Latency

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Latency is a synonym for delay. It is the discrepancy in time between when an audio signal enters a system or electronic device and when it emerges.

For assistive listening systems, no or minimal latency is associated with a positive user experience, while high latency is associated with a poor user experience. Even claimed “low” latency can create a poor user experience.

Latency is measured in milliseconds (ms).

  • Humans are very sensitive to audio and acoustic delays and can detect differences of even a few milliseconds depending on the circumstances. At low values of delay, this tends to occur as a perceived change in audio quality before continuing on to where two separate sounds are heard.
  • Common latency measurements: Classic Bluetooth 100+ ms; hearing instruments < 10 ms.
  • Properly installed hearing loops, FM and infrared systems should have virtually no latency.
  • Latency is additive. Assistive listening system latency + receiver latency + hearing instrument latency

Potential causes of latency in an audio system include analog-to-digital conversion, buffering, digital signal processing, transmission time, digital-to-analog conversion and the speed of sound in the transmission medium. For assistive listening system applications, the latency should be less than 50ms and less than 15ms for hearing instruments where the user may also speak.

The ALS sound MUST be in sync with the PA system. If the ALS is out-of-sync, speech understanding is nearly impossible. What might a user experience with latency?

  • The lip sync is “off.” For people with hearing loss, the speech reading does not match what they hear. This causes extra effort/listening fatigue, as people with hearing loss often supplement with speech reading.
  • Hearing an echo. a) People with hearing loss, who have non-occluding ear domes, will often hear two sounds: the ambient room sound and the delayed sound through an assistive listening system. b) This can also occur when a hearing instrument enables a mix of the microphone (M) & telecoil (T) signals. For example, with an improperly ALS, the telecoil (T) signal may be heard before the acoustic (M) sound. c) People with typical hearing can replicate this “echo” sound by using an ALS receiver and headphones, but only cover one ear (one ear hearing through the ALS and one ear hearing the room’s acoustics).
  • Hearing a mushy, non-clear sound.

What does latency sound like?

  • audio clip, 47 ms latency, speaking (this website, 16 seconds)
  • audio clip, 0 to 300 ms latency, clap (YouTube, 1 minute)
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