Why is there a slight audio delay

To understand the reason behind the audio delay, we first need to learn about Bluetooth codecs.

A Bluetooth codec is an algorithm that compresses and decompresses audio data during Bluetooth audio transmission. It determines the quality and stability of audio transmission between Bluetooth audio devices. Common Bluetooth codecs include SBC, AAC, aptX and LDAC, each with different audio transmission speeds and sound quality performances.

Let's first explain the SBC codec. SBC stands for Sub-band Coding, and it is the mandatory and default codec for all stereo Bluetooth headsets. In other words, all Bluetooth headsets have the SBC codec. Additionally, AAC is typically used for Apple headphones, and LDAC for Sony headphones.

Now let's talk about the codecs supported by Avantree C81: 1) FastStream 2) aptX Low Latency 3) aptX 4) SBC. Please note the order above as they are listed by priority.

Let's take the Sennheiser MOMENTUM 3 as an example. It supports the SBC, aptX LL, aptX and AAC codecs. When connected to the dongle, we go down the line of the dongle's codecs. 1) FastStream is skipped as it's not supported by the Sennheiser headphones. Next, 2) aptX Low Latency is supported by both, so it's used.

The same applies for AirPods Pro. Since it supports AAC and SBC, the codec used between the devices will be SBC.

How about the audio delay when use different Bluetooth codec?

Now the truth has come out - when you notice audio delay, your headphones and dongle are likely using the SBC codec, with audio delay of around 220ms. If you're unsure of the codec supported by your headphones, you can search "[headphone brand] + [model] + codec" and you'll understand the situation.

The 40ms latency that aptX Low Latency and FastStream can achieve is the best Bluetooth can do, and most people won't notice it. Therefore, we now know whether there is delay depends on the headphones - changing the dongle won't help.