1. A Crash Course in Game Design and Production Week 5
- Posted by Michael Packard <lgp at EXO.COM>
Feb 06, 1997
-
Last edited Feb 07, 1997
A Crash Course in Game Design and Production
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Week 5 - Sound and Music Specification
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Welcome back! This is the fifth installment in "A Crash Course in Game
Design and Production. Like last time, this lesson is in multiple parts.
In PART ONE, we'll talk about the basics of sound reproduction on the
computer. In Part TWO, we'll discuss computer music and sound effects, and
what we need to write the Sound and Music Specification. In PART THREE we
will write the fifth section of the Design Spec for our Course Project, the
Sound and Music Specification. This is part 1 of 3
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Part 1 - Basics of Sound Reproduction on the Computer
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Before we can talk about the Sound and Music Specification, we need to get
up to speed on how sound works, at least as far as we're concerned, and
what we need to know to be able to talk about Music and Sound later on.
Sound Terminology
=================
Sound - Anything we can hear. When you drop something and it hits a table,
say, it and the table vibrate, they shake a little. This vibration moves
the air around them in all directions. As the objects vibrate back and
forth, the air around the objects are pushed away or drawn to them in
"pulses" like the ripples in a pond when you drop a rock in it. The air
pulses eventually reach your ear, and move the hairs inside back and forth
at about the same speed (it decreases with distance from the objects).
These vibrations in your ear are converted to electric signals which go to
your brain and you "hear" a sound.
If the vibrations are too fast or too slow, the hairs in your ear won't
respond to them. They either can't go back and forth fast enough to match
the vibrations, or the vibrations are so slow that their force isn't enough
to move the hairs. You can't hear these vibrations. They are not sound.
If you can't hear it, it isn't sound, I don't care what the speaker
propaganda says. Physicists and philosophers argue that. If I didn't
hear it, it didn't make a sound for me.
Amplitude - How far the hairs in your ear move as they vibrate back and
forth. The Amplitude is the total distance from back to forth. The farther
the hairs move back and forth, the louder the sound. If a garbage truck
falls off the Empire State Building and hits the ground, the vibrations
that it causes will stronger than those caused by a Yugo. The sound may be
similar, but it will be MUCH louder. There is no standard measurement for
this as far as I know. When you're trying to reproduce a sound, the
amplitude is relative to the hardware, amplifier and speakers, you are
using.
Frequency - How many times the hairs in our ears are vibrating per second.
The measure for this is Cycles (back and forth) Per Second, or Hertz, named
after the guy who figured this stuff out. The Human ear can "hear" sounds
ranging typically from 100 Hertz (1hz) to 16000 Hertz (16khz - 16 Kilo-
Hertz, 16000 cycles per second)
Tone, Musical Tone, Pure Tone, or Note - A continuous "sound" at a certain
frequency for a certain duration. If your ear hairs are vibrating at 440hz
(440 times back and forth per second) for awhile, you are "hearing" an
"A Note"
A guitar or piano string vibrating at 440hz will make a similar tone.
The won't sound exactly the SAME probably, because in the real world things
can't continuously vibrate at set rate over ANY length of time. There are
a lot of technical reasons why, which are way beyond the scope of this
work. Things that effect the sound you hear include, material that is
vibrating, density of that material, how it started vibrating, artificial
damping of the sound, the transit medium (things sound different underwater
than they do in the air), etc. On an instrument, when you play a "note"
the actual frequency you hear fluctuates many times a second, but they
average pretty close to the "standard" frequency of the note. An "A" on a
guitar will be on average about 440 hz, although at any time the frequency
may be between 437-444hz.
Synthesis vs Digital Sampling
=============================
There are 2 ways to create a sound for the computer to play, Syntheses,
and Digital Sampling.
Synthesis - The computer program itself "creates" the sound by sending
frequency and\or amplitude parameters to an electronic Sound generator. On
the PC, this is generally either to a sound card or PC speaker. On the
Adlib(tm) or SoundBlaster(tm) cards, the frequency determines the musical
tone, and the amplitude determines the volume. This is called FM synthesis
(Frequency Modulation) and is great for producing pure musical tones.
Music, for the most part, is a sequence of distinct musical tones, played
in some sequence with varying rhythms. Unfortunately, FM synthesis is
awful for trying to recreate "real life" sounds, like the human voice.
"Real" sounds are NEVER pure tones. The frequency and amplitude fluctuates
thousands of times a second. They may average at around a standard
frequency, but the parameters of how they vary with time are too
complicated to approximate with any accuracy. Musical instruments are
easier to synthesize, since they are designed to play "notes." You at
least have a starting point. Where do you start trying to synthesize a
cough?
Digital Sampling - The computer "listens" to a real world sound, and
"records" the frequency and amplitude changes over time. The process is
called "sampling" or "digitizing." On the PC, the SoundBlaster(tm)
compatible cards convert the electrical signals from their microphone or
line inputs into digital information. It actually only records the
amplitude many thousands of times a second. When it "plays" the "sampled"
sounds, it converts the data back to an electrical signal which drives the
speaker back and forth.
Digital Sampling is the best way to reproduce real world sounds. Pretty
much anything you can hear can be sampled into the computer. You can sample
your voice saying "Level Completed" and play it in your game. For short
duration sound effects and musical tags for your game, digital samples are
ideal.
The real drawback is that sampled sounds take up a TON of memory and hard
drive space. To accurately reproduce a sound, the computer needs to sample
the sound many thousands of times a second, with a very fine amplitude
scale. Example: If you sample a five second sound 44,000 times per
second, with 2 bytes of amplitude information per sample, the sound will
take about 429.7k of ram. If you want to sample in stereo, that's 859.4k
for five seconds! Digital sampling is awkward for long musical pieces,
they sound great, but the storage requirements aren't usually worth it.
For long musical pieces, FM synthesis is generally better than digital
sampling, but you are creative, and your hardware supports it, you can
merge the two forms. You can, for example, sample every note on a piano,
then tell the computer to play the individual notes in some order, and it
will sound exactly like a piano playing a tune. This is a hybrid between
pure synthesis and pure sampling, and is called "wavetable synthesis"
Good News and Bad News
======================
For Euphoria programmers, there is good news and bad news. The good news
is at long last we have the ability to use digital samples in our games,
thanx to Jacques Deschenes and his sound blaster drivers. The bad news is
* His drivers can only handle small <64k samples without considerable
computing time dedicated to moving memory around
* We can only play mono sounds
* We can only play one sound at a time
* We have no ability yet to do FM synthesis.
Since we can't actually DO FM synthesis in Euphoria (yet), the rest of this
discussion will be geared toward the Hows and Whats of Digital Sampling
More Terms and Definitions
==========================
Sampling Rate or Sampling Frequency - This is how many samples we are going
to record per second. This is how frequently we are looking at signal
coming in. Since its called a "frequency," the sampling rate is denoted in
Hertz or KiloHertz. If you sample a sound 44,000 times a second, it's
called a 44khz sample. This has NOTHING to do with the frequency of the
SOUND being sampled. You can, for example sample a 440hz sample 44,000
times a second, making a 44khz sample of a 440hz sound. I know it's
confusing. Read this about 20 times and you'll get it.
There is a relationship between sampling rate and the kinds of sounds you
can sample. A 22khz SOUND vibrates the speaker in and out 22,000 times a
second, meaning it moves it IN 22,000 times a second, and OUT 22,000 times
a second. In order to sample this sound adequately, you need a high enough
sampling rate that you can record all the IN's and OUT's. You must be able
to sample at least 44,000 times a second to do this. So the maximum SOUND
FREQUENCY you can record is half of your SAMPLING RATE. A 22khz sample can
reproduce any sound up to 11khz. An 8khz sample can reproduce any sound up
to 4khz.
Sound Quality - How close to the actual sound is your sample? As we've
just seen, the higher the sampling rate, the higher the sound frequency we
can reproduce, therefore the closer to the actual sound we get. CD players
sample at 44khz, and they sound great. Your telephone "samples" at about
8khz, and it sounds sucky in comparison. If you have fiber optic phone
cable, the sampling rate is about 15khz so it's better.
Amplitude Resolution - This is how many discrete amplitude levels your
sound hardware is capable of reproducing. Early Sound Blasters could
reproduce 128 amplitude levels for each direction of the speaker vibration
in and out, giving 256 total steps. This entire range can be represented
by 8 bits or one byte of information. The resolution is called 8bit. When
you hear the term "8-bit sound card" or "8-bit sample" it means that the
sound you hear from it will have at most 256 different amplitude
variations. Newer sound cards, like the SoundBlaster Pro(tm) have 16 bits
of amplitude resolution or 32768 different amplitude levels. They drive
the speaker back and forth more accurately, so the sound they can produce
is closer to what you sampled than 8-bit cards. 16-bit samples have better
sound quality than 8 bit samples, because the speaker moves closer to the
way the original sound would have moved it, but they take up twice as much
hard drive space and memory at the same sample rate.
I'm guessing most telephones today have 8 bit resolution. Anyone know?
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End of Week 5 - Sound and Music Specification
Part 1 - The Basics of Sound Reproduction on the Computer
If you have any questions for group discussion post them to the list server.
E-mail any other questions, comments or suggestions to lgp at exo.com
Mail monetary donations large or small to
Lord Generic Productions 1218 Karen Ave Santa Ana, Ca 92704
A Crash Course in Game Design and Production - Euphoria Edition
(C) Copyright 1996,7 Lord Generic Productions - All Rights Reserved
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