Bluetooth is a fascinating technology. For example, when you play music on your wireless
head-phones, your smartphone transmits around a million 1s and 0s to your headphones every
second using Blue-tooth. These 1s and 0s are assembled into 16-bit
numbers which are used to build the electrical waveform that is sent to the speaker and converted
into sound waves. But how are a million or so 1s and 0s wirelessly
transmitted every single second between your smartphone and your wireless earbuds? In order to answer this question, we’re
going to explore the engineering behind Bluetooth and the principles of wireless commu-nication. Before we get into the details and specifics
of Bluetooth, let’s start with an analogy. When you see a traffic light change color,
you recognize what that color change means. The traffic light uses a section of the electromagnetic
spectrum, or light, to convey information. The green light has a wavelength of around
540 nanometers, yellow around 570 nanometers, and red around 700 nanometers. Your eyes can easily distin-guish between
these different wavelengths of light, and your brain interprets these different wavelengths
and the information they convey. Your smartphone and wireless earbuds communicate
using electromagnetic waves in a rather similar fashion but utilizing a different section
of the spectrum. Specifically, Bluetooth uses waves that are
around 123 millimeters in wavelength. They are invisible to the human eye and can
generally pass-through obstruc-tions like walls, rather like visible light passing through
glass. When your smartphone sends a long string of
binary 1s and 0s to your earbuds, it communicates these 1s and 0s by designating a wavelength
of 121 milli-meters as a 1, and a wavelength of 124 millimeters as a 0, similar to the
540-nanometer green and 700 na-nometer red colors of the traffic light. Your smartphone’s antenna generates these
two wavelengths, and switches back and forth between them at an incredible rate of about
a million times a second. With this pro-cess of switching between the
two wavelengths, kind of like switching between the red and green traffic lights, your smartphone
can communicate around a million 1’s and 0’s every single second to your earbuds. And amazingly, engineers have designed the
antennae and circuitry in your earbuds and smartphone to be attuned to sensing and transmitting
these wavelengths back and forth to one another. Before we dive into further details on Bluetooth,
let’s briefly explore and clarify these visualizations because they’re potentially
rather confusing. First of all, electromagnetic waves do not
travel in a single direction in a sinusoidal fashion like this. In fact, the electromagnetic waves that are
transmitted from your smartphone travel out in all directions like an expanding sphere. When your smartphone switches between frequencies,
it’s as if it were a lightbulb that rapidly changes between two different frequencies
of millimeter length electromagnetic waves, which travel out as expanding spheres. As a result, your smartphone and wireless
headphones can work in any di-rection. Thus, this visualization of a directional
sinusoidal wave is lacking, yet there are still merits to the vis-ualization. In order to give you a sense of how Bluetooth
works, we’re going to use 4 different visualizations that are all different perspectives of looking
at the same invisible thing. Here we have the sinusoid waves which give
us a sense of the frequency and wavelength of the electromagnetic wave. What’s moving up and down is not the wave
itself, but rather it’s the strength of the electric field. This perspective just shows us a directional
sliver or ray of the expanding sphere with the electric field going up and down as the
Bluetooth signal propa-gates outwards in all directions. If we were to measure the electric field at
a single point in space, we would find that the strength of the electric field would increase
and decrease sinusoidally, and the number of peaks per second would be the frequency. Furthermore, we’re ignoring the magnetic
field component of the elec-tromagnetic wave, as including it would be too confusing. Let’s move onto the second visualization. Here we have the travelling binary numbers
which give us a sense of the data being sent, however it also doesn’t show the spherical
propagation of the electromagnetic waves or the changing frequency of the wave. Note that it’s possible to send multiple
bits at the same time which we’ll explore later. Third, we have the expanding spheres visualization,
which gives a sense of the true near-omnidirectional emission of electromagnetic waves from your
smartphone and headphones, but it’s difficult to show the frequency or the data that’s
being sent, and it's rather visually complex to process. And last, we have the simplified spheres,
which help us see that these two devices are emitting and receiving electromagnetic waves
along the same frequencies, but it doesn’t show us much else. Different visualizations are useful in different
scenarios, and with that covered, let’s get back to the focus of this video. As mentioned, Bluetooth operates at around
123 millimeters of wavelength, but specifically, it oper-ates between 120.7 millimeters and
124.9 millimeters of wavelength in the electromagnetic spectrum. Note that, these frequencies are more commonly
referred to as having a 2.4 to 2.4835 Gigahertz frequency band-width or range. Just as our eyes see within a range on the
electromagnetic spectrum, Bluetooth anten-nas see or perceive within their own range of
frequencies . Now, at any given time, there might be dozens of people using Bluetooth
devices at the same time in the same room. To accommodate so many users, this section
of the electromagnetic spectrum is broken up into 79 different sections or channels,
with each chan-nel having a specific wavelength for a 1, and another for a 0 and at any given
moment your Smartphone and earbuds communicate across just one of these channels. For example, these are the frequencies for
a 1 and a 0 in channel 38, whereas these are the frequencies for channel 54. Now this begs the question: if dozens of devices
are using the same wavelengths and possibly the same channel, how do your earbuds receive
long strings of binary bits, or messages from your phone exclusively. Well first, the messages are assembled into
packets. In each packet, the first 72 bits are the
access codes that synchronize your smartphone and earbuds to make sure that it’s your
specific earbuds that receive the message. These access codes are similar to the address
words on a postal letter or package. Just a few lines of writing and a stamp can
send a letter, which is seemingly identical to millions of other letters, to the exact
house or address anywhere in the world. The next 54 bits are the header which provides
details as to the information being sent, which in our analogy can be equated to the
size of the letter or the box. And the last 500 bits are the actual information
or payload, kind of like the contents of our postal letter or box, which in this case are
the digital 1s and 0s that make up the audio that you are listening to. If you’re wondering how audio can be represented
by 1’s and 0’s take a look at this episode on audio codecs. Ok, so now let’s add more complexity to
the mix. As mentioned, Bluetooth operates in a set
of 79 dif-ferent channels. However, when your smartphone and earbuds
communicate, they don’t stick to a single channel, but rather they hop around from channel-to-channel
kinda like channel surfing on your TV. In fact, this hopping between the 79 channels,
which is called frequency hopping spread spectrum, happens 1600 times a second, and after each
hop one packet of information composed of the address, header, and payload, is sent
between your smartphone and earbuds. Your smartphone dictates the sequences of
channels it will hop to, and your earbuds follow along. Furthermore if one of the 79 channels is noisy
due to interference or is crowded with other users, then your smartphone adapts and doesn’t
use that channel until the noise clears. This channel hopping also prevents anyone
from eavesdropping on the information that is being sent between the two devices, because
only your smartphone and earbuds know the sequence of channels that they will communicate
across. Interestingly, because the information is
divided and sent using packets, if your earbuds don’t receive one of the thousands of packets,
it says it didn’t receive that particular one , and your smartphone sends the packet
again. It might seem crazy or mind blowing that the
circuitry in your phone can generate pulses of electro-magnetic waves a million times
a second at very specific frequencies and then have these pulses received and decoded
by your earbuds- but hey- it happens. Just think about how your screen has millions
of pixels, also emitting specific frequencies and strengths of the electromagnetic spectrum,
or light at around 30 to 60 or more times a second. Technology is fascinating. One quick side note: We would greatly appreciate
it if you could take a second to like this video, sub-scribe to the channel, comment
below, and share this video with others. A few seconds of your time can help us to
create many more educational videos. Thank you! Okay, let’s move on. One point of interest is that Bluetooth’s
frequency range of 2.4 Gigahertz to 2.4835 Gigahertz is shared by other industrial and
medical devices. For example, your microwave is in this range
and has a frequency of 2.45 Gigahertz. In fact, when your microwave is on, it can
cause your head-phones to lose track of the 1s and 0s being sent by your smartphone, or
in other words your headphones can lose signal. However please don’t think your Bluetooth
headphones are dangerous because they emit a wavelength that’s similar to your microwave’s. That would be like comparing the light output
from stadium floodlights to the light from your smartphone screen, and saying that, because
they both use the same colors of light, they will both cause damage when stared at from
a foot away. Also, remember we mentioned that the electromagnetic
waves from Bluetooth can easily travel through obstacles such as the walls of your house? However, the walls of the microwave are designed
to block waves of this frequency. You can test this by putting your smartphone
in the microwave; the Bluetooth signal from your smartphone to your headphones will be
blocked, and the connection lost. However, make sure NOT to turn on your microwave
with any electronic devices inside of it, I repeat, do NOT turn on your mi-crowave otherwise
it WILL damage whatever electronics you put into it. In addition to microwave ovens, 2.4 Gigahertz
Wi-Fi networks also operate within this range of the electromagnetic spectrum. Similar to Bluetooth, Wi-Fi networks divide
this range or bandwidth into 14 chan-nels in order to accommodate multiple users communicating
via Wi-Fi at the same time. You might be won-dering, if there are a bunch
of different devices all sharing similar frequencies, one of them being a microwave that, if poorly
shielded, can emit stray electromagnetic waves, how is it possible for your smartphone and
headphones to send megabits of data every second, error free? Well, as mentioned earlier, your smartphone
does this by frequency hopping, and utilizing packets. In addition to that, Bluetooth also utilizes
bits for de-tecting errors and the circuitry in your smartphone filters out unwanted noise. For a non-technical under-standing of this,
let’s go back to our traffic light analogy. When you’re driving and you see a traffic
light, it’s not like that’s the only thing you can see. Your eyes perceive a rather complex scene
filled with tons of other objects. Your brain interprets this information-filled
scene and picks out the information important to you, while ignoring all the objects that
aren’t. Similarly, your smartphone and wireless headphones
have rather complicated circuitry inside a specialized Bluetooth microchip that’s designed
and tested by engineers, which filters out unwanted signals, checks for errors, coordinates
the frequency hopping, and assembles the infor-mation into packets thereby enabling reliable and
secure communication. Before we move onto some higher level-engineering
concepts, we’d like to take a few seconds to thank KIOXIA for sponsoring this video. Many Bluetooth devices such as mobile phones
and tablets use KIOX-IA BiCS Flash Memory. KIOXIA also manufactures a wide variety of
SSDs and they have sponsored a couple of our videos that explore the inner workings behind
how SSDs work. Here’s a consumer class SSD, versus this
enterprise class SSD. They look similar from the outside but are
entirely different on the inside. KIOXIA pro-vides these leading quality enterprise
class PCIe NVMe solid state drives, and they can fit in the same space, but have capacities
up to a whopping 30 Terabytes, and use a proprietary architecture built with their own controller,
firmware, and BiCS Flash 3D TLC memory in order to deliver incredibly high sustained
read and write performance and reliability. Check out KIOXIA’s SSDs using the link in
the description. Let’s move on to even more complicated details
regarding Bluetooth. The scheme of sending a digital signal, or
a binary set of 1’s and 0’s by transmitting different frequencies of electromagnetic waves
is called frequency shift keying. Frequency shifting means that we adjust the
frequency, and keying means that a 1 is assigned to one frequency, and a 0 to another, just
like our traffic light colors. Note that the comparison to a traffic light
which emits one color and then another is a little inaccurate because your smartphone’s
circuitry generates one frequency, called a carrier wave. This circuitry shifts the carrier wave to
a higher frequency when it wants to send a 1 or to a lower frequency when it wants to
send a 0. This shifting of frequencies in order to send
information is also called frequency modulation, and it’s closely related to FM radio. That being said, Bluetooth isn’t limited
to using just frequency shift keying; but rather it can also use other properties of
electromagnetic waves to transmit information. A different method that has higher data transfer
rates is called phase shift keying, which is a significantly more complicated to explain
but we’ll try. An electromagnetic wave’s phase is a property
that our eyes can’t perceive, and it shouldn’t be confused with the amplitude or the frequency
or the wavelength. Let’s use an analogy. Imagine you’re at the beach and you see
the waves hitting the shore at a rate of one wave a second. Over a minute you would see 60 wave peaks
reach and break on the shoreline. Changing the frequency would be changing how
many wave peaks reach the shoreline every second and changing the amplitude would be
changing the height of the peaks and troughs of the waves. However, phase shifting would be seen as breaking
up the waves’ locations of the peaks and the troughs within a set of wavelengths. There are still 60 waves over an entire minute,
meaning the frequency doesn’t change, but as the phase shifts, it’s as if the peaks
and troughs shift forward or backward within a set of wavelengths. Bluetooth antennas and circuitry in your smartphone
and wireless earbuds can be designed to emit and detect shifts in the phase of an electromagnetic
wave, and binary values can be keyed, or assigned to dif-ferent levels of shifts in the phase
of the wave. There are a few things to note with our examples
and explanations. We’ve talked a lot about your smartphone
sending information to your wireless earbuds; however, your earbuds also send data to your
smartphone. For example, when you’re on a phone call
using your earbuds, the audio from the microphone in your wireless headphones is obviously sent
back to your smartphone. In order for Bluetooth to accommo-date this
back-and-forth conversation, the smartphone and the headphones alternate transmitting
and re-ceiving data, while maintaining the frequency hopping schedule. During one 625 microsecond timeslot, your
smartphone will send one packet of data to your headphones along one channel, and then
during the next 625 microsecond time slot your headphones will send one packet of data
to your smartphone along the next channel in the frequency hopping schedule. Also, as we mentioned earlier, a Bluetooth
packet is composed of 3 sections: access codes of 72 bits, a header of 54 bits, and for example
a payload of 500 bits. The number of bits in the access codes and
header are pretty close to those mentioned, however the size of the payload which is specified
using the header can vary widely between 136 bits and 8168 bits depending on the requirements
of the data being sent. For exam-ple, simple commands from your headphones
like pause or play the music would require far fewer bits than sending or receiving high
quality audio. An additional caveat is that the electromagnetic
waves sent and received from the antenna in your smartphone and earbuds, and the light
from a traffic light, share the aspect that they both function within the electromagnetic
spectrum. However, the principles that govern how your
smartphone and headphones gen-erate and receive those electromagnetic waves are quite different
from the principles around how traffic lights and your eyes work. It’s kind of like how fire and an electric
radiator both generate heat but using vast-ly different methods. The principles behind Bluetooth fall under
the category of antenna theory and will be explored in a separate episode. Thus far we’ve made a few episodes that
help to explain other parts of these wireless headphones such as noise cancellation and
the audio codec, and we’ve made even more episodes that dive into the dif-ferent parts
of your smartphone. Check them out to learn about these other
fascinating technologies. We believe the future will require a strong
emphasis on engineering education and we’re thankful to all of our Patreon and YouTube
Membership Sponsors for supporting this dream. If you want to support us on YouTube Memberships,
or Patreon, you can find the links in the description. You can also provide additional support by
subscribing, liking this video, commenting below, and sharing this video with others. This is Branch Education, thanks for watching!
Idk how advanced your signals course was in college but Bluetooth is too advanced to cover in an introductory SS course.
This is a fantastic video. I learned a lot.
Specifically what stood out to me was not to turn the microwave on with electronics inside.
I am an electromagnetic wave. Imma go zero kelvin ahhhhhh
If you want to get the gist of the "error detection" he quickly talks about, one of the first algorithms we put in place to do exactly that is called "hamming codes" and gives a good introductory idea to what kinds of things you can do to spot errors with as little redundancy as possible
https://www.youtube.com/watch?v=X8jsijhllIA
Most modern things will now use Reed-Solomon codes which are way more complicated
https://en.wikipedia.org/wiki/Reed%E2%80%93Solomon_code
The guy who made the video likes AirPods lmao
I hope to teach communications electronics next spring at a Junior College (I have before), and will use this Youtube channel for some of it.
Why does it choose to vary the wavelength to send either a zero or a one instead of varying, say, the amplitude ; or sending nothing/something ?
Android + AirPod🥴
Good stuff!!!