BEMF can tell the decoder how fast the motor is turning, which
is related by the gears to how fast the wheels are turning.
In diesel sound decoders, the question is which of the 8
notches of the motor sound should be being generated. Simply,
the decoder will divide the speed ranges into 8 parts and assign
a sound to each one.
But a diesel running in, say notch 5, will sound different if it is
pulling hard or if it is running light. Sophisticated sound decod-
ers use user input and BEMF to decide where in the range of
notch 5 sounds it should be.
Some decoders allow the user to manually move through the
notches, for more realistic operation. All seem to come with an
automatic selection process enabled.
Now comes the conundrum of steam locomotives. The chuff
should be directly related to specific mechanical positions on
the loco, either wheel position on rod locos or motor rotation
on geared locos.
12
12: SoundTraxx 810038
chuff cam mounted on
the inside of an HO
steam driver – this is a
cam for an articulated
style of loco with two
sets of cylinders and
drivers.
DCC Impulses Column - 9
Deciding when the chuff sound should be generated can be
handled by the microprocessor, based on motor speed, once
the user adjusts the chuff rate (via CV). However, this will only
be correct for the single speed where the user calibrated the
decoder to the locomotive.
How correct the chuffs are over a speed range is a big question.
Some locos and decoders play well together and, once the chuff
is adjusted at a medium speed, they will look in sync from a few
scale-MPH until the wheels are turning too fast for the eye to fol-
low. Others are in sync over a very narrow range of speeds.
The one sure way to synchronize the sound is with a switch
that tells the decoder exactly when to make a chuff sound,
based on wheel rotation. These chuff cams are usually very
tricky to install. Most folks who use them do so out of a love of
perfection. The time necessary for an after-market cam installa-
tion makes them very expensive if one is hiring the work done.
All the chuff cam does is tell the microprocessor, “now” when
it needs to initiate a chuff sound. The chuff cam wiring is just
a single wire into the microprocessor block. I left it off figure 3
for clarity.
Keep-Alive Circuitry
With sound decoders came sensitivity to power dropouts on
the track. The loco could be running down the track at 1/3
speed and hit a bit of dirt. If that power dropout was long
enough to reset the microprocessor, then when it woke up, it
thought the loco was standing still and ran through the “fire
up” sequence. Nothing is more concerting than a loco running
down the track but emanating the sound of a diesel motor
cranking over!
MRH-Feb 2013
