Monitoring

Monitoring

Passthrough and heterodyne monitoring modes, output routing, haptic feedback, and background streaming.

Overview

Chirps offers three monitoring modes that control how audio is routed from the microphone to the device’s output. Monitoring is independent of recording — you can monitor without recording, or record silently without monitoring.

Off — no audio output. The stream runs and the display updates, but nothing is sent to the speaker or headphones.

Passthrough (PT) — the input signal is routed directly to the output with minimal latency. You hear what the microphone is capturing in real time.

Heterodyne (HET) — ultrasonic frequencies are shifted into the audible range using frequency mixing. This is the technique used in traditional bat detectors.

Switch between modes using the PT and HET toggle buttons in the control bar. Tapping a button activates that mode; tapping the active button again returns to Off. Only one mode can be active at a time.

Passthrough monitoring

In passthrough mode, Chirps adds the incoming audio signal to the output at the gain level set by the Monitor Gain slider.

Use cases:

  • Listening to bird calls or insect sounds through headphones in the field
  • Feeding audio from a directional microphone to headphones without introducing a separate monitoring device
  • Checking microphone placement — you can hear the signal while watching the spectrogram

Output device: Monitoring plays through the current iPhone or iPad output route, such as the speaker, wired headphones, or AirPods, even when the input is an external microphone or sound card. If the external device has output capabilities and iOS keeps that route active, the output is routed there.

Monitor Gain: Adjust the output volume using the Monitor Gain slider in the control bar. The slider ranges from 0.00 to 1.00 — a value of 1.00 passes the signal at full level; lower values reduce the output. The default is 0.10.

Heterodyne monitoring

Heterodyne monitoring detects ultrasonic signals by mixing the incoming audio with a tunable carrier frequency. When an ultrasonic signal is close to the carrier frequency, the mixing produces an audible beat tone. The further the signal is from the carrier, the higher the beat frequency.

This is the same principle used in analogue bat detectors. A bat call at 45 kHz produces an audible tone when the carrier is set to 45 kHz; the tone shifts in pitch as the bat changes its call frequency.

Setting the carrier frequency: Use the Heterodyne Frequency slider in the control bar. The range spans from 1 kHz to half the preferred sample rate configured in Settings. At a preferred sample rate of 192 kHz, the slider reaches 96 kHz.

You can adjust the frequency while monitoring is active without pausing or muting the output. The pitch of the beat tone changes continuously as you drag the slider.

Recommended approach for bat survey:

  1. Start with the carrier set to the most common frequency for the target species (e.g. 45 kHz for Pipistrellus pipistrellus, 25 kHz for Nyctalus noctula).
  2. Watch the spectrogram to see where calls are appearing.
  3. Adjust the carrier to match the peak call frequency for maximum output sensitivity.

Monitor Gain: The same gain slider applies in heterodyne mode.

Haptic feedback: Chirps can add tactile confirmation while HET is active. See the dedicated Haptic Feedback guide.

Output routing

By default, Chirps uses Automatic output routing. iOS selects the output device — typically wired headphones or the active external output if connected, otherwise the built-in speaker.

For some configurations you may want to force output to the device’s built-in speaker. Set Output Route to Device Speaker in Settings → Output.

Note: Device Speaker is available only when iOS can keep that combination stable. Paired input devices, headset microphones, and some Bluetooth HFP routes require Automatic routing.

Haptic feedback

When heterodyne mode is active, Chirps can add tactile pulses on supported devices when the live input crosses a chosen threshold.

  • Enable it in Settings → Haptic Feedback → HET Haptic.
  • The threshold is tuned for field use over a 0.01–0.50 range.
  • The trigger follows the live input level, so changing Monitor Gain does not change when haptics fire.
  • The feature is available only on devices with supported haptic hardware.

For setup advice, supported-device guidance, and threshold tips, see the dedicated Haptic Feedback article.

Background streaming

By default, Chirps continues the audio session when the app moves to the background (you press the Home button, switch to another app, or lock the device). Any recording in progress continues uninterrupted.

When background streaming continues, Chirps sends a local reminder notification after approximately one minute so you know the microphone stream is still active.