A big thanks to Dave Heap for the following article:
Although the NMRA specification says EITHER Vmin, Vmid and Vmax OR Speed Table should be active, some manufacturers do not adhere to this strictly:
- With Soundtraxx Tsunami decoders, the value in Vmin will be internally added to EVERY value in the Speed Table.
- With QSI decoders, if either of Vmin or Vmax is non-zero, the WHOLE speed table will be internally rescaled (expanded or contracted) so the min and/or max speeds will match the non-zero Vmin and/or Vmax.
- With Loksound ESU V4 and select decoders, the WHOLE speed table will always be internally rescaled so the min and max speeds will match the Vmin and Vmax values. However, since expansion of a speed table (as in the QSI case) causes loss of precision, ESU forces you to shape the speed table between 1 and 255 so the speed table is never internally expanded, only contracted .
All three systems allow control over the full speed range, you just need to be aware of the interaction with Vmin and Vmax.
The procedure with ESU is to first set minimum and maximum speeds using Vmin and Vmax. Then use the speed table to either set a predefined shape (Linear, Log, etc.) or if speed matching, set each individual value to match speed at that step.
There is no problem speed matching ESU and other locos, provided you set minimum and maximum speeds FIRST, then set the other points. The procedure is no more difficult than with any other brand of decoder.
The important thing to remember is that with ESU, QSI or Tsunami decoders, the values above each slider are not absolute but relative to their neighbours (as they may be subject to scaling or offsetting by Vmin and/or Vmax).
The NMRA standard S9.2.2 specifies that either CVs 2, 5, 6 or CVs 66 to 95 are used, governed by Bit 4 of CV29.
Bit 4 = Speed Table: "0" = speed table set by configuration variables #2,#5, and #6, "1" = Speed Table set by configuration variables #66-#95
ESU has chosen instead to use the Märklin mfx® standard, where CVs 2 and 5 are active in both modes, with the values in CVs 67 to 94 scaled to fit between CVs 2 and 5.
The same degree of control can be achieved by either method. The advantage of the Märklin mfx® method is that you can subsequently tweak starting or top speed by adjusting CV2 or CV5 and the curve will be retained. With the NMRA method, you would have to redo the curve to change starting or top speed.
However, if you are speed matching step-by-step with an NMRA-standard decoder, you must match starting speed (by CV2 in one and CV67 in the other) and top speed (by CV5 in one and CV94 in the other) BEFORE you match the intervening steps.