Hardware
Audio Input & Hardware
Connecting USB microphones and audio interfaces, sample rate negotiation, channel selection, and input gain.
Built-in microphone
Every iPhone and iPad has a built-in microphone. Chirps uses it automatically if no external device is connected.
The built-in microphone on current Apple devices supports up to 44.1 kHz or 48 kHz sample rates. This gives a Nyquist ceiling of approximately 22–24 kHz — sufficient for bird calls, frogs, insects, and general acoustic monitoring, but not for bat echolocation which typically spans 20–120 kHz.
External USB microphones and audio interfaces
For ultrasonic work or higher-quality recording, connect an external device via USB-C (modern iPad/iPhone) or Lightning (older devices, with a Lightning-to-USB-A adapter).
Chirps supports iOS-compatible USB Class Compliant audio devices — no special driver is required. This includes:
- Ultrasonic bat microphones: Pettersson M500-USB, Dodotronic Ultramic series, Audiomoth USB Microphone
- USB condenser microphones: Rode NT-USB Mini, Blue Yeti, Shure MV7, and similar
- Audio interfaces: Focusrite Scarlett, Universal Audio Volt, MOTU M series, and similar
How to connect:
- Plug the device in before starting the stream.
- Open Settings in Chirps — the device should appear in the Input Device picker.
- Select it, confirm the input channel, sample rate, record file bit depth, and output routing, then start the stream.
When an external input is connected, Chirps scans the route, probes supported channels and sample rates, and remembers authentic capabilities for that device. Repeated use of the same device should therefore be faster and more consistent.
If the device does not appear, tap the refresh icon (↺) next to the Input section header to re-query available devices, channels, sample rates, and cached capabilities. Plugging in while the stream is running is not supported — stop the stream first.
Bluetooth microphones
Bluetooth audio devices use the HFP/SCO profile when used as microphone input on iOS, which caps the sample rate at 8 kHz or 16 kHz. This is insufficient for meaningful spectrogram analysis above a few kilohertz.
Chirps will use whatever input iOS provides, but wired USB is strongly recommended for any bioacoustics application. Bluetooth output (headphones) works normally for monitoring.
Input device picker
Open Settings → Input to see all detected input devices. The list includes:
- Built-in microphone
- Any connected USB device
If a previously cached device is listed but not physically connected, its entry might be greyed out. Use the refresh button (↺) to re-probe active connections.
Channel selection
For stereo or multi-channel USB interfaces, Chirps captures one channel at a time. Select the input channel in Settings → Input → Input Channel. The available channel numbers are populated automatically from the connected device.
If you switch to a channel that does not exist on the selected device, Chirps adjusts to the first available channel and logs the adjustment.
Sample rate
Chirps tries to use the sample rate you select. If the connected device does not support that rate, Chirps adjusts to an available supported value and logs the adjustment.
Preferred sample rate is set in Settings → Input → Sample Rate. The picker is based on iOS route discovery, device probing, cached authentic capabilities, and actual runtime formats observed during streaming. Rates shown for the built-in microphone are typically 44.1 kHz and 48 kHz.
Custom sample rates discovered during probing or actual streaming are kept with the device. Rates outside the standard scan list are shown with a Custom tag instead of being hidden or silently rounded to a nearby standard rate.
Common rates for ultrasonic work:
| Device family | Typical sample rates |
|---|---|
| Built-in microphone | 44.1, 48 kHz |
| USB condenser (standard) | 44.1, 48, 96 kHz |
| Ultrasonic USB (bat detection) | 192, 250, 300, 384 kHz |
Bit depth for recording
The Recording File Bit Depth setting (Settings → Input → Record File Bit Depth) controls the bit depth of the WAV file written to disk.
| Bit depth | File size | Notes |
|---|---|---|
| 16-bit | Smallest | CD quality. Sufficient for monitoring and general recording. |
| 24-bit | Medium | Standard for professional audio. Recommended for archival. |
| 32-bit | Largest | Maximum dynamic range. Use when processing in post. |
The engine format shown in Diagnostics is separate from the file bit depth and may not be the same as a USB device’s hardware converter depth. This setting controls the saved WAV PCM format.
Hardware input gain
Some USB audio devices expose adjustable hardware gain to iOS. When this is the case, a Hardware Input Gain slider appears in Settings → Input. This controls the device’s own ADC gain using Apple’s normalised 0.00–1.00 scale.
If the slider does not appear, the selected device does not expose hardware gain on iOS. Use the device’s own physical gain control (dial or software mixer) instead.
Input gain settings are remembered per-device. Switching devices does not reset the gain for the previously selected device.
What happens when a USB device is disconnected mid-session
If a USB device is unplugged while the stream is running, the audio session is interrupted. Chirps stops the stream automatically and logs the event. Any recording in progress is closed and saved up to that point. Reconnect the device and start the stream again.