There are two reasons for this – one due to the repeated use of EQUISAT and the other to do with how the TRANSYT signal optimiser works.
- Looking first at EQUISAT.Some TRANSYT 13 users may have noticed that when a file is imported from TRANSYT 12 that has EQUISAT switched on, they are informed that EQUISAT has been switched off. The reason for this is that in TRANSYT 12 the selection of EQUISAT did not always result in EQUISAT being run, while in TRANSYT 13 it is always run if requested. It is assumed that a file being imported will have already been run in TRANSYT 12, so will contain either EQUISAT, optimised, or fixed timings already. This means that in TRANSYT 12 if the file was re-run EQUISAT would not run – to match this behaviour in TRANSYT 13 the program switches off EQUISAT.
If optimisation is selected, the resultant optimised timings will become the starting point for the next run. Although EQUISAT now always runs when selected and it will always overwrite the existing timings, EQUISAT is still influenced by which stage comes first in the cycle which might have changed during optimisation. This means that even when EQUISAT is ON and overwriting the timings these timings may not be exactly the same as those produced by the previous EQUISAT calculation. This different starting point can occasionally produce different optimised results. Note however that all the results are ‘good results’ and the differences are likely to be small. The effect usually reveals itself as a set of two or sometimes even three slightly different results which loop round as TRANSYT is repeatedly run. If this occurs you can simply select your preferred result when it appears and then switch off EQUISAT. Alternatively, you could choose to only ever run EQUISAT once in order to get a set of timings.
- The second reason that repeatedly pressing the run button can result in better results is nothing new to TRANSYT, and is simply due to the optimiser being given a second chance to determine improved timings.With EQUISAT switch off, the optimised timings from one run automatically become the starting set of timings for the next run. If they are different from the previous timings then again the optimised results from this run can again be better. N.B. This effect will never produce worse timings than the previous run. This procedure can be automated by selecting the new “Enhanced Optimisation”option in the Signals Options screen.
TRANSYT 15 and TRANSYT 14:
Pressing Run twice in TRANSYT can produce better results as described above for reason 2, but for reason 1 the effect won’t happen as the EQUISAT routine has been largely replaced by a new “Re-distribute” option.
Furthermore, regarding what is said above regarding different starting points, a new optimiser called “shotgun hill-climb” now exists in TRANSYT. This allows multiple runs of TRANSYT to be carried out, each with a different starting point (in terms of timings). Since the starting point can have an influence on the final timings, this allows TRANSYT to produce better timings for those who are happy to let TRANSYT take longer to produce a set of results. This feature is described in full in the TRANSYT User Guide.