The world of audio processing has changed over the last few decades.
What was once an elaborate, time consuming and generally expensive discipline of engineering
has seen a major shift towards digitization, which has made itself widely available to
essentially everybody.
Back when there were no computers or digital systems, analog audio signals had to be recorded
onto magnetic tapes.
Any effects or processing were electronic circuits that modified the signal, and these
had to be placed beforehand in the signal chain and the effects would be irreversible
once recorded.
And if you had to edit or trim the end result, you’d have to take a pair of scissors, and
carefully slice the tape reel, and reattach it by glueing it to a different section of
the tape.
And if you wanted to save the audio recording safely for a long time, you’d etch the analog
footprint of the signal onto a vinyl disk, for example.
With the advent of digital systems and the personal computer, engineers devised ways
of converting the analog signals into a form that these systems could operate over.
This proved incredibly resilient and flexible because it provided control and manipulation
capabilities in ways they could never have achieved before.
It gave them fine control over every aspect of the signal, right from processing to storage.
You could now record several audio signals straight into your computer, apply effects
when needed, store it internally into a hard drive, and come back to it the next day and
press control + V to undo everything.
The digital world was born, but underneath it all, it's just an abstraction for really
complicated arrangement of electrical pathways.
A good example to quote is the saying that everything in the universe boils down to physics,
and everything in physics boils down to mathematics.
A biological organism is just a really complicated system of chemical reactions, and chemistry
is a really complicated system of nuclear forces in physics.
When we talk about signals in the digital domain, we’ve abstracted away the complexity
by introducing new terms like memory or bits and bytes.
But underneath the hood, it’s just really complicated paths electrical signals can take
through diodes and transistors.
The digital domain offers a much higher vantage point, where you can talk about complicated
concepts like input, output and memory, without needing to understand the underlying complexity.
But the digital representation of analog signals, though they sound really convenient, come
with a cost, and a set of limitations.
For better or for worse, we are deeply seated in the digital world today, and understanding
the digital domain is key when you are setting up a project to record on a DAW, or when designing
signal processing algorithms for a certain effect, or when just making choices that could
either make or break your signal path.
So lets start there, the signal path.
It’s a path a signal takes before reaching its intended target, usually our ears.
The simplest signal path is the one that doesn’t contain any transformation or processing of
the audio signal.
We seem to be throwing around the term audio processing, quite a lot.
Audio can exist in one form or another in 3 broad domains.
In the physical world where we live and breathe, audio exists as physical disturbances of a
medium or more simply as pressure waves.
These pressure waves are the only way for us to experience the auditory sensation of
hearing.
In the analog and digital domains, audio exists as information via electrical signals.
These signals would need to be converted or transformed back into the physical domain
for us to hear it, and until then, it’s merely just information and nothing else.
Conversion would need to happen at each step of transforming a wave from the physical domain,
to signals in the analog and then to the digital domain, and similar conversion would need
to occur in the opposite direction when transforming the signal back into physical waves.
Each step of transformation has a cost associated with it, and none of the steps can offer lossless
conversion.
Meaning, that the audio quality will be degraded and there will be a loss of resolution at
each stage, and it’s unavoidable.
The scale of degradation however depends on a lot of factors, which we’ll look at later.
But we generally want to avoid unnecessary transformation stages.
Audio processing on the other hand is the modification of audio within the same domain,
mainly within either the analog and digital domains.
Examples of audio signal processing are amplification where you boost the strength of the signal,
or filtering where you attenuate or remove certain bands of frequencies from the signal.
In a lot of the cases, this processing is lossless and reversible.
Let's get back on track with signal paths.
I’ll present 3 scenarios where varying levels of transformation is needed.
In scenario 1, the simplest scenario, a natural source produces sound, and we hear it.
I hear a friend screaming at me.
The sound exists purely in the physical domain, and no transformation is applied.
Here’s a philosophical question for you.
Imagine a tree falling to the forest floor deep within a forest, where no human or animal
was around within the vicinity to have heard it.
The question is, did the falling tree produce any sound if no one was around to hear it?
Think about it.
Moving onto scenario 2, a person is singing into a microphone on a stage, and her voice
is being heard to several people through an array of speakers.
Here, the vibration of her voice is converted into electric analog signals through an input
transducer, in this case a microphone, The signal is then fed to a pre amplifier where
the signal from the microphone is boosted up and then fed into an output transducer,
in this case a powered speaker, which tries to faithful convert the analog signal into
physical pressure waves by vibrating its cone.
So all in all, there are 2 stages of conversion.
Scenario 3, let’s imagine the same vocalist, is in a studio trying to record herself.
The vocalist sings into a microphone, the microphone is connected to an audio interface
which houses a pre amplifier stage which we saw earlier, but more importantly it houses
an analog to digital convertor or an ADC which convert voltage to digital data and is transferred
to a computer via a USB cable, where it’s recorded and stored.
When the vocalist wants to listen back to what she sounded like before inevitably requesting
another take, the data is transferred back to the audio interface, which is responsible
for converting the data back to it analog form using a circuit called the digital to
analog converter or a DAC and then is transferred to the powered studio monitors for playback.
There are 4 stages of conversion in all, 2 stages to get the physical wave into a digital
form and another 2 in the other direction.
There is a lot we can talk about about how the physical wave is converted into an analog
signal through transducers, how different microphones work, what determines a frequency
response for a mic, and all the good stuff.
But since this module is focused towards audio in the digital domain, we’ll stick to exploring
just that.
Specifically the analog to digital conversion, the crux of the transformation and all the
nuances associated with it.
Alright, let's get into it, lets see what we need to do to transform voltage into data.