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Fader resolution issues in control surfaces

“I’m looking into different control surfaces for my software DAW, and in some cases I’m seeing wildly different specs for fader resolution. Some manufacturers even contradict their own specs from place to place. Can you give me any advice for making heads or tails of the different fader resolution specs?”

Fader resolution is the number of different values produced by the fader as it moves from the bottom of its travel to the top of its travel. Because high resolution equals a more exact mirror of your intended fader movement (in theory), the rule of thumb here is essentially “more is better.” However, there are a few things to be aware of when looking at fader resolution specs.

A system that has actual 6-bit resolution delivers a total of 64 different steps, while a system that delivers 8-bit resolution delivers a total of 512 different steps. In other words, a few bits of resolution make a significant difference in the overall accuracy of the fader movement. The majority of on screen mouse controlled faders tend to have 127 to 256 different positions (7-bit and 8-bit).

One commonly overlooked aspect of fader resolution is the distinction between actual fader resolution and the transmitted data resolution over MIDI. A particular device may claim to have 10, 12, or even 16 bit fader resolution, but often times this spec isn’t referring to the actual A/D converter that reads the fader position (yes, faders have A/D converters on them. How else would you convert analog values [voltages] to digital data?), but instead is referring to data sent over the MIDI (or other data format) cable. The actual fader resolution is often much less. Manufacturers can get away with this for a variety of technical reasons, one of which is that it is possible to compute or interpolate intermediate values. So while a fader’s A/D converter may only generate 6 bits of data (64 steps), the computer in the device can interpolate between those values and effectively generate smooth fades that actually transmit many more than 64 steps. However, once the fader stops moving there are still a limited number of volumes it could come to rest on. This could be more than 64 possible values (for reasons that are too in depth to get in to here), but may not be – it depends on the implementation in the system. Clearly more bits will translate to more positions at which a fader can come to rest, and more accuracy in reproducing those positions in moving fader systems. Also, keep in mind that an A/D converter of a given rated bit depth (i.e. 16 bits) can never really produce valid data for all those bits. It’s always something less than what is rated. A GOOD 16 bit converter will likely only produce 13 or 14 bits of valid data.

Remember that 10-bit resolution has a maximum of 1024 different steps available and theoretically (if it truly is a 10 bit system throughout) this would correspond to 1024 different fader positions. If you look at that in the context of a 100mm travel for a fader it is 1 different value for every .097mm (.0038 of an inch). Precision this fine is hard to pull off mechanically (even many machine tools used in manufacturing have a hard time achieving repeatable tolerances finer than .005″ – and they use geared motors with screws and braking systems), not to mention the value of such resolution from a human interface perspective. Further, systems that produce more MIDI automation data tend to also have more latency; so generating more data beyond a certain point can actually become detrimental to the overall performance of the system. So when we say that more is generally better, there is a point where more can be too much. While it’s still open to argument, 8-bit resolution (512 steps), if it truly is 8-bit throughout, is probably more than enough for nearly any project.

It’s sometimes hard to know whether a manufacturer’s spec is based on the “rated” bit depth of the converter, the actual accuracy of its resolution in practice, or the data as it is output through MIDI (after possible interpolation). All specs are ever going to give you is a general idea of a system’s capabilities – it’s best to talk to a competent sales person before making final decisions. Your Sweetwater Sales Engineer can help give you a real world idea how different surfaces perform in the context of your circumstances and needs.

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