Electronics notes/Bluetooth notes
Bluetooth works in 2.4 - 2.4835 GHz, in 79 separate 1Mhz channels
Bluetooth continuously hops channels, to lessen the probaility of consistent interference with other devices and other protocols (in particular 2.4GHz WiFi). Busy areas still have congestion problems, though.
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Versions and speed
- Bluetooth 1.1 and 1.2 versions is 1mbit in theory, ~700Kbit/s in practice
- Bluetooth 2's EDR can reach maybe 2Mbit in practice,
- Bluetooth 3 plain is still that ~2MBit
- Bluetooth 3 HS can do ~24Mbit -- if and when it can basically hand over to WiFi(verify)
- Bluetooth 4 didn't really change speed(verify), but changed low power details
- BLE (max 1Mbit) is a new stack alongside the classic Bluetooth 1..3 one, because it was actually developed elsewhere and merged into bluetooth 4
- Bluetooth 5 ups BLE to 2 Mbit/s at shorter range
Range is primarily a function of output power, and is either:
Class 2: ~9m, which is what many portable devices do.
Class 1: Up to ~100m, though in practice it may be more like ~30m
Dongles may well be class 1, but many devices are Class 2, which lessens collision/congestion, lowers power use, and is enough for devices like PDAs and in particular headsets.
Headphones (and speakers)
When looking for wireless headphones, look at the profiles.
Headphones meant for music may (and bluetooth speakers and such usually will) support A2DP instead to get music-quality sound (not audiophile guaranteed, but good enough for the rest of us). (Note that A2DP also supports the lower-quality modes, and the devices may also do HFP and/or HSP for compatibility, so configuration can matter)
A2DP devices tend to also support remote volume control via AVRCP.
When considering bluetooth sound (e.g. music around the house), keep in mind that many transmitters are class 2 and so do at most 10 meters (...both ends matter, since both are transceivers), and that limited bandwidth may may degrade quality (instead of cutting out. In particular some headsets seem to do this).
There are some Class 1 headphones out there. They (almost per definition) use more power, so tend to have shorter battery life.
If you like your own headphones, you can buy a battery-powered A2DP-capable receiver and plug your own favourite headphones into those. These are usually class 2 devices. Class 1 variants are regularly mains powered (their point apparently being less cable clutter).
Some choices in bluetooth itself means that for audio, it's never going to get below 20-40ms or so.
And for a few designs it may actually be this low, but this is fairly rare. A lot of bluetooth headphone are in the 100-200ms range.
Add unknown bluetooth audio transmitters, or smartphones that are trying to avoid choppines from momentary cpu load, the system-total latency may be 200-300ms. Even 500ms isn't crazy.
Compare this with latency of wired headphones is necessarily almost nothing.
Note that RF used to be all analog and basically isntant, but RF is now often digital. Typically better than bluetooth, but
So, you probably won't notice audio is 30ms late for movie watching, that's one or two frames.
And when listening to just music, there is nothing to be off.
But there are uses where it quickly matters.
Gamers will care as soon as they hear anything is over a few milliseconds (whether it's gearheadery or actually matters, frankly).
If you're a musical artist, then you have microphone or instruments you probably want to match rhythm/offset. Brains are good at compensating and not even hearing small amounts (mic'd guitar cabs are also a few few ms just because physics). And in DAWs yo can offset recordings. Yet "150ms" and "by ear" in the same sentence is going to make many people unhappy.
There are specific fixes. For example, smart TVs may intentionally play the video late (configurable, possibly detectable - see e.g. iOS Wireless Audio Sync) so that things match up again. Great for movies and streaming and all that.
But you can't do that for gaming or music production, because you can't really delay the physical input that you do.
The codec stuff is a little confusing. aptX itself is isn't low-latency (often still 50-150ms), aptX LL is.
For many, basic latency is high and aptX LL is low, though for a few latency without LL is already low(verify).