All About... Bluetooth music

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No, not another site(!), but a genuine attempt to dig into Bluetooth music, i.e. hooking up your smartphone to Bluetooth headphones and the gradual increase in audio quality over the last decade. When did it get so good and what are the underlying protocols and numbers? Here's where you need to know your codecs from your acronyms and your kilobits per second from your profiles...

Author's note: the technicalities behind wireless audio are legion. So I've simplified things below to an extent, for ease of reading and understanding.

Back in the day (2006!), Bluetooth audio was itself quite new and it was unusual to listen to music wirelessly (most people plugged in via Nokia Pop-port or something else proprietary, or - increasingly - a 3.5mm audio jack, with the Nokia N95 being the first mass market smartphone that was so equipped). And when you did hook up over Bluetooth, stepping through the pairing process and typing in the PIN numbers on your keypad, results weren't great. Quality was low, breakups were common, and even the tempo of your music would vary as the encoders and decoders struggled to keep up.

Fast forward to today and, connected wirelessly to modern smartphones, we have Bluetooth headphones that are small enough to fit inside your outer ear, have great battery life, rock solid conmections, and audio quality that rivals a wired connection. 

How has this been achieved? Through protocol evolution, codec improvement, and chipset speed/data rate increases.

First and foremost, Bluetooth itself has evolved over the years, evolving to add more capabilities, better security, and easier connections, fuelled by faster chipsets and more varied uses. With a slight focus on audio, here are the main changes (heavily summarised from Wikipedia!):

Bluetooth 1.2

  • Faster Connection and Discovery
  • Adaptive frequency-hopping spread spectrum (AFH), which improves resistance to radio frequency interference by avoiding the use of crowded frequencies in the hopping sequence.
  • Higher transmission speeds in practice than in v1.1, up to 721 kbit/s.

Bluetooth 2.1 + EDR

  • Addition of secure simple pairing (SSP): this improves the pairing experience for Bluetooth devices, while increasing the use and strength of security.

Bluetooth 3.0 + HS

  • This provided theoretical data transfer speeds of up to 24 Mbit/s, though not over the Bluetooth link itself. Instead, the Bluetooth link is used for negotiation and establishment, and the high data rate traffic is carried over a colocated 802.11 link. Not relevant to audio here, but interesting nonetheless.

Bluetooth 4.0

  • Added Bluetooth Low Energy (BLE) protocols, aimed at very low power applications powered by a coin cell (e.g. tags). Again, not relevant to audio, but interesting and notable.

Bluetooth 4.1

  • A pure software change, this improved a lot of behind the scenes protocols, including audio architecture updates.

Bluetooth 4.2

  • More software improvements, mainly for Bluetooth LE.

Bluetooth 5

  • This was a major step up in reliability, in my experience, improving speed and reliability (though not at the same time) according to use case.

Bluetooth 5.1

  • Many changes related to IoT and (again) reliability.

Bluetooth 5.2

  • This introduced 'LE Audio', running on the Bluetooth Low Energy radio and lowering battery consumption. It added features such as one set of headphones connecting to multiple audio sources or multiple headphones connecting to one source.

Bluetooth 4.x from 2010 onwards was the point at which I found Bluetooth audio to be good enough for music, for the most part. I still preferred a wired connection (and often still do), but it was ostensibly 'good enough'.

Yet there's more to the Bluetooth audio story than just the underlying protocol improvements. The second part is the maxing out of data rates, i.e. the amount of audio data being sent out by the phone per second, allied to fancy new codecs, i.e. using more sophisticated ways of compressing and decompressing the digital music streams in order to sound closer to the original 'perfect' source.*

* Of course, when talking about digital music one also has to think about the source. A decade ago, you could use a 128kbps MP3 file and issues in Bluetooth transmission and reception would mask any minor issues in the source compressed file. In 2021, the wireless channel has improved enough that for anyone serious about their music you'll either need more data per second in the source material or a better way of compressing it in the first place. For example, using 320kbps is common now for MP3, or using AAC+ encoding at 128kbps or 192kbps when ripping or creating digital music.

Bluetooth music, Marshall

All Bluetooth audio travels according to 'A2DP', which you'll have seen referenced in phone specs, this is the 'Advanced Audio Distribution Profile', but what you might like to also watch for is support of codecs and bitrates over and above the current basic SBC, see the table below.

Codec Specs, notes

Low-complexity codec, mandatory for all Bluetooth audio installations - everything supports this. Transfer rates vary according to device and signal strength between 192 and 320kbps. The latter is, of course, pretty good in terms of keeping music fidelity, though note that this compression (at the phone end) and decompression (at the headphone end) is on top of the existing source compression, so the audio artefacts do start to add up. 

Interestingly, SBC divides the signal into multiple frequency bands and encodes each one independently, a system which has worked better and better over the years as more horsepower is available to use higher quality algorithms.

aptX One of Qualcomm's popular codec schemes, this supports data rates of 352kbps, so 10% higher than SBC.
aptX HD Because the basic aptX wasn't that far ahead of SBC, Qualcomm went one better with 'aptX HD', supporting bitrates of 576kbps, and also handling data at 24-bit depth if needed. In short, well over twice as much compressed music per second, resulting in better fidelity to the trained ear.
AAC  Short for 'Advanced Audio Coding', this is sophisticated and uses 'psychoacoustic modelling', apparently. Ahem. Its Bluetooth transfer cap is 'only' 250kbps, but there's greater audio fidelity than SBC, at the expense of needing more horsepower at both ends to encode and decode. Mainly used in iOS on iPhones (to compatible equipment), which have horsepower to spare. Note that a variant of AAC is also used for original music encoding, especially on Macs, where it offers similar fidelity to MP3 at around half the bitrate. Or twice the fidelity at the same bitrate, etc.
LDAC A Sony variable bitrate codec that has seen some take-up in the Android world, with bitrates up to 1000kbps and theoretically higher quality. In practice, the limitations are usually the phone or headphones in terms of encoding or decoding this bitrate, and LDAC only achieves its potential when both ends are up to speed.

Audiophiles will champion aptX HD and even LDAC, but in practice these only have benefits when you take care to have well matched phone and headphones that also support the codecs fully. Which is rarer than you might think. In practice, I contend that a decent music source (say 320kbps MP3) encoded again at 320kbps over the standard SBC into modern and high quality (think £100+) headphones will sound superb and close enough to a wired solution.

So it's the usual law of diminishing returns - you can take more care and pay a lot more to get a little bit more fidelity, but on the whole things have improved markedly. Our 2021 phones have more than enough horsepower to decode any audio file (even if it's 'lossless'), and then re-encode at maximum bitrate for Bluetooth; and then modern Bluetooth headphones have themselves enough horsepower in their circuitry to decode these streams in high quality fashion.

So, while a debate over which codecs to research was appropriate for most people even five years ago, in 2021 I don't think it makes much difference for most people - some manufacturers have even stopped quoting codecs in spec sheets, for either phones or accessories. The quality of the music source and - even more - the speaker size, quality and seal of the physical headphones (in-ear, outer ear, on ear, over ear, etc.) are a bigger factor.

Comments welcome though. Have you researched different Bluetooth codecs and made them a part of your buying decision, and how reliable and enjoyable do you find Bluetooth audio in 2021?

PS. See also my tests on wired DACs - the other side of the audio coin, as it were!

PPS. The central photo shows my current Bluetooth set-up, the Marshall Major III Bluetooth - these 'only' support aptX (+SBC) but sound fantastic, thanks to large 4cm drivers. They used to be well over £100 but are now regularly £75 on Amazon UK. And when the batteries run out of charge (weeks, not days) I can just plug in a 3.5mm cable and carry on listening(!) Note that I'm also getting in the Marshall Major IV Bluetooth, which have no less than three big innovations over the III, though are considerably more expensive. Update: review now live.