The SCV feature operates by measuring vehicle speed and raising and lowering the volume at programmed speeds.   The DCI has 14 “levels” of volume boost so you’re sure to find one that suits your preference.  Actually, you get to choose 2 levels and switch between them any time – just hold the MODE button and tap CH to toggle between the two SCV levels.  Most owners use the second SCV level to assign a more aggressive volume setting for Top Down driving.  The DCI will remember the setting you last used until you change it.  If you want to completely disable the SCV function, don't connect the vehicle speed sensor wire to the DCI or choose Level 0 as one of your two SCV settings and disable it via the MODE + CH buttons.

For all SCV levels the first Volume Up command occurs at 15 mph.  Subsequent Volume Ups are at different speeds depending on what SCV level you choose.  Looking at the below chart you can see that higher SCV settings provide more frequent volume adjustments.  The spacing between volume adjustments is called the 'interval', which is measured in mph.  Higher SCV settings use smaller intervals, which provide more volume boost as you speed up. 

Note:  Even though the chart only shows three “Volume Up” commands the DCI will continue to raise the volume no matter how fast you go.  Default settings are shown with a gray background.  DCI-2 software revisions prior to .316 only have SCV levels 0 to 8.  This chart describes the SCV levels for the current version of the DCI.

The default programming is set for SCV levels 4 and 6 with level 4 being active when you first install the DCI.  If you want to change these settings see the programming instructions in the manual.  The 14 settings for the SCV function work pretty much the same - the only difference is how closely spaced the volume changes occur.  It is easier to understand if I describe one setting in detail, and I'll use the ‘6’ setting in my example.

When the DCI determines that vehicle speed has reached 15 mph, it sends one VOLUME UP command to the head unit.  At every succeeding interval of 15 mph (for the ‘6’ setting) it will send an additional VOLUME UP command, so that by the time you reach highway speeds (60 mph) the volume will have been increased 4 times.  (15, 30, 45, and 60 mph)

As you slow down, the volume will be reduced, and if you were to stop, the volume will return to its initial setting.  The difference is that when slowing down, the VOLUME DOWN commands occur at different speeds than when you speed up.  There is a good reason for this, as I'll explain.

Let's assume you are driving on the Interstate at 60 mph.  The volume will have been increased 4 times, at 15, 30, 45, and 60 mph.  As you cruise at 60, it would be annoying if every time your speed wavered between 59 and 60 mph the volume lowered and then increased again.  So the DCI was designed to wait till your speed slows significantly before dropping the volume.  It must see the speed drop by 1/2 of the speed 'interval' before it makes a volume adjustment.  In this example, the speed interval is 15 mph, so the volume will not decrease till you slow below 52.5 mph (half of 15 is 7.5).  The effect is that if you are cruising at 60 mph, you will need to either speed up to 75 or slow down to 52 before a volume change occurs, which eliminates annoying volume changes if your speed varies just a little.

Bear in mind the DCI checks vehicle speed about once a second, so if you accelerate quickly you may notice the volume adjustments lag a little behind.  Don't be concerned, it will catch up within a second or two and adjust the volume appropriately.