Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC (MacBook Air M4)
Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC explained, with the MacBook Air M4's own measured figures as the worked example: the codec determines how audio is compressed for transmission....
Tom Halvorsen
The gist of it
MacBook Air M4, runtime 16h 20m mixed use — Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC. Measured on hardware rather than argued from theory, on a device we rate for the best trackpad on any laptop.
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 MacBook Air M4 as a worked example. A ten-minute 4K export finished in 4m 12s with no fan to spin up. That is the difference this makes once you stop reading specifications and start using the thing.
| Specification | MacBook Air M4 |
|---|---|
| Display | 13.6in, 2560 x 1664, 60Hz |
| Chip | Apple M4 |
| RAM | 16GB |
| Storage | 512GB |
| Battery | 53.8Wh |
| Weight | 1.24kg |
| Runtime | 16h 20m mixed use |
The same concept across four 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 |
|---|---|---|---|
| MacBook Air M4 | 13.6in, 2560 x 1664, 60Hz | £1199 | 5 / 5 |
| HP Spectre x360 14 | 14in OLED touch, 2880 x 1800, 120Hz | £1499 | 4 / 5 |
| Acer Aspire 14 AI | 14in IPS, 1920 x 1200, 60Hz | £649 | 3 / 5 |
| Dell XPS 14 | 14.5in OLED, 3200 x 2000, 120Hz | £1749 | 3 / 5 |
What good actually looks like
It helps to have a number in mind before you start comparing. Across the laptops 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 MacBook Air M4 reports 13.6in, 2560 x 1664, 60Hz for display, which sits comfortably on the right side of that line, and Apple M4 for chip. Treat anything materially below those as a real compromise and anything materially above as a number you are unlikely to feel.
Common misconceptions
- LDAC always sounds better, when it drops to lower bitrates under congestion
- Codec choice matters more than tuning or fit — it does not
- You can hear the difference on any source material
Checking it yourself
- 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 MacBook Air M4 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.
Where it shows up on a spec sheet
On a laptop listing this usually appears as the display line — the MacBook Air M4 quotes 13.6in, 2560 x 1664, 60Hz. That figure is necessary but not sufficient: the chip entry (Apple M4) 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 laptops 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 (Razer Blade 16) and Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC (ThinkPad X1 Carbon Gen 13).
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.
