understanding speed step CVs 67-94 28/128 speed steps

I believe you are confusing two different scenarios. CVS 67 and 68 are a part of CV29 and have nothing to do with the speed steps 28 or 128. To understand CVs67 through 94 study CV29.

Understanding the motor controlling CVs is too involved in a short thread in my opinion and should be explored in articles and you tube videos. Speed steps are simply dividing your overall max speed by 14/28/128 steps. You can also change the increments of each speed step.

I believe you are confusing two different scenarios. CVS 67 and 68 are a part of CV29 and have nothing to do with the speed steps 28 or 128. To understand CVs67 through 94 study CV29.

I might be confused, but I thought (assuming CV29 bit1 is on) if I crank CV67 to a very high number then my train will go faster when I'm at speed step 1.  Am I wrong about that?  If that is the case, its really clear how these 28 speed matching CVs (67-94) correspond when decoders were all 28 step, but a little murkier now that all decoders are 128 step (particularly since 126 does not divide evenly into 128).  I imagine they interpolate, but I'm hoping to understand that interpolation better.   I know "Google it and research the topic" is the best way to learn (which I will), but I was hoping someone would have some insight.

I found an interesting discussion here:  https://cs.trains.com/mrr/f/744/p/296708/3469241.aspx

CV2 is for motor starting voltage. CV3 is for acceleration delay,CV4 is for de acceleration speed. CV5 and CV6 are your speed curves. Each manufacturer of the decoders will enable different CVs. I use air wire and they have a motor bump and a motor bump duration. Being new i suggest you don’t use the speed curve from CV29 and use CVs 2,3,4,5, 6 plus any motor control CVs. That’s what I do and find that’s enough motor control

Although I don’t use CV67-94 my understanding that it is a speed curve and not speed steps. Some decoders also have 14 speed steps

To follow up what Bob has said, CV5 is top speed and CV6 is midrange - so how fast the engine goes at full throttle and how fast it goes at 50% throttle. The decoder sets a straight line between the amount in CV2 (start power) and CV6, and then a straight line between CV6 and CV5. (Many early decoders only had CV2 and 5; that's why they had to use CV6 for midrange.)

So if you have CV2 set to zero, CV 6 to 90, and CV 5 to 180, it would essentially be a straight line, so each increase in speed step would increase the speed the same amount, regardless of whether you went from step 3 to step 4, or step 23 to step 24. In effect, the decoder/programmer fills in the in-between speed steps automagically.

IF you choose to, you can turn that off and use a speed curve using CVs 67-94. Decoders and DCC systems can be set to use 14, 28 or 128 speed steps, but decoder's speed curves can only be set using those 28 CVs.

Although you can use them to create a straight speed curve (which seems like an oxymoron but is correct) but most people opting to use them want to set up a curve - so like for example, the first 14 steps only go from 0 to 30% and the last 14 go from 30% to 100%.

I set my engines to all go a scale 64 MPH at speed step 128, in a straight line 'curve'. So each speed step increase equals 1/2 scale MPH increase in speed. If I re-set my throttle (CVP) to use 28 speed steps, each increase in speed step is about 2.25 scale MPH. If I set it to 14 steps, each step increase would increase speed about 4.50 scale MPH. So 50% power is still 32 scale MPH, but using 128 speed steps allows me greater control to slowly increase or decrease speed.