Since the launch of the Apple AirPods back in December 2016, the wireless earbuds market has become pretty crowded — more than 15 different brand name and new-to-the-world hardware companies have launched their own wireless earbuds in the past year, with varying levels of success. On occasion, I’ll spot someone sporting the Sol Republic Amps Air or the Samsung Gear IconX earbuds, but the current dominance of AirPods is quite clear.
If any company could take down Apple’s lead in a smartphone accessory, it would have to be Google, with the ability to fine-tune its Android operating system and push out smartphones with custom silicon of its own. On October 4, 2017, Google finally rose to the challenge, simultaneously pushing out new smartphones with no audio jacks (the Pixel 2 and Pixel 2 XL), and wireless earbuds to match — the Pixel Buds.
While a plethora of reviews exist comparing the user experience between the Pixel Buds and the AirPods, there hasn’t been a hardware comparison yet. Together with our trusty Banana-ghini compatriots from Fictiv, we tore down a set of Pixel Buds and another set of AirPods, for a side-by-side comparison. Electrical design and manufacturing observations will be addressed in this blog post, while Fictiv will cover mechanical design and manufacturing observations in their corresponding post. In both posts, credit for the sweet photography goes to Fictiv.
Bottom view of the Pixel Buds. Note the two charging contacts on either earbud.
Left earbud – disassemble! We slowly apply heat and exacto knife along the outer plastic cap rim, prying it off to reveal a coin cell battery. Strapped to the battery back side is its corresponding protection circuit module (PCM for short). Lithium-ion batteries typically include tightly integrated protection circuits, keeping them from exploding when accidental short-circuits occur. This coin cell, however, doesn’t have an internal protection circuit, so the external circuit keeps it safe.
Judging from battery dimensions and capacity, this is Varta’s largest cell, the 120mAh CP 1654 A3. Varta sells three sizes of compact coin cells for hearing aids, and a large number of competing wireless earbuds are designed around Varta’s smaller 60mAh and 85mAh cells. The AirPods went an alternate route and created their own custom-sized cylindrical battery, a pricey strategy that requires large volume orders, which puts it out of reach for hearables startups.
Protection Circuit Module (PCM) board welded to a Varta CP 1654 A3 battery. We disconnected the short flex from the ZIF connector to get a better view.
As shown below, the PCM is sparsely covered. A small four-pin ZIF connector (with the white tab) holds onto a flex that carries the audio signal to the speaker and from the charging contacts, and the one large component near the top is probably a dual-FET package, taking care of the bulk of the protecting. Soldered along the bottom are six wires, which run between the left and right earbuds. From left to right:
- The light pink and green wires serve a mystery purpose: on the PCM, they are tied together by a 105 kOhm resistor, and each is capacitively coupled to ground. On the right earbud side the pink wire connects to a diode on the main board, but is untraceable after that. Do they provide some method of accounting for voltage drop across the long, thin cables? Or are they a vestigial remnant from a previous iteration?
- The blue and coral middle wires carry the left earbud audio, and are routed to the left earbud speaker via the ZIF connector and flex.
- The black and orange wires on the right are battery ground and power, respectively, carrying power from the PCM to the right earbud during usage, and from the right earbud charging circuitry back to the battery when the Buds are charging in their case.
Judging from the insulation warp and the uneven amounts of solder on each pad, I would venture to say these wires were hand-soldered to the PCM. On the assembly line, there probably was a fixture to hold the wires and board in place during soldering, and a precision solder dispenser of some kind, but the variation suggests there was human intervention required in this step. Ultimately, this doesn’t bode well for scaling up mass production.
PCM board close-up: Uneven solder quantity, wire alignment, and insulation warping indicate the six wires were hand-soldered.
That short flex cable we disconnected earlier was fed through the plastic housing from the in-ear part of the buds, which we now pry apart from the back side. The round face of a dynamic speaker driver emerges.
Dynamic speaker drivers in both the Pixel Buds (left) and AirPods (right)
When you need to squeeze sound into small spaces, you have two driver options: dynamic or balanced armature. Dynamic drivers are cheaper and produce a more “natural” sound, but they take up a lot of space. Also, dynamic drivers produce sound by physically moving a volume of air, and so require vents in the housing on either side for air to flow in and out. The AirPods also chose to use dynamic drivers, managing to squeeze all other components behind the driver. Balanced armature drivers are much smaller but more expensive, and require more sealing to the ear to sound good. Most other space-constrained hearables opt to go this route, squeezing the balanced armature driver into the little nubbin that extends into your ear canal. Occluding your ear canal blocks out the sounds of the outside world, for better or worse. Both the Pixel Buds and AirPods chose the open ear design, which requires the use of dynamic drivers.
Balanced armature driver wedged into the housing in the Here Ones earbud
One more piece of the left earbud left – we peel apart the plastic to pull out the flex holding the charging contacts, and discover an interesting point we’ll come back to later: the charging contacts on the left earbud are, incredibly, grounded together. These two metal points don’t actually do anything. Yes, if the metal contacts in both earbuds both ran to charging circuitry, that would be overkill, since there is only one battery and the earbuds are tethered, but I would have expected the charging circuitry to be in the left earbud with the battery, to reduce voltage drop across the cable. Instead, looks like the left earbud has just dummy charging contacts.
Pried apart the inner plastic housing (left), then the speaker driver (right), to show the flex holding the left earbud charging contacts, grounded together
Alright, enough putzing around with the battery. Let’s see the brains and controls of the Pixel Buds, located in the right earbud.
The Pixel Buds support four types of control inputs, all through this right earbud. A single tap turns the Buds on, controls play/pause for music, or allows you to answer an incoming call. When not on a phone call, a long press turns on the microphone and streams what you say to the Google Assistant. Forward swipe turns volume up, and backward swipe turns volume down. This easy volume control is a definite user perk over the AirPods, where the only hands-free volume adjustment is asking Siri out loud to adjust it for you.
Under the cap of the right earbud, you see how this is done in the Pixel Buds: four capacitive touch pads in the center. Like the Here Ones and any other earbud based around the CSR8675, the Pixel Buds make use of the six capacitive touch lines that chip supports. Although the AirPods do boast a capacitive touch supporting Cypress PSoC4 chip, there isn’t a good, non-crowded inner surface to which a capacitive touch sensor could stick. Instead, the AirPods rely on an internal accelerometer to sense user input.
Four capacitive touch pads on the inside of the right earbud cap allow the Pixel Buds to detect swipes and presses.
Around the rim of the cap is the Bluetooth antenna, laser-direct sintered (LDS) onto the plastic. A single pogo pin connects the antenna to the inner circuit board. For optimum reception, you want your antenna as far away from the conductive water sack that is the human body, so into the cap it goes! Creating the antenna using LDS also saves precious space, and most existing wireless earbuds on the market use this method. The AirPods were unique in placing their antenna in the stem – they were also unique in being able to get away with having that odd stem dangling outside the ear.
LDS antenna around the perimeter of the Pixel Buds right earbud cap, and the back of the capacitive touch flex in the center (left). For comparison, the AirPods antenna runs along the back of the stem (right).
Along the same flex we have a squishy foam blob. I’ve never seen a flex with a foam piece built in before. It seems primarily mechanical in function, but it is grounded, so likely also doubles as extra electrical grounding, either for the capacitive touch sensors or for the antenna (otherwise a regular chunk of foam would have been more cost-effective). The foam blob and the pogo pin are the only things occupying the vertical space between the board and the antenna. That’s a surprising amount of unused space…
A tiny bit of foam-in-flex. That’s a rare sight!
The flex attaches to the main processing board through a board-to-flex connector, which occupies a large chunk of space here in the center. We carefully detach the connector, and after picking off some glue around the edges and breaking a few heat stakes, the main PCB comes out easily. Much less glue here than in the AirPods.
Outward-facing side of board, with the CSR8675 Bluetooth/audio SoC commanding the center.
The outward-facing side of the board has mainly passives, neatly placed around the CSR8675 chip along the bottom and an unidentified QR-coded chip on the left side. The CSR8675 is the main Bluetooth + audio chip, managing the audio streaming from your phone to the speakers, from the microphones to your phone, and possibly the capacitive touch readings. Next to it, the white rectangle with a dot is the balun for the antenna line, which you can see runs along the top layer of the PCB to the shiny pogo pin on the left.
Most non-Apple wireless earbuds are based around the best compact and integrated options available on the market, the Qualcomm CSR8670 or (newer) CSR8675 Bluetooth/audio SoCs. They are so commonly used that Goertek, the contract manufacturer that made the Pixel Buds, offers its own wireless earbuds based around the CSR SoCs, that can be customized for their clients. A more powerful Bluetooth + audio chip is coming, though: Qualcomm recently released the CSRA68100, the next generation of the CSR8675. If wireless earbuds companies geared up to begin integrating that chip last fall, we might find earbuds with the updated performance coming out in 2019.
In comparison, the AirPods’ Bluetooth is handled by Apple’s W1 chip, which grants both a seamless integration with Apple products and a huge power-savings on the Bluetooth wireless communication. Bluetooth streaming and transmit are the most power-hungry activities of wireless headphones, and the power savings helped propel the AirPods to their extended 5-hour battery life, a record that other wireless earbuds needed a year to match. As might be expected, Apple only licenses its W1 for use in Apple and Apple-related (aka Beats) products.
Before we leave this side of the board, one more empty volume question: What is that unpopulated flex-to-board connector footprint along the right edge? Probably remnants of a connector formerly used for debugging, but it sure takes up a lot of prime real estate, just sitting there.
Now we flip the board over, and whoa, look at that empty space! They even had room to leave the RF test point, that circle-in-a-circle in the bottom left of the board. RF test points are the easiest non-destructive way to check antenna performance during the manufacturing process, and we’ve seen this kind of test point left on larger boards, like the Xbees and some WiFi kits, but not in small earbuds, where that space is a premium.
Here are descriptions of the ICs labeled above:
- CY8C4146FNI-S433: the main MCU inside the Pixel Buds, this Programmable System on Chip (PSoC) is part of Cypress’ PSoC 4 family, incorporating an ARM Cortex-M0 and reconfigurable analog/digital blocks.
- This was harder to deduce just from part markings, so kudos to this post for their in-depth sleuthing, which identified it as a memory chip.
- Rclamp3324T: quick Digikey search for “3324T” brings up a Semtech ESD protection chip with a matching footprint. ESD stands for “electrostatic discharge” – all the static in your fingers and hair on dry days can fry tiny ICs, and this chip prevents that from happening.
- This 4-ball BGA component could be some kind of load switch, regulator, or amplifier.
- MAX8971G-EWP: Maxim Integrated Li-Ion Single Cell DC-DC charger. This controls the current/voltage coming in from the charging case, safely charging the Pixel Buds battery.
- BQ27421: Texas Instruments Fuel Gauge IC. This monitors the battery level during charge/discharge and sends its best estimate of battery state-of-charge to the PSoC via I2C.
- TPS743: unknown Texas Instruments chip that, judging by the naming scheme, is a flavor of LDO or buck regulator, bringing the battery voltage to a safe 3.3V to power the other components. No datasheets readily available online – but this exact same chip can be found nestled in the AirPods as well.
Nice – the Pixel Buds MCU is a PSoC, in fact the same PSoC as in the AirPods! We’re a big fan of the versatility of the PSoC platform ourselves, and have used internal PSoC blocks in previous projects to shrink circuit boards down by >50% area. The fuel gauge is also a nice touch – Li-Ion battery discharge curves have a plateau in the middle, so you’ll get more precise readings on your remaining battery charge with an integrated fuel gauge than trying to extrapolate from reading battery voltage. Otherwise, though, the extra empty space is admittedly confusing.
Almost there — just one more flex to go!
Under the board, nestled into the plastic, are the two MEMS microphones – one facing forward, one facing back. The forward microphone picks up your voice, and the back-facing microphone helps with removing ambient noise from the front microphone signal. The six cables from the left ear are also soldered down to this assembly. In a mirror to the left earbud, a small six-pin ZIF connector connects the rigid chunk of this flex to a smaller extension, which feeds through the plastic from the speaker and the charging contacts.
Front and back-facing microphones inside (left) and removed from (right) plastic housing.
The right earbud charging contacts are actually functional, unlike the left, and the two tiny components between them, judging from the pin 1 markings, are diodes.
Right earbud charging contacts, with diodes in between
Whew, done! That was a lot of connectors. Yes, they were low-profile flex-to-board and ZIF connectors, but still, having connectors takes up a lot of premium space. Let’s take a look at how compact it can get when there are pretty much zero connectors, as in the AirPods:
The single connector in the AirPods – a tiny coaxial button for the antenna. Everything else is on one single, very tightly populated rigid-flex assembly with many careful folds.
To be fair, the AirPods are probably the most compactly packed wireless earbuds out on the consumer market (Bluetooth-enabled Hearing Aids are more compact, for 10x the price). In terms of design, the Apple AirPods had to meet a specific form factor — they wanted to resemble the wired Apple Earpods as closely as possible. The Google Pixel Buds, with free rein on their industrial design, went with a shape and form closer to that of other wireless earbuds on the market, but, surprisingly, unlike other earbuds, they left a large amount of free volume inside.
From an electrical perspective, the Pixel Buds have more empty space than other earbuds for several reasons. In deciding to keep a cable between the two earbuds, the Pixel Buds were able to use only one Bluetooth radio and one battery, instead of two of each. Also, with the tether, they no longer needed to solve the problem of wireless synchronization between earbuds, eliminating the need for an NFMI chip and antenna coil in each earbud. The Pixel Buds didn’t promise any special audio filters or noise cancellation, so they didn’t need to include any additional DSPs or codecs. They didn’t implement in-ear detection – saving them the need for another sensor in the left earbud. Instead, the Pixel Buds left themselves plenty of space for relatively large connectors between boards, dummy charging contacts, and even empty space on both sides of their main board.
Google has been quoted explaining that they chose to keep the cord between the buds based on user research showing people wanted the cord, so they wouldn’t lose their earbuds. The market for Bluetooth headphones is definitely growing, but I would be curious how much of that is going towards completely wireless earbuds (other than the Airpods) vs neckbuds or over-the-ear wireless headphones. Perhaps this will prove a wise gamble.
The Pixel Buds electrical hardware didn’t innovate more than other wireless earbuds out there. Instead, it’s seems the primary feature fueling the Pixel Buds design was the EE/software integration. The hardware teams needed to produce something on which the software could be run. Google’s strength is in its software, and its software pulled through to provide a more seamless Google Assistant experience, and the bonus of Google Translate. To enable the real-time translation, specific audio routing between phone and earbuds had to be baked into the Pixel and Pixel 2 software, which is why that feature doesn’t work on other phones. In the grand voice assistant battle between Apple’s Siri, Amazon’s Alexa, and Google’s Assistant, Google is definitely trouncing Siri. Leveraging that user experience may have been more worthwhile to Google than trying to implement additional hardware features. Perhaps future iterations of the Pixel Buds will have more hardware focus. Let’s pause for a moment and imagine how crazy awesome the Google Translate feature could be with completely wireless, shareable Pixel Buds.
The Pixel Buds and AirPods represent opposite ends of the wireless earbuds spectrum. The Pixel Buds have a ton of extra volume, and their key selling point is their software integration with Google Assistant and Google Translate. The AirPods, on the other hand, are probably the most compactly packed earbuds in existence, using their W1 chip for a much-lauded seamless pairing experience. The table below summarizes some of the most salient differences and similarities.
I enjoy working at Mindtribe for many reasons, but one of my favorites is how much in-depth knowledge I find amongst my teammates. Special thanks to our in-house hearables experts Sam Evans, Rosemary Chang, Kyle Tucker; teardown enthusiasts Brian Cherbak, Freddie Temperton; and overall tech trends guru Kerry Scharfglass.