Online Monitor

The Online Monitor provides real-time visualization of incoming signals during hardware acquisition. It allows you to verify signal quality, check levels, and observe spectral content before and during recording. The monitor operates independently of the recording process and does not affect the recorded data.

Display Modes

Level Meter

The level meter displays the instantaneous and RMS signal level for each active channel as a horizontal bar graph. Key features:

RMS level — smoothed average level in engineering units or dB.
Peak level — instantaneous peak value, with configurable hold time.
Peak hold indicator — retains the maximum peak for a user-defined period (default 3 seconds) to catch transient peaks.
Overload warning — the bar turns red when the signal approaches the full-scale range of the ADC (within 1 dB of clipping).

Display Parameter	Options	Default
Level Unit	Engineering units / dB re reference / dBFS	dBFS
Time Weighting	Fast (125 ms) / Slow (1 s)	Fast
Peak Hold Time	1 s / 3 s / 10 s / Infinite	3 s
Update Rate	10 Hz / 25 Hz / 50 Hz	25 Hz

Use the level meter to set input ranges before recording. All channels should show adequate signal level (at least −40 dBFS for broadband signals) without triggering the overload indicator.

Spectrum View

The spectrum view computes and displays a real-time FFT of the selected channel. This is useful for identifying dominant frequencies, checking for electrical interference (50/60 Hz hum), and verifying transducer response before committing to a recording.

Parameter	Options	Default
FFT Size	512 / 1024 / 2048 / 4096	2048
Window	Hanning / Flat Top	Hanning
Averaging	None / Exponential (time constant)	Exponential, 0.5 s
Y-Axis Scale	Linear / dB	dB
Frequency Range	Full / Zoom	Full

Waveform View

The waveform view shows a scrolling time-domain trace of the selected channel. Display parameters include:

Time window — visible duration on screen (10 ms to 10 s).
Y-axis range — auto-scaling or fixed range.
Scroll mode — continuous scroll or triggered sweep.

In triggered sweep mode, the waveform display refreshes when the signal crosses a configurable threshold, similar to an oscilloscope. This is useful for visualizing repetitive events such as impact hammer hits during modal testing.

Multi-Channel Layout

When monitoring multiple channels simultaneously, V-Listen offers several layout options:

Stacked — each channel in a separate vertical strip (default).
Overlay — all channels superimposed on a single plot with color-coded traces.
Grid — channels arranged in a customizable grid layout.

Recording Status Indicators

During an active recording, the Online Monitor displays additional status information:

Indicator	Description
Recording Duration	Elapsed time since recording started (HH:MM:SS.ms).
Samples Recorded	Total number of samples written to disk per channel.
Disk Usage	File size of the current recording and available disk space.
Buffer Status	Fill level of the acquisition ring buffer. Should stay below 80%.
Overrun Count	Number of buffer overruns (data loss). Should be zero.

If the buffer status consistently exceeds 80% or overrun count is non-zero, the disk write speed is insufficient. Switch to an SSD, reduce the sampling rate, or reduce the channel count.

Waveform Capture

While the Online Monitor is running (whether recording or not), you can capture a snapshot of the current waveform buffer for offline inspection:

Click the Capture button (camera icon) in the monitor toolbar.
The current buffer contents (typically 1–10 seconds depending on settings) are saved as a temporary time-domain segment.
The captured segment appears in the Data Explorer and can be analyzed immediately.
Captures are not automatically saved to the project — use File → Save to persist them.

Waveform capture is useful during setup to quickly check signal quality without starting a full recording session. Captured segments can be used for trial FFT analysis to verify frequency content before the actual measurement.

Performance Considerations

The spectrum and waveform views are computed from a decimated copy of the signal stream. They do not affect the recorded data quality or introduce processing load on the acquisition thread.
On systems with limited GPU resources, reduce the update rate to 10 Hz if the display appears sluggish.
The monitor can remain active during recording without impact on data integrity. However, on low-end systems with many channels, consider reducing the display update rate.