01 · Anatomy of an earbud

In the story: before the first byte, we had to know what we were talking to. A Mac sees “a pair of headphones”; inside each bud is a computer with sensors, a radio and memory where every setting lives.

Before we send a single byte, we should know what we’re talking to. A true-wireless (TWS) earbud is a complete computer, with a battery, a radio, a processor, memory, sensors, microphones and a speaker, small enough to sit in your ear canal. Every setting Earshot changes ends up somewhere in this small machine.

The model in these plates was built in Blender for this manual (tools/earbud_blender.py). It is stylised, but every part is one a real stem-style bud has.

Pulled apart

Going from the ear outwards:

The same parts, as a catalogue

The case is a computer too

The charging case has its own battery, charger chip and a small controller.

• A Hall-effect sensor notices the lid magnet. A Hall sensor is a chip whose output voltage changes in a magnetic field. Opening the lid wakes the buds, which is when they reconnect to your phone or Mac.
• The case reports its own battery level to the buds.
• The buds relay that level to whatever is connected, which is why Earshot can show a case battery at all.
• When the lid is shut and the buds are inside, the case often can’t be read. Earshot treats a case level of 0 with no charging flag as “unknown”, not “empty” (see chapter 07).

Who talks to whom inside

The SoC is the centre of everything. It connects to the other parts through a handful of standard on-board buses:

• SPI (Serial Peripheral Interface): a fast clocked bus to the flash chip. Firmware and settings are read and written over it.
• I²C (Inter-Integrated Circuit): a slow two-wire bus for configuration chips like the PMIC and sensors.
• I²S / PDM: digital audio buses. Digital mics usually send PDM, a fast stream of single bits whose density encodes the sound. The SoC turns that into ordinary samples.
• DAC → amplifier → speaker: numbers become a voltage that drives the voice coil.
• GPIO / ADC: general-purpose pins and analog-to-digital converters for the touch pad, IR sensor and LED.
• RF: the radio front end and antenna.

You never see these buses, and Earshot never touches them. The firmware does. What Earshot sends is a request like “set ANC to deep”, in the firmware’s own language, over Bluetooth. The firmware then writes the flash and reconfigures the DSP itself. That separation is why talking to earbuds is safe: we can only ask for things the firmware already knows how to do.

How noise cancelling works

Sound is a pressure wave: tiny fast rises and falls in air pressure (chapter 02). If you add a second wave that falls exactly when the first rises, the two cancel. Active noise cancelling (ANC) works like this:

1. The feed-forward mic hears outside noise before it reaches you.
2. The DSP computes the inverted wave (180° out of phase), predicting how the noise will change on its way through the shell, and plays it through the speaker along with your music.
3. The feedback mic inside measures what is left and corrects the filter continuously.

Noise cancelling works best on steady low sounds (engines, fans) and worst on sudden high ones (voices, clatter). Those are blocked mostly by the ear tip’s physical seal.

Transparency (“Ambient” in Earshot) does the opposite on purpose: it plays the outside world through the mics so you can hear it. Off turns both processes off.

realme buds expose four ANC strengths, which are simply different filters:

The codes aren’t in order of strength, a reminder that protocol codes are labels, not numbers to compare.

What this means for software

Everything Earshot shows or changes maps onto these parts:

Next: 02 · Sound to signals to bits →