Transcript dc501 sound talk presented by FirmWarez.

Phun With Sound
Basic Concepts
For
Hardware (and other) Hackers
You Don’t Know What This Is
But you want to!
This Circuit Board
Made this famous riff
Phun With Sound
• Combo talk, covering:
– Intro to sound
– Some basics of synthesizers
– Making sounds with microcontrollers
What is Sound?
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Modulated air pressure
Frequency of modulation is pitch
Amplitude is volume
Waveforms can be defined (and created) with
math
More Sound Basics
• Unit of frequency is Hertz (Hz)
• Unit of sound energy is decibel
• Human hearing is around 20 Hz to 20 kHz
• Phreak bonus: who is the unit of sound level
named for?
Basic Waveforms
• Sine
• Triangle
• Square
Some Math
• Sine is the “purest” waveform both in math
and acoustical presentation: f(x) = sin(x)
• Square waves are an infinite summation of
sine waves:
• All those terms beyond the fundamental
frequency are “harmonics”
More Square Wave
Some Examples
• Using the SH101 analog synthesizer
– Sine
– Square
– Triangle
– PWM
• Microcontrollers driving motors often use
PWM…
Pulse Width Modulation
• PWM: fixed period, changing duty cycle
• Used as motor drive, DAC, and in sound
synthesis (and others)
• Back to the square wave function…
• …does changing duty cycle change harmonics?
YES!
• Bonus: what’s a 100% duty cycle PWM called?
Phun With Harmonics
• Subtractive Synthesis
– Start with a signal with lots of harmonics
– Filter it!
• Low pass filter
More Filtering
• Traditional analog synths
use voltage controlled filters
VCF
• Modern synths use digital
filters running on DSPs
The Filter Sweep
• Classic electronica music effect
Triangle Man (Delta)
• Non-changing waveforms sound dull, even
irritating
• ∆ Sounds more interesting
• How to implement change?
– LFO, repeatable change
– “Random” change
Art of Noise
• Noise is random
• Pseudo-random isn’t random, nor noise
• What does it sound like?
– Noise on the SH101
– Pseudo-random on the SH101
– How the pros use pseudo-random influences
• InfoSec tie-in: see how limited pseudorandom is a bad choice for seeding crypto?
Making Sound With
Microcontrollers
• Two basic approaches:
– Synthesis
– Sample playback
• Two areas of concern:
– Hardware
– Firmware
Making Sound With
Microcontrollers: Firmware
• Synthesis options
– Hardware based (waveform into VCF)
– Math based (calculations in firmware)
• Sample option
– Look up table
– Can be waveform data (sine, triangle, DTMF)
– Can also be “sample” in the sense of digital
recording
Microcontrollers: Hardware
• Synthesis options
– Output waveform and control of external
elements (VCF, VCA)
– Heavily filtered square wave
– DAC…
Microcontrollers: Hardware
• Sampled sound needs DAC
– Filtered PWM, single output pin, HW controlled
– DAC Ladder, multiple output pins, simple interface
– DAC hardware, multiple i/o pins, complex
interface
Example: DEFCON 18
Badge Hack
• What happens when you combine:
– DC18 Badge
– Beer
– Pr0n
– Top engineers from famous comm corp
– And me?
• The Optimized Tomfoolery DC18 badge hack
entry! (Kingpin approved!)
• The Hacker Jeopardy answer is: “Hey, what’s
the input impedance of an LM386?”
DEFCON Badge Hack
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Limited microcontroller code space…
…and limited hardware options (no PWM)
Sampled desired sound to get .wav
Edit .wav to get C #include
Crude R-2R ladder DAC (considered other
methods)
• Play samples back from codespace
• 4-bit low sample rate, was recognizable!
DC18 Badge Hack
• Tools used:
– Audacitiy: sound recording manipulation tool
– Lame: mpg recorder option for Audacity
– HxD: hex editor
– Code Warrior: microcontroller IDE
• Search youtube for “DC18 Theater” for
Kingpin’s video of our entry (NSFW!)
¿Questions?