Table of Contents

1 · Overview

MetricsMonitor is an FMDX Webserver Monitor plugin for displaying RDS and RF information, volume, equalizers, spectrum analyzer, and oscilloscope for FM audio, FM baseband, and MPX signals.

It extends the FM-DX-Webserver with a suite of professional-grade signal analysis tools. It captures MPX audio from a sound card or USB audio device, performs real-time FFT analysis on the server, and streams the results to all connected browser clients via WebSocket.

The plugin replaces the standard signal panel with a configurable multi-module display. Six display modes are available; they can be freely ordered, filtered by login state, and switched interactively by the user without a page reload.

2 · Requirements & Hardware Configuration

Server Requirements

• FM-DX-Webserver (latest version recommended)
• Node.js ≥ 16
• Linux (x64 / ARM / ARM64), Windows (x64), or macOS (x64 / ARM64)
• A stereo audio input device capable of 48 / 96 / 192 kHz sampling
• The MPXCapture binary (included in the bin/ folder)

Browser (Client) Requirements

• Any modern browser with WebSocket and Web Audio API support (Chrome, Firefox, Edge, Safari)
• Chart.js + chartjs-plugin-streaming (loaded automatically by FM-DX-Webserver)

2.1 · MPX Equipment Settings

ESP32 Receiver

Make sure the correct PE5PVB firmware is installed. You can either use the software-based MPX switching ("MPXmode": "auto" or "on") or you can permanently activate the MPX output in the audio settings menu.

Headless TEF

If the Headless TEF has a line-level audio output, the MPX output can be permanently enabled via a jumper on the board. It is recommended to output the signal to a 192kHz compatible sound card, which is then configured as "MPXInputCard": "plughw:CARD=Device" (Linux) or "Microphone (HD USB Audio Device)" (Windows). Normal mono/stereo sound output continues to be handled by the i2s USB sound interface.

MPX Tool & Co.

Anyone wishing to perform stereo decoding using MPX Tool or similar should use the settings "MPXmode": "on" and "MPXStereoDecoder": "on". This will result in permanent MPX output and stereo signaling on the display.

3 · Installation Instructions

1. Download: Download the latest repository as a zip from the releases page.
2. Unpack: Unpack all files (MetricsMonitorPlugin.js + MetricsMonitor Folder) to ..fm-dx-webserver-main\plugins\.
3. Stop Server: Stop or close the fm-dx-webserver.
4. Start/Restart: Start/Restart the fm-dx-webserver with npm run webserver or node . on the node.js console, check the console information.
5. Activate: Activate the MetricsMonitor plugin in the settings UI.
6. Stop Server: Stop or close the fm-dx-webserver.
7. Start/Restart: Start/Restart the fm-dx-webserver again.
8. Configure: Configure your personal settings in the automatically created metricsmonitor.json (located in the folder: ../fm-dx-webserver-main/plugins_configs).
9. Stop Server: Stop or close the fm-dx-webserver.
10. Final Start: Start the fm-dx-webserver. Check the console information to confirm successful startup.

4 · Display Modules

MetricsMonitor provides six numbered display modules (IDs 0–5). You control which modules appear and in what order via MODULE_SEQUENCE in the config file. The user can cycle through them by clicking the panel or pressing number keys.

4.1 · 0 · Audio Meter

5-Band Audio Meter (metricsmonitor-audiometer.js)
Stereo L/R VU meters using the Web Audio API (Float32 precision), an RF signal bar, and five isolated frequency-band meters centred at 64 Hz, 256 Hz, 1 kHz, 4 kHz, and 10 kHz. Corrects the treatment of signals with different phases. Peak hold with configurable decay. Logarithmic dB scale (+5 dB to −26 dB). Completely browser-driven — no MPX capture needed.

4.2 · 1 · MPX Meters

MPX Meters (metricsmonitor-meters.js)
Six segment bar meters: LEFT, RIGHT (stereo audio), RF (signal strength), PILOT (19 kHz deviation in kHz), MPX (total composite deviation in kHz), and RDS (57 kHz subcarrier deviation in kHz). Values are derived from the MPX capture binary. Each meter shows a numeric value. Peak hold with 2 s decay.

4.3 · 2 · MPX Spectrum Analyzer

MPX Spectrum Analyzer (metricsmonitor-analyzer.js)
Canvas-based FFT spectrum and revised waveform display of the FM composite signal. Horizontal range depends on sample rate: 48 kHz → 0–24 kHz, 96 kHz → 0–48 kHz, 192 kHz → 0–76 kHz. Smoothed with configurable attack/decay envelopes. Supports horizontal and vertical zoom, pan, hover cursor with frequency/dB readout.

4.4 · 3 · Signal Meter

Signal Meter (metricsmonitor-signalmeter.js)
Combined panel: LEFT/RIGHT stereo audio bars, ACI and CCI meters (replacing the previous RF/Multipath bars, based on R100DX/SignalGuard), a large numeric signal readout with highest-seen tracking, and status icons (RDS, Stereo, TP, TA, ECC flag).

4.5 · 4 · Signal Analyzer

Signal Analyzer (metricsmonitor-signal-analyzer.js)
A Chart.js realtime line chart plotting signal strength over time. Includes a graphical Channel Condition display plotting Signal value, Multipath (MP), ACI, and CCI. Supports horizontal zoom (time window), vertical zoom (dB range), and drag-to-pan.

4.6 · 5 · MPX Oscilloscope

MPX Oscilloscope (metricsmonitor-scope.js)
Time-domain waveform viewer of the MPX composite signal (up to 1024 samples/frame). Includes a switchable waveform display. Phosphor afterglow with up to 23 persistent traces. Peak-hold amplitude band in the background. REF level lines indicate 100 % modulation reference. Computes a 60-second windowed MPX power value (dBr).

5 · Interaction & Navigation

5.1 · Module Switching

Clicking the Panel

Click anywhere inside the signal panel (#level-meter-container) to advance to the next module in the configured MODULE_SEQUENCE. A 150 ms fade transition is applied between modes.

Keyboard Shortcuts

You can quickly switch between modules using the numeric keys. Which numbers are active depends on the number of activated modules. The counting always starts with the first value.

Key	Action
1	Switch to the 1st module in MODULE_SEQUENCE
2	Switch to the 2nd module
3 … 9	Switch to the Nth module (if it exists)

5.2 · Zoom & Pan Controls

Press the CTRL key to select different zoom options in MPX + Signal analysis modes.

Spectrum & Oscilloscope

Action	Effect
Scroll wheel	Horizontal zoom in/out
Ctrl + Scroll	Vertical zoom in/out (dB / amplitude axis)
Left-click + Drag	Pan the view horizontally (and vertically when Y-zoomed)
Right-click	Reset all zoom to 1.0×
Ctrl + ↑	Zoom in horizontally
Ctrl + ↓	Zoom out horizontally
Ctrl + ←	Pan left (when zoomed in)
Ctrl + →	Pan right (when zoomed in)
Ctrl + Space	Reset zoom to 1.0×

Signal Analyzer (module 4)

Action	Effect
Scroll wheel	Horizontal zoom (time window: 1 s – 120 s)
Ctrl + Scroll	Vertical zoom (dB range)
Left-click + Drag	Pan dB range up/down (only when vertically zoomed)
Right-click	Reset all zoom
Ctrl + ← / →	Decrease / increase time window

5.3 · Canvas Panel (Overlay)

A button labelled MPX/Signal toggles a full-width canvas overlay above the tuner that displays one of the canvas modules defined in CANVAS_SEQUENCE (IDs 2, 4, 5). Each click cycles through them. If EnableSpectrumOnLoad is true, the first canvas module starts automatically.

5.4 · Header Icons & Status Indicators

Icon	Description
ECC flag	Country/ECC badge cloned from the page's .data-flag element.
Stereo/Mono icon	Reflects the current stereo lock state. Clicking it sends B1 (force mono) or B0 (release) to the server. When MPXStereoDecoder = "on" it toggles between B1/B2.
PTY label	Shows the current RDS Programme Type as human-readable text.
TP / TA icons	Traffic Programme and Traffic Announcement flags from RDS.
RDS icon	Green when RDS lock is active. Also changes the signal panel background colour.

6 · Configuration Reference (metricsmonitor.json)

The following variables can be changed in the metricsmonitor.json config file. After making changes, a server restart is only necessary for selected settings; a browser reload may also be sufficient!

Audio & MPX Hardware Settings

"sampleRate": 48000,             // Supported sample rate of input audio card (48000 / 96000 / 192000)
"MPXmode": "off",                // MPX behavior: "off" / "auto" (switching) / "on" (always output)
"MPXChannel": "auto",            // Fixed channel for MPX signal ("left", "right", "auto")
"MPXStereoDecoder": "off",       // Set to "on" if decoding stereo signal from MPX
"MPXInputCard": "",              // Exclusive MPX input (e.g., "plughw:CARD=Device")
"MPXTiltCalibration": 0,         // Adjust input slope from -1000 µs to 1000 µs
"VisualDelayMs": 250,            // Client-side delay in ms to sync with audio playback

Calibration Offsets & Scales

"MeterInputCalibration": 0,      // Global input gain offset in dB for meters
"MeterPilotCalibration": 0,      // Offset for Pilot deviation
"MeterMPXCalibration": 0,        // Offset for MPX deviation
"MeterRDSCalibration": 0,        // Offset for RDS deviation

"MeterPilotScale": 400,          // Scale factor for Pilot deviation
"MeterMPXScale": 100,            // Scale factor for MPX deviation
"MeterRDSScale": 750,            // Scale factor for RDS deviation

FFT, Spectrum & Scope Settings

"fftSize": 512,                  // Frequency sampling rate (higher = better resolution, more CPU)
"SpectrumInputCalibration": 0,   // Input gain offset in dB for spectrum
"SpectrumAttackLevel": 3,        // Response rate as signal increases
"SpectrumDecayLevel": 15,        // Smoothing decay (higher = stronger smoothing)
"SpectrumSendInterval": 30,      // MS between WebSocket spectrum frames
"SpectrumYOffset": -40,          // Curve offset
"SpectrumYDynamics": 2,          // Curve dynamic multiplier
"ScopeInputCalibration": 0,      // Amplitude gain offset for oscilloscope

Meter Gains & Layout

"StereoBoost": 2,                // Gain factor for stereo L/R VU meter
"AudioMeterBoost": 1,            // Gain factor for equalizer display

"MODULE_SEQUENCE": "1,2,5,0,3,4",// Click-cycle order of modules inside panel
"CANVAS_SEQUENCE": "2,5,4",      // Cycle order for the canvas overlay
"MultipathMode": 0,              // 1 = TEF Radio smooth, 0 = Raw MP Data
"LockVolumeSlider": true,        // Lock main volume slider to prevent measurement distortion
"EnableSpectrumOnLoad": false,   // Automatically open canvas overlay on load
"EnableAnalyzerAdminMode": false,// Restricts canvas modules to admin users only

Colors & Peaks

"MeterColorSafe": "rgb(0, 255, 0)",       // Safe range color
"MeterColorWarning": "rgb(255, 255, 0)",  // Warning range color
"MeterColorDanger": "rgb(255, 0, 0)",     // Danger range color
"PeakMode": "dynamic",                    // "fixed" or "dynamic"
"PeakColorFixed": "rgb(251, 174, 38)",    // Custom color for peak if PeakMode is fixed

7 · Calibration Guide

Proper calibration ensures that the meter readings match real broadcast standards.

Step 1 – Input Level (MeterInputCalibration)

Inject a 100 % modulation test tone (typically ±75 kHz deviation at 1 kHz) into your tuner. Observe the MPX Total meter. If the Pilot, MPX, and RDS meters show no signal level, increase the MeterInputCalibration in increments of 5. If the meters are in the red zone, decrease the MeterInputCalibration in increments of 5.

Step 2 – Roughly adjust the meter readings

• MeterPilotScale: Adjust this up/down (100/50 steps) until you reach approximately 7 kHz (Standard is 9%).
• MeterMPXScale: Adjust this up/down (100/50 steps) until you reach approximately 75 kHz.
• MeterRDSScale: Adjust this up/down (100/50 steps) until you reach approximately 2.0 kHz.

Step 3 – Fine-Tuning with the Calibration Tool (Recommended)

Now you can use the calibration tool to calculate the exact scale values ​​so that the calibration offset values ​​(MeterPilotCalibration, etc.) remain at 0:
Launch Metrics Monitor Calibration Tool

Step 4 – Manual Fine-Tuning (Alternative)

If you prefer not to use the tool, you can adjust the calibration variables directly in the decimal range (0.x). Note: The calibration variables should only be used for direct fine-tuning, as otherwise they will distort the measurement result.

• Pilot: Display: 7.2 kHz. Target: 6.7 kHz. -> MeterPilotCalibration: -0.5
• MPX: Display: 78.2 kHz. Target: 78.0 kHz. -> MeterMPXCalibration: -0.2
• RDS: Display: 2.5 kHz. Target: 2.0 kHz. -> MeterRDSCalibration: -0.5

Step 5 – Spectrum Visibility (Spectrum-Y-Offset)

The spectrum curve baseline (noise floor) should sit near the bottom of the display. Decrease Spectrum-Y-Offset (make more negative) to move the curve down if the noise floor is too high. Typical working value: -40 to -60.

Step 6 – Pre-Emphasis Tilt (MPXTiltCalibration)

If the spectrum shows a rising slope toward high frequencies, set MPXTiltCalibration to 50 (Europe) or 75 (Americas) to apply an inverse de-emphasis tilt. Set to 0 to disable.

8 · Advanced Technical Details

8.1 · Audio Backend (Web Audio API)

Modules 0 (Audio Meter), 3 (Signal Meter), and parts of module 1 (MPX Meters) use the browser's Web Audio API to measure the live audio stream directly — entirely independent of the MPX capture.

The plugin connects to Stream.Fallback.Player.Amplification (the gain node of the FM-DX-Webserver audio player). A ChannelSplitterNode feeds separate AnalyserNodes for left and right channels. Amplitude is extracted as Float32 time-domain data, converted to dBFS, and mapped to the 0–100 % meter scale (+5 dBFS to −26 dBFS).

8.2 · WebSocket Protocol

MetricsMonitor uses the existing FM-DX-Webserver WebSocket endpoint (/data_plugins) for real-time communication.

// Server → Client: MPX Packet
{
  "type": "MPX",
  "peak": 74.3,          // MPX total deviation in kHz
  "pilotKHz": 8.97,      // Pilot tone deviation in kHz
  "rdsKHz": 3.82,        // RDS subcarrier deviation in kHz
  "s": [ ... ],          // Spectrum magnitudes array (float32)
  "o": [ ... ]           // Oscilloscope waveform samples array
}

8.3 · Admin Mode

Setting EnableAnalyzerAdminMode to true restricts canvas modules (IDs 2, 4, 5) to authenticated admin users only. Non-admin visitors will still see the non-canvas modules (0, 1, 3) normally.

8.4 · Debug & Logging

• Server-Side Debug: In metricsmonitor_server.js, set let ENABLE_EXTENDED_LOGGING = true; to enable detailed console output of MPX/RDS/SNR values.
• Client-Side Debug: In the browser console, run localStorage.setItem("mm_meters_debug", "1"); for throttled output, or "all" for all WebSocket messages. Use localStorage.removeItem("mm_meters_debug"); to disable.

9 · Important Notes & Troubleshooting

• Audio Playback: Press the play button to activate the audio output and equalizer. If the Stereo bars are silent, ensure you have clicked Play first.
• Volume Control: To avoid distorting the measurement results, the volume control is disabled after the plugin is installed! You can re-enable it via the configuration (LockVolumeSlider) if strictly needed, but it affects metered values.
• Sample Rate Dependencies: The function of the modules depends on the input signal and the data rate used:
  • 48 kHz: Sufficient for Modules 0 and 3. Displays FM audio spectrum up to 19 kHz.
  • 96 kHz: Required for the pilot tone display (Module 1). Displays FM baseband up to 38 kHz.
  • 192 kHz: Required for the full MPX and RDS displays. Displays MPX spectrum up to 56 kHz. For both 96 and 192 kHz, the receiver must support MPX output.
• CPU Load: Due to the high data rate (up to 10 Mbit/s) and CPU load, it is recommended to restrict the display of the MPX Spectrum and Oscilloscope to the administrator account using "EnableAnalyzerAdminMode": true on slower connections or hardware.
• Compatibility: Compatibility with all hardware components and platforms cannot be guaranteed. The receiver's output volume, as well as the technical characteristics of the hardware components, affect the display behavior and must be taken into account!

10 · Full Changelog

v2.7

• RF and MP meter displays have been replaced by ACI and CCI in signal meter module 3. The calculation basis comes from R100DX/SignalGuard (Thanks to mateusz100).
• Added a new graphical display of signal value, MP, ACI and CCI to the Canvas Signal Plot Display (Thanks to GOR for the template and inspiration).
• The 5-band audiometer corrects the treatment of signals with different phases.

v2.6

• Added dBr power display to the Canvas Scope display (thanks to Bojcha for the code).
• Added CRT monitor effect to the Scope Monitor.
• Removed ghost peak display from the VU meter.
• Fixed a bug when switching modules in the VU meter display with the Canvas Scope Monitor enabled.

v2.5a

• Peak display in the 5-band audiometer corrected for the "fixed" setting.
• Fixed a bug in the spectrum/scope display in the module display when Canvas was started.
• Revised scope update rate.

v2.5

• Numeric and meter display for multipath value built into signal meter module (calculation from UIAddonPack).
• MultipathMode Variable added to adjust the multipath measurement value.

v2.4b

• Patch against spamming detection updated.
• 5-band audiometer and VU meter adjusted.
• Fixed memory overflow issue.

v2.4a

• New variable ScopeInputCalibration added for calibrating the oscilloscope.
• The frame rate of the oscilloscope was significantly increased.
• Bandwidth of the 5-band audiometer adjusted.
• MPXcapture optimized for Linux.

v2.4

• Vertical marker lines at 16, 22, 54 and 60 kHz were added to the analyzer.
• 5-band audio display calibrated and adjusted.
• VU meter calibrated and adjusted.
• Fixed duplicate tooltip display on multipath icon.
• RF level meter now matches module 3.
• Tilt correction improved (thanks to AmateurAudioDude for the code).
• Variable built in for delaying the analyzer, scope, and MPX output (necessary for client-side audio synchronization).

v2.3d

• Channel selection added for the MPX signal (left, right or auto).

v2.3c

• Alignment of scale values adjusted.
• Remove duplicate peak displays.

v2.3b

• Oscilloscope and spectrum analyzer treated separately for resource reasons -> reduction of CPU and network load.

v2.3a

• Access rights to MPXCapture under Linux are granted automatically.
• VU meter display now also starts with "MODULE_SEQUENCE": "1".
• Frequency display at cursor line in spectrum corrected.

v2.3

• Oscilloscope now has its own Canvas and Module element -> Analyzer and Oscilloscope can now be displayed simultaneously.
• Maximum and minimum peak markers in the oscilloscope graph (line and area).
• Added detection and insertion of the multipath icon to the header bar.
• Fixed a false start issue with the audio display in the signal meter.

v2.2

• Improved Pilot, MPX, and RDS calculations.
• Enabled display of other signal patterns in the scope display.
• Refined tilt correction and scaling adjustment.
• Expanded horizontal zoom in scope mode and revised cursor alignment in analyzer mode.
• CPU/Network Optimization: Spectrum/Scope calculation is gated (only active when accessed via a browser).
• New variable EnableAnalyzerAdminMode restricts analyzer/oscilloscope display to administrators.

v2.1

• Integration of an oscilloscope (click on MPX Spectrum title in the analyzer!).
• New variable for adjusting the tilt at the sound card input.
• Sound card reconnect logic built in.

v2.0

• Using our own program library for spectrum analysis and display calculation -> more resource-efficient and higher measurement accuracy.
• New scale variables for PILOT, MPX and RDS.
• Automatic detection of the MPX channel.
• Fixed issues with relative image paths.

v1.5

• Improved PILOT/MPX/RDS detection and corrected faulty spectrum display on smartphone screen.
• Separate input control added for spectrum and meter display.
• Added custom color adjustments for meter displays and highest peak.
• Adjusting the RDS meter scale to the pilot display.
• Configuration variables restructured. Automatic backup of JSON configuration file.

v1.4

• Added button with additional MPX/Signal Canvas display.
• Added improved FFT support for modern CPUs.
• Added startup activation for Spectrum Graph plugin.
• Added MPXBoost to enhance the MPX spectrum.
• ESP32 MPX Auto Switching synchronized with Headless TEF command.

v1.3

• Signal diagram with time display and frequency marking added (new module), press CTRL for individual zoom options.
• Quick module switching possible using numeric keys.
• White border removed from the ECC flag and tooltip for module switching revised.

v1.2

• Additional sound card for only MPX encoding can be activated.
• New variable "ExtStereoDecoder" added.
• Optimization & fix calibration of level and analysis displays.
• Volume control can now be activated if needed.
• MPX Encode Script for 96/192 kHz sampling under Linux.
• Headless TEF software switching Mono <> Stereo <> MPX.
• Various zoom options for analyzer display.
• Discreet real-time display of measured values above the bar graph display. MPX Mode display integrated.

v1.1 & v1.1a

• Fixed incorrect display of TA, TP and RDS.
• MPX software switching for ESP32 TEF receivers added.
• Multiple WebSocket connections revised.

v1.0 & v1.0a

• Three display modes: Audio + PILOT/MPX/RDS spectrum analysis / Audio + equalizer.
• Unit of measurement corrected at MPX level. Variables for individually adjusting the spectrum added.

11 · Credits & Contact

MetricsMonitor is developed by Highpoint.
GitHub: https://github.com/Highpoint2000/MetricsMonitor

Special thanks for support, testing, and ideas:

• Jeroen Platenkamp
• Bkram
• Wötkylä
• AmateurAudioDude
• GOR
• Bojcha
• mateusz100

Released under the MIT License.
FM-DX-Webserver is an open-source project for FM radio DXing over the internet.

Contact

If you have any questions, would like to report problems, or have suggestions for improvement, please feel free to contact me! You can reach me by email at highpoint2000@googlemail.com. I look forward to hearing from you!

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