Causes of #DuoLingo#Bluetooth / #BluetoothAudio#latency
Not very timely, as DuoLingo starts laying off translators, but it might have implications for other apps and bluetooth audio.
Duolingo has noticeable latency issues on some bluetooth headphone setups, with threads on reddit and stack exchange like "Duolingo app Bluetooth delay", but no real solutions. On general bluetooth latency there is a Soundguys article (https://www.soundguys.com/android-bluetooth-latency-22732/) which suggests it should not be this bad in 2023, and a Pocketnow article suggesting Bluetooth 5.0 should improve things. I got my hands on a variety of bluetooth headsets and tried testing.
Testing on Samsung S23 Galaxy, supports Bluetooth 5.3. All tests with default connection settings (not adjusted via developer options, although for most of these devices bluetooth developer options don’t seem to offer many choices other than default).
Using phone audio or wired headphones via USB-C adapter (Anker USB C to 3.5mm Audio Adapter, A8195)
There is no noticeable latency. For each question where there is speech it plays without delay when the question comes up (if it is a spoken question) or when a word in the course language is pressed (for multiple selection word banks). Similarly the correct and incorrect answer sounds and lesson complete sounds play on tapping the corresponding "next" button without appreciable delay.
Anker Soundcore Spirit, A3403 (original spirit, ~2018). Bluetooth 5.0 (connects SBC). Sport style earbuds (with connecting wire and control bar). Very noticeable latency. In questions where speech plays it is slow to start, and the fade up is slow, so the start of the phrase might not be heard. For word-banks the word plays noticeably after the word has been tapped, similarly for correct/incorrect answer sounds and lesson complete.
Sony WH-1000XM3 (2018). Bluetooth 4.2. Over ear noise cancelling. Snappy response, very hard to differentiate from device audio or wired connection.
Kitsound/KS Harlem (~2017). Bluetooth 4.1. Over ear budget. Response similar to Sony, maybe a little more latency.
Belkin Soundform Bolt (2023). Bluetooth 5.2 (connects AAC, offers SBC). Completely wireless earbuds (Samsung Galaxy Bud style). A slight noticeable lag compared to the wired and over ear headphones, but less than soundcore spirit. Usable, possible to forget the lag is there.
Overall, bluetooth version seems less important than format. The over-ear Bluetooth models are more responsive and the Sony WH-1000XM3 is hard to differentiate from a wired connection, despite being Bluetooth 4.2. More aggressive power saving in earbuds compared to over-ear models may lead to more latency. The longer volume fade up of the Soundcore Spirit may also contribute to perceived latency and difficulty hearing the start of sentences, but the time to start playing still seems noticeably longer than for the Soundform bolt (based on word bank sound where the fade up doesn’t occur if words are already playing).
(As an aside, I got the Soundform Bolts recently to see if they'd address this issue, they do, but I wouldn't wholeheartedly recommend them: reluctant to switch pairing between devices, no volume control via the buds, can only turn off by replacing in the case.)
It's a bit buried, 'Broadcasts' under the 'watch' menu. Live ones are at the top, you may need to scroll down a bit of they're not currently playing. @lichess