CV133 and CV134 work together to control chuff timing.
CV133 and CV134 can be adjusted to create the desired chuff rate in your locomotive, or you can configure your sound decoder with CV133 and CV134 to use an external cam to create chuffing sounds.
CV133 Steam Chuff/CAM configurations:
CV 133 set to a value between 1-127 determines the driver size in inches.
(Default Value = 63)
CV134 Steam gear ratio trim
CV 134 set to a value of 32=100% ratio.
Setting CV133's value from 1-127, puts the sound decoder into 'Autochuff' mode.
Autochuff mode tells the decoder to simulate driver chuff timing in software.
The CV133 default value is 63, approximating a real-world loco driver diameter of 63 inches.
If you halve this figure you will effectively double the chuff rate for a given loco speed.
CV134 (gear ratio) also effects the Autochuff rate.
The CV134 default value of 32 assumes no gear reduction.
Doubling this value to 64 tells the decoder to simulate a 2:1 gear reduction thereby doubling the chuff rate.
External Cam:
Setting CV133 to a value of 128 tells the sound decoder that you want to configure an actual cam input on your locomotive.
CV134=128 tells the decoder to activate the white cam input wire on the 10 pin sound harness of the sound decoder.
(see diagram below)
A chuff will be triggered when a pulse of over 6 volts or DCC track voltage is seen on the white CAM input lead.
This voltage has to go off (0 volts) before the next chuff can be triggered.
The external cam turns on and off as the wheel it is connected to rotates.
For purposes of estimation:
A typical switching locomotive will have 42" drivers or similar.
Mainline locomotives had larger drivers, with passenger locomotives having larger drivers than most freight locomotives.
At one time, conventional railroad thinking was that the driver diameter of a locomotive equated to the maximum speed of that locomotive.
This was ultimately disproved by the Milwaukee Road F-7 Hudson (84" drivers, 100+ mph top speed) and numerous others.
In the case of CV134, you can increase the chuff rate for a geared locomotive, such as a Shay or Climax, by changing its value.
As noted in the related article about cams, this matter is handled by the cam itself if you have enabled the cam function.