The NMRA DCC provides a method of establishing a relationship between a given speed step value and the desired locomotive scale speed (MPH or KPH). It can be used to implement speed matching between locomotive regardless of their gear ratios or wheel diameters. If one establishes a "speed standard" for all locomotives, it will allow multiple locomotives of different brands, models or type to work well together in a consist.
However, many decoders manufactures for various reasons, good/bad, have implemented variations of the "speed tables" that does not conform to NMRA Standards. This has lead to differences in how to properly setup the speed tables. Failure to understand these difference will result in the locomotives not working correctly. Simply put incompatibilities.
1) NMRA SPEED TABLE TYPES.
The relationship between speed step value and physical speed a "speed curve". There are two ways to create a speed curves. However in the venacular, speed curves are being called Speed Table even if there is no table of values used.
1) Basic Table: 3 CV's are used to define a 3 point speed table. Using this data, the decoder mathematically figures out the speed curve for all the speed step values.
2) Speed Table: 28 CV's that defines a 1:1 relation between a 28 Speed step value and a speed. This is commonly called the "Speed Table. If one is operating in 14 Speed step or 128 Speed step modes, the decoder will mathematically interpolate, down or up respectively, to get the speed curve for the given mode.
1) NMRA CV DEFINTIONS.