Bluetooth is a flexible standard. It defines various profiles that operate on top of the Bluetooth networking protocol stack and implement specific services, such as hands-free communications. Bluetooth devices each support a small subset of profiles, typically only one or two, according to their designated function.
Bluetooth headphones and speakers implement the advanced audio distribution profile (A2DP). The A2DP profile transports encoded audio streams from one device to another. To guarantee compatibility between devices, any device implementing the A2DP profile needs to offer a common codec, SBC. However, A2DP supports additional codecs that may increase audio quality or reduce latency compared to SBC. One of those optional codecs is aptX. In this article, I am looking at aptX benefits and I am describing how to get aptX on Windows 10.
What is aptX?
aptX is an alternative codec for the Bluetooth A2DP protocol. It comes in multiple flavors:
- aptX LL (low latency)
- aptX HD
Is aptX better than SBC?
Qualcomm, who acquired the aptX company CSR in 2015, claims that aptX offers “superior audio”. Figuring out whether that is actually true is harder than expected. Let’s break this quality question down into two different aspects: fidelity and latency.
As this overview shows, the technical specifications of the base aptX code are similar to that of SBC. Specifically, the maximum bitrate of 352 kb/s is not much higher than SBC’s 320-345 kb/s (SBC depending on implementation).
The bitrate by itself does not tell us anything about a codec’s fidelity, however. Different codecs can be very different in encoding efficiency, as are H.264 and its successor H.265 for video (the latter only needs about half as many bits for the same visual quality). Fidelity can only be determined by double-blind listening tests. Unfortunately, nobody seems to have performed such listening tests in a scientific, reproducible manner (at least I could not find any information). Please let me know by commenting below if you know of any publications.
Another aspect influencing fidelity is that implementations do not always use a codec’s maximum bitrate. SBC, for example, comes with three quality modes, low (201 kb/s), middle (229 kb/s) and high (328 kb/s). Devices may select lower-quality modes to favor a stable connection over sound quality (example: these Sony headphones).
When watching a video, you do not want the audio to lag behind. In other words: the audio latency should ideally be small enough to not be noticeable. Unfortunately, that is often not the case.
Bluetooth audio introduces significant latency, the exact amount of which depends on the codec as well as its implementations in the sending and receiving devices. Most Bluetooth headphone reviews do not include latency measurements, with the notable exception of rtings.com. They even have a list with latencies for all the headphones they ever tested. Plain aptX seems to be slightly better than SBC, but only aptX LL seems to be good enough to not be noticeable.
Audio Quality Summary
There do not seem to be any objective comparisons between the aptX variants and SBC. From my experience it should be safe to say the following:
- SBC offers pretty good fidelity at maximum bitrate.
- aptX might be slightly better than SBC at maximum bitrate.
- Both SBC and aptX introduce significant latency in the range of 150-200 ms.
- aptX LL drastically reduces latency but is only supported by a handful of devices.
aptX on Windows 10
If Microsoft wanted to hide this piece of information, they could not be doing a better job. A single page on microsoft.com mentions aptX. Apparently, Windows 10 has supported the aptX codec since the first release (1507). According to this Reddit thread, Windows 10 aptX support does not require any drivers in addition to what is part of the OS. I am mentioning that because numerous forum posts state you need to install special drivers. That does not seem to be true.
As for aptX HD or aptX LL (low latency): those codecs do not seem to be supported. If you would like to have them in Windows – especially aptX LL would be great when watching video – make sure to vote for this item in Feedback Hub: please add aptX Low Latency codec support to the Bluetooth A2DP driver.
For a list of supported Bluetooth versions and profiles see this page.
External USB Adapter
The Avantree Audicast is a flexible Bluetooth transmitter that can be connected to a PC (via USB) or a TV (via optical input or headphone jack). The Audicast has several neat features:
- Support for aptX LL (low latency) in addition to regular aptX and, of course, SBC
- LEDs indicate which codec is being used
- Two receiving headphones can be connected
- Small and light
- No additional power source except for USB required
- All cables are included
Please note that when connected to a PC, the Audicast does not work as a generic Bluetooth adapter (in which case the OS drivers would be used). Instead, it registers as a USB audio device. No drivers are required.
Which Codec and Bitrate are Being Used?
Amazingly, Windows does not provide any tool or API for monitoring the codec used by A2DP. Whether it is SBC, aptX or something different – users are left completely in the dark. To help change that and encourage Microsoft to provide more visibility, please vote for please let users see what Bluetooth A2DP codec is used.
For the sake of completeness I sent several hours capturing and analysing ETW logs as indicated at the following source, but none of the generated logs seemed to indicate the A2DP codec being used.