Four Chuffs or Two

[DCB]

I just watched another YouTube "How to do it" DCC video, and again off goes the loco chuff chuff.  etc   Two chuffs per revolution.
Steam railway locos have with almost no exceptions,(the   double acting cylinders which give four chuffs with two cylinders, Most three cylinders give six chuffs and 4 cylinders 4 or eight chuffs.  "Aerolite" at the NRM is AFAIK the last 2 "Chuff" per rev loco left in the UK.   Is this endemic in the DCC sound files or just ignorance on the part of the modellers?

[Hamburger]

With a few exceptions, locomotives with 2 and often even 4 cylinders have 4 chuffs per revolution and with 3 cylinders they have 6.
Decoders such as TTS produce a "fake" number of chuffs due to their primitive technology.
Fewer chuffs than the original are preferably played at higher speeds because the sound sampling of the chuffs is simply poorly done. When the models run at higher speeds, the sounds usually sound very choppy.
In fact, model railroaders often ignore reality because they just want to hear a nice "puff-puff" and that's it."
There are exceptions, such as the A4 sound project by Locomansounds or European projects by D&H, but such things are rare.

More sophisticated decoders than TTS have the ability to individually adjust the number of chuffs.

Edited March 31 by Hamburger

[Nigelcliffe]

I've been fitting sound to UK prototype locos for approx 15 years.   Generally Zimo or ESU chips, with a few cases of "others".  
Steam have always been 4-chuffs-per-revolution (2-cylinder locomotives), with a small difference in emphasis/sound on each (because standing trackside will give a difference).    They'll always "coast" with just rod-noises, representing the regulator being shut, and thus slow and stop with just rod-clank and the odd wheeze - certainly don't "chuff to a stop".   

Low speed chuffing rate would be set extremely carefully, so at all lower speeds there are precisely the four-per-revolution.   

There is a reproduction issue at high running speeds, even with different chuff recordings at different speeds (which will improve things), eventually the sounds will appear to merge into a mush.  The pragmatic solution is to slightly reduce the frequency of chuffs at higher speed; decoders usually have things to do this.  Most people can't tell that's happening as the model's movement is too fast to attribute individual noises to wheel/rod movement (plus one is usually viewing a fast running locomotive at a distance, when the sound of the real thing would be changed further by observation distances). 

I've had the benefit of building models which are used by current drivers of real steam locomotives.  They will say if things are wrong.   

There always has been a large toy-train element in model railways,  I guess DCB has tripped over one of those.  

 

 

[Harlequin]

On 31/03/2024 at 02:53, DCB said:

I just watched another YouTube "How to do it" DCC video, and again off goes the loco chuff chuff.  etc   Two chuffs per revolution.
Steam railway locos have with almost no exceptions,(the   double acting cylinders which give four chuffs with two cylinders, Most three cylinders give six chuffs and 4 cylinders 4 or eight chuffs.  "Aerolite" at the NRM is AFAIK the last 2 "Chuff" per rev loco left in the UK.   Is this endemic in the DCC sound files or just ignorance on the part of the modellers?

 

Ignorance.

 

And when self-proclaimed experts get basic stuff like this wrong you have to wonder what else they got wrong...

 

[Bob83a]

I don’t think it is ignorance on the side of the sound file developers.

 

The issue is more to do with timing and the construction of the drive chain.

 

The cycling of the cylinders which are responsible for the chuff is directly linked mechanically to the revolution of the wheels.

However the decoder in general has no way to know the rate at which the wheels turn.

The decoder only has control of the voltage supplied to the motor. But the motors have different characteristic even between individual motors, depending on the brushes/ commutator and even the lubrication. So that the RPM is not always constant for a particular voltage.

Different manufactures of the same model use different motors.

Then there is the gearing, if you have two models of the same loco from different manufactures then it is unlikely that gearing will be the same.

there is also the effects of lubrication and dirt on the gear train and wheels.

so it is very unlikely that the same voltage applied to two different models will result in the same wheel rotation.

 

the only way to get perfect synchronisation of the chuffs is to have some sort of feed back from the wheels back to the decoders, which currently very few models have.

 

so getting the chuffs rate right is one big compromise. Especially as most decoders only have chuff rates for a sample of voltage’s applied to the motor and interpolate between those.

 

most decoders try to get a good approximation and some are better than others and none is perfect.

 

the sound file developers will produce an algorithm that is reasonable for the model that they test with but is never going to be right for every other model of the same type.

[Jeremy Cumberland]

Hornby's "Exhaust Steam Sound" was always perfectly synchronised. One chuff per revolution of the tender wheels.

[Nigelcliffe]

15 minutes ago, Bob83a said:

I don’t think it is ignorance on the side of the sound file developers.

 

The issue is more to do with timing and the construction of the drive chain.

 

The cycling of the cylinders which are responsible for the chuff is directly linked mechanically to the revolution of the wheels.

However the decoder in general has no way to know the rate at which the wheels turn.

The decoder only has control of the voltage supplied to the motor. But the motors have different characteristic even between individual motors, depending on the brushes/ commutator and even the lubrication. So that the RPM is not always constant for a particular voltage.

Different manufactures of the same model use different motors.

Then there is the gearing, if you have two models of the same loco from different manufactures then it is unlikely that gearing will be the same.

there is also the effects of lubrication and dirt on the gear train and wheels.

so it is very unlikely that the same voltage applied to two different models will result in the same wheel rotation.

 

Not really correct, and an excessively pessimistic view of the what is possible and normal for a sound decoder. 

  
There are two basic ways of getting the rate of cylinder movement from the model: 
    -   one is difficult to fit to many models - a timing switch (usually done with a hall-sensor) on the axle feeding into the decoder, tends to be more practical at 7mm scale and upwards. 

    -   the easier method is by measuring the motor speed from the BEMF algorithm:  the motor is control in decoders a lot more complex than "turn up the volts". 

 

Decent sound decoders offer both of these options (ESU, Zimo, D&H, etc.. etc), and have done so for ages.  The decoder makers publish manuals which include the settings to alter the decoder to match the specific installation (motor choice, gearing, etc.. ).       

I can get four chuffs per revolution from a decoder over a range of speeds in a huge variety of models (2mm scale, 4mm scale, 7mm scale).   

It takes a bit of time to setup the values, but once set for a particular model it remains set and will still work exactly the same on that model years later.