Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC (Pixel 9a)
Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC explained, with the Pixel 9a's own measured figures as the worked example: the codec determines how audio is compressed for transmission. It mat...
Aisha Bello
In brief
Pixel 9a, screen-on time 7h 05m — Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC. Measured on hardware rather than argued from theory, on a device we rate for better battery life than the Pro.
The codec determines how audio is compressed for transmission. It matters less than fit and tuning, but it is not nothing.
Why this matters: Codec support is one of the few hard compatibility facts worth checking before buying wireless audio.
The mechanism
SBC is the mandatory baseline every device supports. AAC is efficient and what Apple devices use. aptX variants target lower latency. LDAC carries the most data but needs a supporting source and drops back under interference. A codec sets a ceiling on quality; it does not guarantee it.
What it looks like in practice
Take the Pixel 9a as a worked example. In daylight we struggled to tell its photographs from the 9 Pro. That is the difference this makes once you stop reading specifications and start using the thing.
| Specification | Pixel 9a |
|---|---|
| Display | 6.3in OLED, 120Hz |
| Chip | Tensor G4 |
| RAM | 8GB |
| Battery | 5100mAh |
| Charging | 23W wired |
| Weight | 186g |
| Screen-on time | 7h 05m |
The spread across devices
One worked example is an anecdote. Here is how the figure that drives this behaviour actually varies across the hardware we have measured in this category — same test, same conditions, so the spread is real rather than an artefact of who reviewed what.
| Model | Display | Price | Our score |
|---|---|---|---|
| Pixel 9a | 6.3in OLED, 120Hz | £499 | 5 / 5 |
| Pixel 9 Pro | 6.3in LTPO OLED, 120Hz | £999 | 4 / 5 |
| iPhone 16 Pro | 6.3in LTPO OLED, 120Hz | £1099 | 4 / 5 |
| Galaxy S25 Ultra | 6.9in LTPO AMOLED, 120Hz | £1249 | 4 / 5 |
What counts as good here
It helps to have a number in mind before you start comparing. Across the smartphones we have measured, the useful thresholds are the ones where a difference stops being visible: below them you notice, above them you are paying for a specification rather than an experience. The Pixel 9a reports 6.3in OLED, 120Hz for display, which sits comfortably on the right side of that line, and Tensor G4 for chip. Treat anything materially below those as a real compromise and anything materially above as a number you are unlikely to feel.
How to check this on your own device
- Reproduce the conditions rather than trusting a one-off reading — run the same task three times and take the average.
- Note the figure your device reports and the figure it actually achieves; on the Pixel 9a those diverge under sustained load.
- Change one variable at a time. Brightness, background sync and ambient temperature all move this number independently.
- Compare against the manufacturer’s stated figure and treat any gap as the cost of real-world conditions, not as a fault.
- Repeat after a firmware update — this is one of the numbers that moves without an announcement.
Reading it off the spec sheet
On a smartphone listing this usually appears as the display line — the Pixel 9a quotes 6.3in OLED, 120Hz. That figure is necessary but not sufficient: the chip entry (Tensor G4) constrains it in practice, and the two are almost never presented together. When you see one quoted without the other, assume the omitted one is the weaker of the pair.
Putting it to work
Check your phone supports the codec before paying extra for it, and prioritise fit and tuning over the acronym.
Buy for fit and tuning first. Treat the codec as a tie-breaker.
Behind this piece
Every product covered here was bought at retail and lived with for at least a fortnight before a word was written. Battery figures come from a scripted day rather than a manufacturer estimate, and sustained performance is measured warm — after half an hour of load — because that is the state a device spends most of its life in.
Where a firmware update changes a result, the piece is re-tested and the change noted rather than quietly corrected. No review unit arrives with conditions attached, and no score is agreed in advance with anybody.
Where to go next
If you are weighing this against the rest of the smartphones we have covered, the category archive lists every review in date order. The closest comparisons we have published are Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC (Xiaomi 15 Ultra) and Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC (iPhone 16 Pro).
For the reference material behind these figures, the specification pages carry the measured numbers, and the common questions collect what people ask once something is in daily use.
