Have you ever noticed that a loudspeaker is the opposite of an eardrum?
Eh, probably not.
But it’s true!
See our ears work by concentrating changes in air pressure onto a small diaphragm that
will move back and forth with the pressure changes.
This vibration causes stimulation in the heary bits of the ear which your brain can, assuming
you have normal hearing ability, turn into what we perceive as sound.
A loudspeaker does the opposite--its diaphragms (the driver cones) vibrate to create pressure
changes in the air.
This vibration gets transferred to our eardrums so we can hear it.
We’re sticking to simple stuff today because the rabbit hole is just too deep.
All you need to know is that things vibrate, which causes air pressure to fluctuate, which
causes our eardrums to also vibrate, which stimulates the brain so that we can perceive
that vibration as sound.
This channel started as a series exploring the history of artificial sound, and it’s
been over TWO YEARS since I last touched on it at all.
Finally we’re finishing this up with the introduction of
DIGITAL SOUND (emphasis added with obnoxious reverb).
Since it’s been forever, let’s go over a brief history of sound recording technologies.
The first device which could reproduce a sound recording was the phonograph.
Thomas Edison’s invention consisted of an artificial eardrum, which would vibrate along
with changes in sound pressure, and with the aid of a collecting horn, the vibration is
transferred into this stylus, creating an up-and-down motion.
This carves a groove into a wax cylinder, and the vibrating stylus creates an imprint
of the sound wave.
The depth of that groove becomes a literal analog of the original sound vibrations.
Then, when the stylus is run over the now bumpy groove, the bumps cause the diaphragm
to vibrate in the same way as it did when it first made the bumps, and the result is
that you hear the same sound as before.
Or at least, a barely passable imitation of that sound.
(sad sounding violin music)
Commercially produced discs and cylinders were molded from master
recordings, and wouldn’t wear down like the original wax cylinders.
They were played back using devices like this.
This device is called a reproducer, and for decades all phonographs were based on
simple acoustic devices like this.
For nearly a century, this is how artificial sound recording technologies worked.
Something (like this horn) would collect sound waves, and recreate them onto a physical analog.
Then, that physical analog could recreate the original sound waves when played back.
While it all started with simple acoustic devices like this phonograph, eventually improvements
were made.
The development of the electronic microphone was perhaps the most important.
Now, sound waves cause a receiving diaphragm to move a coil of wire around a magnet, and
a voltage is produced in the wire as the diaphragm moves.
This time, sound waves are recreated as a voltage coming from the microphone, and by
amplifying this voltage and sending it into a new record cutting device which moves its
cutting stylus as a function of the voltage it receives, a more accurate carving of the
sound wave could be made into a disc or cylinder.
This greatly improved the fidelity of the recorded sound, even on acoustic reproduction
devices like this.
With the proliferation of radio--which I feel I must explain is a sound transmission technology,
not sound recording.
Just so we don’t get confused too much here--
the loudspeaker became a big deal.
Loudspeakers are the opposite of microphones--instead of producing a voltage as a reaction to a
sound pressure wave moving its diaphragm, a loudspeaker will move its diaphragm and
create a pressure wave as a reaction to incoming voltage.
With loudspeakers all the rage, record players could now use a phonograph cartridge, which
acts like a microphone for records.
The movement of the stylus as the groove vibrates it generates a voltage which can be amplified
to drive a loudspeaker.
This gets very meta very quickly.
An artificial ear turns sounds into voltage, and a cutting stylus turns this voltage into
a groove on a record.
Then, a playback stylus playing the record generates a voltage as the stylus vibrates.
This voltage is then amplified to drive a loudspeaker, which causes pressure changes
in the air around the loudspeaker, which your ears concentrate down to your eardrums, and
now your real eardrums are vibrating in roughly the same way that the original artificial
eardrum moved in the microphone in the first place.
Yeah.
In essence, the record becomes a way to recreate the original pattern of voltage created by
the microphone, so that the sound can be heard again in a different place
at a different time.
Let’s cut out the middle bit because that’s what’s most confusing.
A microphone like this creates an electrical signal of fluctuating intensity based on how
its diaphragm moves.
I can just amplify that signal and send it straight into a loudspeaker, which will reproduce
the sound in real time.
Radio accomplishes this wirelessly, but the sound isn’t recorded.
To capture the sound coming from the microphone to be played back later, it has to be converted
into an analog of the signal.
And that’s why it’s called analog recording technology.
No matter if it’s a record, a cassette tape, an open reel tape, or even a wax cylinder,
the sound information is recorded “doorectly”...
Doorectly.
Doorectly?
The sound information is recorded directly onto something, which can then be used to
recreate a copy of the original sound information.
That something is an analog of the original sound waves.
Improvements in sound technology were for many years simply incremental.
Wax cylinders became shellac discs.
Shellac became vinyl.
Magnetic recording wire allowed for a reusable, electronic recording medium.
This was improved into magnetic tape, allowing for a high fidelity, versatile recording medium
enabling multi-track recording and editing.
And to improve on the noise of magnetic tape, different particle formulations were developed,
and noise reduction technologies matured.
But we were still just taking some signal from a microphone, then slapping it basically
as is onto some sort of physical medium.
And that medium was never perfect.
Poorly made tape would cause signal dropouts.
Discs would be plagued by dust and scratches, and would slowly wear down with each play.
Because the analog medium contained the sound in its physical properties, it was inherently
prone to wear, damage, and distortion.
Which of course would wear down, damage, or distort the sound recording itself.
If only there were some way to encode the sound, perhaps a way to store sound logically
rather than analogously.
Maybe if the signal weren’t the sound itself, but instead were a set of instructions on
how to recreate it, we could get lossless, near-perfect sound reproduction.
And thus, digital sound was born.
The heart of uncompressed digital sound is pulse-code modulation, or PCM.
PCM’s roots can be traced back to the telegraph days, but its invention as we know it today
for sound came from British Engineer Alec Reeves.
I feel I must compliment Mr. Reeves on his given name, it’s excellent.
Very good.
He first devised this digital method of transmitting and receiving voice communication in 1937,
though it required extremely complex circuitry for the time.
However, PCM transmission was used during World War 2 as a way to encrypt extremely
important voice conversations, such as those between Winston Churchill
and Franklin Delano Roosevelt.
This encryption system was called SIGSALY,
“SIGSALLY”?
“SIGSALIE”?
Or Project X, X System, Ciphony 1, or Green Hornet.
Anyway, Project Green Sally X System Hornet 1 was much more complicated than simple Pulse
Code Modulation, but PCM was a large part of its encryption.
So how does PCM work?
It’s actually simpler than it might seem at first.
It’s rather like a system for repeatedly asking what the instantaneous amplitude of
a signal is many thousands of times per second, then simply writing that down.
Let’s look at a simple sine wave.
If this were to be encoded on a vinyl record, the groove of the record would start out straight
in the center, then move to the left as the signal intensity reached peak, then it would
start to move to the right, keep moving, keep moving, and then it would pull back to the
center.
When it’s played back, the movement of the stylus as the walls of the groove wiggle it
back and forth will recreate this signal.
And audio tape does the same thing, except the intensity isn’t recorded as a physical
movement, but as a degree of magnetization on the tape.
But with PCM, we aren’t even trying to recreate the wave.
Instead, we want to quantify it and play connect-the-dots.
Let’s say I want to take 20 samples of this waveform.
OK, I’ll divide it up into 20 chunks.
Now I just need to define the detail I can have within each sample.
Let’s put this on a scale of 0 to 15.
That's 4 bits of resolution.
Now, at each sampling point, we can take the closest value.
This sine wave can now be represented as the following string of numbers.
To get the sine wave back, we simply plot those numbers on a graph.
Then, connect the dots.
Tada! A sine…
wave?
Well, a sloppy sine wave.
But that’s only because we weren’t very specific.
We only took 20 samples, and each one could only be one of 16 values.
But now we know the two most crucial parts of digital sound--the sample rate and the
bit depth.
Perhaps the most common sample rate and bit depth of digital sound is 44.1 kilohertz,
16 bits.
This means that every second, 44,100 samples are taken, and each sample can be one of 65,536
values, or 2 to the power of 16.
And that’s how devices like this, a Tascam DR-05, record sound.
It’s looking at the voltage coming from the microphone, and taking precise measurements.
Every 44.1 thousandth of a second, it takes a voltage reading, and, well, writes it down.
It’s furiously quantifying and logging the voltage it measures with 16 bits of accuracy,
and the result is a string of numbers that logically represent the shape of the sound
waves that exerted pressure on the microphone’s diagram.
Pretty neat, huh?
And it can actually write down two numbers at a time, since this has two microphones
and records in stereo.
Inside this recorder is what’s called an analog-to-digital converter, or ADC.
The “ADC” is the actual device responsible for creating the stream of samples.
It takes the analog signal coming from the microphones themselves and converts it into
a stream of discrete numbers.
If you open the files it makes in audacity, you see what looks like a waveform of the sound.
It is a waveform, but a waveform that’s been plotted precisely on a graph.
Zoom way, way, way in on the waveform,
and eventually you can see the individual samples themselves.
And that’s all digital sound is--
it’s a huge list of numbers strung together in order.
To get these numbers back into sound we can hear, we need to use the opposite of an analog-to-digital
converter, or “ADC”.
So, we’ll use a DAC, or Digital-to-analog converter.
I like it when names make sense.
A DAC will read the string of numbers, and generate an analog voltage based upon their
values.
The DAC will smooth out the choppiness of the samples a bit to make the resulting sound
a little more natural, and now you’ve got an analog signal to send into an amplifier
and drive a loudspeaker.
The result is a near-perfect reproduction of the originally recorded sound.
Here’s a very crude analogy to explain the difference between analog and digital sound.
A vinyl record’s walls generate an analog signal by moving the stylus left and right...
as well as up and down.
It’s diagonally moved for stereo, but just imagine for a moment that it’s just left
and right.
A record directly creates the analog signal via the motion of the stylus.
But a digital sound source is instead sort of like a virtual stylus riding in a virtual groove.
The sound samples are snapshots in time of where the stylus was.
A DAC will then create an analog signal by running a virtual stylus through this virtual
groove and placing it at exactly the correct location--and thus generating the appropriate
voltage level--as defined by the samples.
By using a giant list of numbers to recreate sound, instead of the physical properties
of a plastic disc, the sound can be reproduced flawlessly and accurately with no reliance
on the record player’s cartridge properties, the integrity of its stylus, it’s motor,
the quality of the vinyl etc.
The biggest boon of digital sound was that it eliminated all of the little nuances that
might change how a recording sounds.
Digital sound is in a sense, absolute.
But getting digital sound into the hands of the average consumer took a long while.
DACs and “ADCs” were expensive components, and the amount of raw data generated by sound
recording was immense for the standards of the time.
Although 650 megabytes, the data equivalent of the first compact discs, is a paltry sum
of data in the 21st century, it was unimaginably huge in the early 1970’s, when the first
commercial digital sound recording took place.
For context, the Commodore 64, released the same year as the compact disc, has 64 kilobytes
of ram, and that was considered huge for the time.
A compact disc held roughly ten thousands times as much data.
64 kilobtyes of CD quality audio lasts this long;
(clip)
That’s not super helpful.
When we continue, we’ll look at the methods that were used to store data from digital
recordings, and we’ll discuss the rise of the compact disc as a robust, consumer-friendly
format for digital sound reproduction and distribution.
Thanks for watching, I hope you enjoyed the video!
If this is your first time coming across the channel and you liked what you saw, please
consider subscribing to Technology Connections.
Don’t forget you can also follow me on Twitter @TechConnectify, and you might enjoy the second
channel, Technology Connection 2, where I talk about stuff and don’t prepare for anything.
Also, thanks to Lord Telaneo on Twitter, there is also a Technology Connections Subreddit.
I really don’t know reddit at all, but you will also find me there as TechConnectify.
As always, thank you to everyone who supports this channel on Patreon, especially the wonderful
folks that have been scrolling up your screen.
It is with the support of people like you that I’m able to make these videos.
Thank you.
If you’d like to you join these awesome people and support the channel too, why not
take a look at my Patreon page.
Thank you for your consideration, and I’ll see you next time!