the receiving input to know whether it should scale the corresponding control to lin or log). In the MIDI world it goes to the number of times per second that an input is sampled in order to get a value. Too little resolution and you get jumps because the sampling keeps "seeing" a voltage that doesn't represent enough change to 'think' that a new value should be sent. When it finally crosses that non-linear threshold it sends a value and BAM your new MIDI value "yanks" the control input in a way that is not smooth and not musical. Most people refer to it as 'jumping' or 'chunking' and if enough of those connect over a range it can sound like zipper noise. For certain types of modifiers the change might be imperceptible (and smarter control inputs have smoothing algorithms that can mask this at the cost of introducing some latency).

But I digress...

Where you *really* helped me was in rescaling the pedal output in order to give a more linear MIDI output scale. I used the "5 Logarithmic" table in the Key Modulation lane to compensate for the non-linear behavior of the Yamaha Expression pedal. It is an absolute BEAR getting that thing to calibrate against the SoftStep input, and it ALWAYS has an exponential behavior. This does a pretty good job of what I'm looking for - sending 64 in the middle of the pedal range. This means that if I set this to CTRL7 (Main Volume) that I can trust the heel of the pedal range to give me silence - the toe of the pedal range to be full volume - and the middle to be in the Goldilocks zone.

:)

I hope I didn't confuse or conflate things for folks - just me expounding a bit as I figure out the nooks and crannies of making this setup robust and repeatable.

Thanks again for the help!
Houston Haynes - LPHovercraft Music
Desktop - Intel i7 920 12GB RAM - RME HDSP 9652
Notebook - Intel i5 8GM RAM - ASIO4All/internal chipset
Arturia Origin, Spark - KMI SoftStep, QuNeo
Mobius Megatar, Roland VG-99/VB-99
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lphovercraft
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Re: Logarithmic vs Linear Setting for Expression Pedal

Postby lphovercraft » Fri May 25, 2012 5:44 pm

After a few exchanges with support (thanks, Carson!) I confirmed my suspicion that the Yamaha pedal is simply the wrong type (logarithmic pot, even though it's TRS). I went to the local music purveyor and bought a Roland EV-5, which works perfectly at the default scaling. Go figure. :)

Now that I've figured this out, I'm going to go get a pair of linear pots and repkace the ones inside the Yamaha units. these things are so well built (and the throw is nice and long) that I can't let them go to waste. :)
Houston Haynes - LPHovercraft Music
Desktop - Intel i7 920 12GB RAM - RME HDSP 9652
Notebook - Intel i5 8GM RAM - ASIO4All/internal chipset
Arturia Origin, Spark - KMI SoftStep, QuNeo
Mobius Megatar, Roland VG-99/VB-99
soundog
Posts: 54
Joined: Fri Apr 15, 2011 5:21 pm

Re: Logarithmic vs Linear Setting for Expression Pedal

Postby soundog » Tue May 29, 2012 9:08 am

Glad I could send you off in some direction or another.

Also, I did replace the pot in my pedal, used an off-the-shelf linear one. See this thread:

viewtopic.php?f=10&t=149&p=921&hilit=pedal+log#p921
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lphovercraft
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Re: Logarithmic vs Linear Setting for Expression Pedal

Postby lphovercraft » Wed May 30, 2012 1:17 pm

Thanks - I got the word from support on that thread. I really like the 'throw' of the Yamaha pedal more than then Roland. I'm going to try a few different linear pots (100K, 250K) and try to find one that has the right range for the amount of 'wipe' that it has in the design (about half of the total 'throw' of the pot).
Houston Haynes - LPHovercraft Music
Desktop - Intel i7 920 12GB RAM - RME HDSP 9652
Notebook - Intel i5 8GM RAM - ASIO4All/internal chipset
Arturia Origin, Spark - KMI SoftStep, QuNeo
Mobius Megatar, Roland VG-99/VB-99

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