
In
this document we will discuss the various aspects of a sound-set as used
to program the CT SL51-4 decoder (SL75/GE75 are identical for programming).
Although I have not experimented with every area of it yet, I hope this will
provide a useful insight into how it fits together and how the various sound
files relate to what actually gets played during operation of the loco.
Basic Concepts
The
decoder uses digital recordings of sounds stored in flash memory and plays them
back through an integrated amplifier and attached speaker. Most sounds are played
back 'as is' without any modification, such as whistles, brakes etc. Other sounds
are modified on-the-fly by the decoder to adjust what the loco sounds like -
for example, the pitch of a chuff is modified depending upon the loco's current
speed, the load, and whether it is accelerating, braking or moving at a constant
speed.
It might seem complex, but actually there isn't much going on!
The
SL51-4 has the ability to play up to 3 different sounds at the same time (this
is what we mean when we say 3 channel playback). Up to 3 sounds effectively
have their waveforms merged before sending the signal to the amplifier/speaker.
3 is just enough for a loco sound decoder - any less and you'd have to cut out
1 sound while the whistle blew, for example, and it would give a stuttered,
disjoint effect. LokSound 3.5 decoders and Zimo sounds decoders provide 4 channels, and LokSound V4 decoders
provide a whopping 8-channels. More channels would be nice, but in practise 3 is adequate.
As
mentioned, the decoder possesses some flash memory, which means that
you can download your own sound recordings onto it, and it will remain stored
there until you replace it with something else (or you swing a big magnet near
it!!). Computers use this type of technology extensively nowadays to store data
which wont be lost when you turn it off.
The
flash memory of the decoder also stores the CV values,
but that's a topic for a separate discussion!
The
sound recordings are stored individually into things called slots. You
can think of the slot essentially as equivalent to a file on a computer's
file system. Rather than having names though, they are referred to by
their slot position. Each slot is used for a different purpose, which
we'll talk about next.
What Sample Rate Should I Use?
The sample rate of a digital sound indicates the number
of chunks of information recorded per second of the sound. The higher the
sample rate, the finer the sound quality... at least in general.
CT sound decoders support 22khz and 11khz sample rates and if there
is enough storage room remaining on the decoder, you should always go for the higher
rate - at least keep the 'master' copies of your sound files at the higher rate.
However, in reality there is little perceiveable difference between
22khz and 11khz for many sounds - particularly those of low-to-mid range frequency.
High-frequency sounds, with little metalic clinks and clicks will suffer most or
high pitched hissy noises. Experiment with the effects of reducing the sample rate
on the computer to hear the loss, if any.
Good candicates for a lower sample rate might include long
platform announcement sounds. Whistles and horns are often largely unaffected too.
What Sample Size Should I Use? 16bit or 8bit
The sample size of a digital sound defines the accuracy of each sound sample.
Let's imagine you had a 2bit sound, this would mean that the individual sound sample values
could only be ON or OFF - nothing in between. The more 'bits' you have, the finer the description of
the sound can be. 16bit sounds more accurately describe the sound than an 8bit sound.
The reality is that actual sound is totally analogue - it is an infinitely fine curve of
'values', and any digital respresentation of the sound is at a loss with the original real-world sound.
Of course the ear can be easily fooled, in the same way that the eye can be fooled by
a TV picture - which displays at 25, 50 or 100 frames (pictures) per second, rather than a
real-world continuous image. The 'sample rate' is a bit like this, and the 'sample size' defines
the number of digital values that a sound can be represented by.
In a high-quality home hi-fi system you can easily tell the difference between 8bit
and 16bit sound (CDs use 16bit), but in a tiny model train speaker the difference becomes blurred.
This is why some manufacturers choose to use 8bit (Zimo), and others choose to use 16bit (CT). The
reality is that there is no discernable difference in a model train!
What Sound Slots Are Available?
So,
a slot is an individual sound recording, but how does the decoder use
each slot? Well, each slot is numbered, so let's simply list the available slots
and put a description by them...
Slot Number(s) |
Purpose |
How Sounds Are Used |
0-3 * |
Chuffs during acceleration |
STEAM: 4
slots used for 2, 3 or 4 cylinder engines during acceleration. CV 49 allows
you to choose the number of cylinders. These chuffs should be slightly
stronger than the other chuffs to give the effect that the loco
is working harder to build up speed.
ELEC/DIESEL:
recommended that these are not used at all - use AUTO slots (see further down)
|
4-7 * |
Constant speed chuffs |
STEAM: 4
slots used for 2, 3 or 4 cylinder engines during constant speed. See CV49
for cylinder count selection.
ELEC/DIESEL:
recommended that these are not used at all - use AUTO slots (see further
down)
|
8-11 * |
Chuffs during deceleration |
STEAM: 4
slots used for 2, 3 or 4 cylinder engines during deceleration. CV 49 allows
you to choose the number of cylinders. These chuffs should be slightly
weaker than the other chuffs to give the effect that the loco
is not doing so much work.
ELEC/DIESEL:
recommended that these are not used at all - use AUTO slots (see further
down)
|
12 * |
Sound between acceleration chuffs |
STEAM:
If there is a time gap between the playing of chuffs (slots 0-3) then this
sound is used to 'fill' the gap. It might be silence, or perhaps a quiet
hiss (white noise). |
13
* |
Sound
between constant speed chuffs |
STEAM:
Same as slot 12, but used between chuffs during constant speed (slots 4-7). |
14
* |
Sound
between deceleration chuffs |
STEAM:
Same as slots 12 and 13, but used between chuffs during deceleration (slots
4-7). |
15-17
* |
Switching-on |
This sequence
of sounds is played when you switch the main running sounds on. It is
typically used to produce a big steam release as the engine builds up
pressure. For diesel, this is the ideal place to put your engine startup
sequence - ignition, sparking, chug-up to idle.
|
18-20
* |
Switching-off |
This
sequence is played when you switch off the main running sounds. You can
use it for final release of pressure, or some other dying downdown.
For diesel, a shutting-off sequence can be put here. |
21-23
* |
Transition
from standstill to moving |
This
sequence is played when you lift the throttle from standstill. It could
be used for some additional creaks/squeaks for example, as the couplings
tighten up. For diesel, this is a great place to put some sort of engine-rev-up
sequence. In fact, setting CV137 bit 5 on (value 32), and this sound will
complete before the motor starts to physically spin. |
24-26
* |
Transition
from standstill to moving (2nd) |
This
is an additional sequence played during the same time as those above in
slots 21-23. You might want to blow a guard's whistle on startup, for example. |
27-29
* |
Transition
from moving to standstill |
Use
this sequence to trigger a sound when the loco grinds to an absolute stop.
Perhaps some clanking as the couplings squash together. Also good for a
final braking squeal. |
30-32
* |
Transition
from moving to standstill (2nd) |
An
additional sequence of sounds played on halting, same as above slots 27-29.
Maybe the sound of doors opening, or the driver shouting something to the
platform. |
33-36
* |
Stationary |
Use
this sequence of sounds while the loco is at a standstill, such as a continuous
hiss of steam, or the gentle sound of a popping valve! For diesel, this
is the ticking over at idle of the engine. |
37-41 |
Effect
1 ('normal' bank of sounds) |
The
first of a number of auxillary effects which can be triggered by
a function key on the controller. These effects may also be played through
random play - see CVs 112 and 113 for details. |
42-46 |
Effect
2 ('normal' bank of sounds) |
2nd
auxillary effect - similar to slots 37-41. |
47-51 |
Effect
3 ('normal' bank of sounds) |
3rd
auxillary effect - similar to slots 37-41. |
52-56 |
Effect
4 ('normal' bank of sounds) |
4th
auxillary effect - similar to slots 37-41. |
57-61 |
Effect
5 ('normal' bank of sounds) |
5th
auxillary effect - similar to slots 37-41. |
62-66 |
Effect
6 ('normal' bank of sounds) |
6th
auxillary effect - similar to slots 37-41. |
67-71 |
Effect
7 ('normal' bank of sounds) |
7th
auxillary effect - similar to slots 37-41. |
72-76 |
Effect
8 ('normal' bank of sounds) |
8th
auxillary effect - similar to slots 37-41. |
77-81 |
Effect
9 ('alternate' bank of sounds) |
9th
auxillary effect - 'Alternate' sound effect to effect 1. Effects 8 through
to 16 are only accessiblie by switching the sound effects to the 'Alternate
Bank'. You can either access sounds 1-8, OR sounds 9-16. To switch between
them, you'll need to assign a Function Key to activate the 'Alt' action
- see the Extended Function Mapping for how to do this. Even though you
have to sacrifice a Function Key especially for the 'Alt' action, it is
worth it as you then have easy access to double the number of sound effects! |
82-86 |
Effect
10 ('alternate' bank of sounds) |
10th
auxillary effect - 'Alternate' sound effect to effect 2 |
87-91 |
Effect
11 ('alternate' bank of sounds) |
11th
auxillary effect - 'Alternate' sound effect to effect 3 |
92-96 |
Effect
12 ('alternate' bank of sounds) |
12th
auxillary effect - 'Alternate' sound effect to effect 4 |
97-101 |
Effect
13 ('alternate' bank of sounds) |
13th
auxillary effect - 'Alternate' sound effect to effect 5 |
102-106 |
Effect
14 ('alternate' bank of sounds) |
14th
auxillary effect - 'Alternate' sound effect to effect 6 |
107-111 |
Effect
15 ('alternate' bank of sounds) |
15th
auxillary effect - 'Alternate' sound effect to effect 7. Beware of this sound on SL75, where a bug
appears to cause this sound to be played whenever the loco starts to move off from standstill. |
112-116 |
Effect
16 ('alternate' bank of sounds) |
16th
auxillary effect - 'Alternate' sound effect to effect 8 |
117-119
* |
Direction
Change |
This
sound sequence is played when you switch direction of the loco. Suggested
sounds might include gear crunch, or clunk for diesels. |
120-122
* |
Rapid
Braking |
Sequence
of sounds played during 'motion' braking. How hard you have to brake to
trigger this sound is defined by CV 107. Use slots 27-29 and 30-32 for a
final braking sound as the loco actually comes to a halt. |
123-125
* |
Rapid
Acceleration |
If
a sudden burst of power is injected, this sequence of sounds plays. Use
it to trigger wheel slip, or a rev-up sound. How hard you have to turn up
the speed to trigger this sequence is determined by CV177 |
AUTO
* |
Electric/Diesel
engine sounds |
The CT decoders
provide 2 methods for you to supply diesel or electric engine sounds (not
relevant for steam) - either via slots 0-11, or via special 'AUTO' slots.
YouChoos recommends the AUTO slots which are quite simple to configure
and provide reasonable flexibility on the tone and pitch of the engine,
varying with speed. You can have anything from 1 to a large number of
AUTO sections which are automatically associated with different speed
regions. For example, if you have 3 AUTO sections, then your total speed
range is divided equally into 3 to decide which of the 3 AUTO definitions
your sounds come from for each speed band.
Effective
differences between each section could be a slight pitch or volume change
(louder at higher speed, for example). It may also produce a realistic
effect if you change the balance of different components of the engine
sound as the speed increases, such as a louder whine/whistle at higher
speeds, and less 'chug'. Imagination is the limit!
It is best
not to vary the pitch substantially, but instead to use CVs 135 and 136
with CV135 set to 128 and CV136 set to anything between 128 and 255 to
achieve engine pitch changes.
It is important
to merge and balance the first AUTO slots with the standstill sound in
Slots 33-36 so that the sound makes a smooth transition from idle to motion. In fact
I've found that it is useful to make the 1st AUTO section use the standstill sounds
themselves in some case.
AUTO slots
consist of 3 sounds in this order:
- Acceleration
sound from lower speed up to this slot's speed (transition) e.g. standstill
up to slow movement
- Constant
sound for this speed
- Deceleration
from this slot's speed down to the next speed down (transition) e.g.
movement down to standstill
It is recommended
to keep each of these 3 sounds between 1 and 2 seconds in duration, or else
the engine sounds may struggle to 'keep up' with the actual motor speed, but it is
long enough to provide realistic transitions between speed steps.
If this appears to be a problem for you, try modifying the acceleration
and deceleration rates in CVs 3 and 4 to cause acceleration and decelaration
of the motor to take longer.
Between 3
and 7 AUTO slots seems to work quite well, depending upon how much of
the tone you want to vary for different speeds.
For simplicity you could get away with just a single AUTO slot, and rely on CVs 135 and 136
to adjust the pitch of the engine, but it wont be as effective as designing individual
engine tones for different ranges in speed.
|
AUTOn * |
Electric/Diesel engine sounds |
An
alternative to the AUTO sounds described above, you can number the AUTO
slots explicitly to define which speed step that group of engine sounds
relates to. I'm not sure there is a huge benefit to this, and I have not
used it enough myself to understand how it differs from normal AUTO slots. |
*
Sounds marked with an asterisk above indicate that they are part of the main
running sounds, rather than additional effects.
As
you can see, many of the slots are grouped together, and the decoder will play
them in sequence within that group, often providing the ability to loop
the sound for a number of repetitions during the middle slot of the group.
For example, Effect 1 could be a long whistle sound, which comprises of the
following component sounds:
- Slot 37 = start of whistle sound
- Slot 38 = continuation of whistle
- Slot 39 = middle of whistle sound (which loops until the function is switched off again)
- Slot 40 = start of the end of the whistle (you know what I mean!?!)
- Slot 41 = end of the whistle sound
When
you activate this sound, say from a function key on your controller, it would
first play Slots 37 and 38 sequentially, followed by Slot 39 which is would
continually loop until you switched the function off, at which time it would
then continue through Slots 40 and 41 until the sound is finished. The decoder
decides if the middle slot is a looping sound or not based on CV 145
- a different bit of CV145 is used to specify if looping is activated
for each of the first 8 Effects.
Effects
1 to 8 may also be sounded by the decoder's random play feature (effects
9-16 are not possible to include in random play). Any sound activated because
of random play may also be looped, but because it is not user-controlled directly,
you need to program the decoder to specify how many times the middle
slot is looped before it moves onto the final slots. This definition is done
using CVs 122 to 128, bits 2-4 and bits 5-7. Please refer to my CV
Commentary guide for the SL51-4 for examples and a fuller explanation. The
choice of which of the first 8 Effects are included in random play are decided
by the values in CVs 112 and 113 (CV 112 for those at standstill, and CV 113
for those during movement).
The
CVs mentioned above are interesting and useful, and you can tweak these in a
loco that already has a CT SL51-4 decoder installed. However, in order to download
your own sounds onto the decoder you will need the right equipment for the job
- please read my Commentary on the CT
Programmer for more information.
Where Can I Obtain Sound Recordings?
Not everyone can
get to a place where the real engine is running, and even if you do, there is
no guarantee that the conditions will be suitable for recording, or that the
engine driver will kindly blow the whistle for you when you ask! Of course,
sometimes this can be arranged - and I would suggest that if you are serious
about getting your own original recordings, that you ask permission before you
set off for your visit.
For most however,
you will need to source your recordings elsewhere, and the obvious place is
the Internet - YouTube in particular is full of useful videos. It does
of course involve spending A LOT of time sitting in front of the PC watching
immensely boring stuff, and if you have a spouse-in-the-house you should probably warn them that
you are going to be 'a while'! No kidding though, you can spend many hours just
watching clips to find the right ones, with suitable sounds that you can make
use of.
Always check that
you are allowed to download/record the clip. If you are using it for yourself
then this is usually no problem at all, but if you intend to resell the loco
then you need to be quite wary about copyright. I would never dream, for example,
of reselling a Thomas The Tank Engine sound-set!
Pay particular
attention to background and ambient noises when searching for good recordings.
It is hard to find completely crystal clear recordings, but the louder the target
sound, and the quieter the ambient noise, the better. Of special interest is
often the whistle, which, although relatively easy to come across, is rare to
find on its own without overriding steam hiss and chuff sounds. It is worth
continuing the search in the hope of something better.
Sounds can
be cleaned up quite substantially later on, but like many end-products, the
better the original recording, the better the eventual results.
As well as the
Internet, there are numerous libraries becoming available specifically of steam
train recordings, such as those on www.steamsounds.org.uk.
Take your pick, and use whatever suits you best.
Of course there
is also a plethora of sound-sets out there already for specific decoders. LokSound
in particular have a huge library of sounds for many steam engines which you
can simply request from them when you purchase the decoder. Zimo is similar,
providing a reasonable selection of sound projects, albeit mostly European.
CT is very limited on this front, so you'll most likely want to create your
own sound-sets. Beware though that many of these sound projects are copyright
to that company, or the individual who compiled it. As we can see, it takes
a lot of effort and skill to put together a good set, and the authors generally
like to be rewarded for their efforts!
How Should I Edit Sound Files?
This is where I
could get very technical, and this discussion could get very opinionated. There
are so many ways to edit your sounds that it really is down to personal preference.
I would recommend
using editing software on the PC to do the job, although there are other, more
traditional methods (don't go there!). My experiences with different
software packages is pretty limited, as there is so much choice. Lots of freeware
of shareware is out there which can do an adequate job, or if your budget stretches,
then you can go right up to professional-grade studio packages which have every
bell and whistle (no pun intended) you could dream of.
I'll tell you what
I currently use, and leave it at that, without the heavy opinion: GoldWave.
This is a relatively low-cost editing package which has a lot of good features.
It misses a few features I would like, but in general it is packed with effects
and filters, and most importantly is quite easy to learn and use. It costs around
£30 to buy online. If you are interested, take a look on www.goldwave.com.
How you actually
go about editing the sounds is again largely a matter of preference. There are
a wide variety of filters and effects that you can play with until you improve
the sound enough to be happy:
- Cropping
- once you find some sounds, you can grab them from source (GoldWave
for example can record any sound playing through the PC), but then you need
to chop the start and end off accurately of the bits you don't want.
This is cropping.
- Stitching
- sometimes it is necessary to stitch 2 or more sounds together to
form a longer sound, or to achieve a seemless loop. For example, something
like a looping whistle which you switch on, then loops in the middle until
you switch it off, requires a central sound which loops neatly. This is actually
one of the hardest tricks to achieve without getting clicking noises
on the join. You'll need to zoom right into the sound to chop/stitch at exactly
the right cross-over points in the wave form. An alternative to stitching
involves a combination of stitching and cross-fading i.e. you overlap the
end of the first sound with the start of the next sound, and cross-fade them
so that they blend into each other. Sometimes this works better, and sometimes
you end up with an even bigger mess! Experiment to get experience!
- Filtering
- Many live recordings will have lots of background noise, often referred
to as white noise. This is essentially a hissing sound from the microphone
that took the recording, perhaps of the wind blowing across it. It is often
possible to use filtering features in your editing software to reduce this
sound, but beware - do not reduce it too much or you lose the crispness of
the main sound that you are trying to keep.
- Change pitch
- often it is useful to adjust the pitch of a sound to make it better
suit the model. I often tweak whistle pitches so that they translate better
into the smaller scale, but even more often I play around with the pitch of
the chuff and engine sounds according to the loco.
- Change volume
- again, a very useful feature is to be able to adjust the volume of a sound.
Although many sound decoders have CVs to allow you to adjust the volume of
each sound, it is best to make sure that your original sound-set is good before
you start. Try to begin with the loudest possible recording, and turn the
volume down on each sound individually as you test it in the loco. I often
go through a repeating cycle of loading onto the decoder, testing the loco
in action, then changing the volume back on the PC for anything which sounds
wrong or distorts a little. It is worth going through a number of cycles like
this until you have a good balance. It is common to find that each speaker
has different tolerrances, and what sounds clear on one speaker may distort
on another, so you just need to turn down the volume a little for that one.
It is important to avoid clipping of sounds by increasing the volume too high - it will just
sound distorted if you go too far.
As you can imagine,
it is a complex business, and a very subjective one, and you'll really only
get good at it over time with lots of practise. This document is just meant
as a starting point giving you some of the basics to kick you off.
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