Audio Frequency Analyzer

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The Audio Frequency Analyzer operator detects and verifies the presence of a specific audio frequency (such as a test tone) in an audio signal. It analyzes incoming audio to determine if a target frequency is present, measures consistency over time, and reports whether the signal matches expected parameters. This is useful for broadcast signal verification, quality control, and automated monitoring of audio feeds.

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Overview

Audio frequency detection works by analyzing the incoming audio signal to:

  • Measure frequency using zero-crossing detection to count wave cycles per second
  • Verify waveform shape by calculating the crest factor (peak-to-RMS ratio) to confirm a clean sine wave
  • Track consistency over a configurable time window to report signal stability

A pure sine wave has a crest factor of approximately 1.414 (√2). By checking this ratio, the operator can distinguish between a clean test tone and random noise or complex audio that happens to cross through the target frequency.

Quick Start

  1. Add the Frequency Analyzer operator to your audio processing chain
  2. Set the Frequency (Hz) to match your expected test tone (e.g., 1000 Hz)
  3. Adjust Frequency Tolerance to allow for minor drift (e.g., ±20 Hz)
  4. Set Minimum signal level to ignore background noise
  5. Monitor the Match indicator and Consistency (%) output

Settings

Configuration

Setting Description
Frequency (Hz) The target frequency to detect, in Hertz. Range: 50–5000 Hz. Common test tones include 400 Hz, 1000 Hz, and 1200 Hz.
Frequency Tolerance Allowable deviation from the target frequency in Hz. For example, ±20 Hz means a 1000 Hz target will match frequencies between 980–1020 Hz. Range: 1–100 Hz.
Shape Tolerance How strictly the waveform must match a pure sine wave. Lower values require a cleaner signal. A pure sine has a crest factor of 1.414; this setting defines how far from that value is acceptable. Range: 0.1–20.0.
Minimum signal level (dB) The minimum amplitude threshold in decibels. Signals below this level are ignored to prevent false positives from background noise. Range: -36 to 0 dB.
Window length (s) The time span in seconds used to calculate the consistency percentage and average frequency. Longer windows provide more stable readings but respond slower to changes. Range: 1–60 seconds.

Result

Output Description
Measured frequency The actual frequency detected in the audio signal, displayed in Hz. This is a weighted average over the analysis window.
Match Boolean indicator showing whether the current audio matches the target frequency within tolerances and passes all validation checks.
Consistency (%) Percentage of the analysis window duration where the signal matched the target. 100% means continuous match; lower values indicate intermittent signal.

Actions

Action Description
Reset Restores all configuration settings to their default values: 1000 Hz frequency, ±20 Hz tolerance, 0.1 shape tolerance, -12 dB minimum level, and 1 second window.

Workflow Guide

Step 1: Identify Your Test Tone

Determine the frequency of the tone you need to detect:

  • Standard broadcast test tone: 1000 Hz
  • EBU alignment tone: 1000 Hz at -18 dBFS
  • SMPTE alignment tone: 1000 Hz or 400 Hz
  • Custom facility tones: Check your technical specifications

Step 2: Configure Basic Settings

  1. Set Frequency (Hz) to your target
  2. Start with Frequency Tolerance at 20 Hz
  3. Set Minimum signal level based on your expected tone level (typically -18 to -12 dB for alignment tones)

Step 3: Adjust Shape Tolerance

  1. If Match shows false when you know a tone is present:
    • Increase Shape Tolerance (try 1.0, then higher)
    • The tone may have harmonic distortion
  2. If Match shows true on non-tone audio:
    • Decrease Shape Tolerance (try 0.1 or lower)
    • The detection is too lenient

Step 4: Set Analysis Window

  1. For real-time monitoring: Use 1–5 seconds
  2. For stable long-term verification: Use 10–30 seconds
  3. For quick detection response: Use 1 second

Step 5: Interpret Results

  • Match = True, Consistency = 100%: Stable, continuous tone detected
  • Match = True, Consistency < 100%: Tone present but intermittent
  • Match = False, Consistency > 0%: Tone was recently present but not currently
  • Match = False, Consistency = 0%: No matching tone detected in the window

Common Scenarios

Broadcast Signal Verification

Monitoring a live broadcast feed for presence of a test tone before program start:

  • Set Frequency to your station's standard tone (typically 1000 Hz)
  • Use Minimum signal level of -20 dB to catch standard alignment levels
  • Set Window length to 5 seconds for stable readings
  • Shape Tolerance of 1.0 allows for minor transmission artifacts

Equipment Testing

Verifying that audio equipment is passing a test tone correctly:

  • Use tight Frequency Tolerance (±10 Hz) to catch frequency drift
  • Set Shape Tolerance low (0.5 or less) to detect distortion
  • Window length of 1 second for quick response
  • Compare Measured frequency against the source to check for pitch shift

Automated Failover Detection

Triggering alerts or switching when a tone disappears:

  • Set Consistency (%) threshold in your automation system
  • Use Window length of 3–5 seconds to avoid false triggers from brief dropouts
  • Minimum signal level should be set just below your expected tone level

Quality Control for Recorded Media

Checking that test tones in recorded files meet specifications:

  • Use Shape Tolerance of 0.5 or lower to verify clean recordings
  • Frequency Tolerance of ±5 Hz for precise verification
  • Review Measured frequency to catch subtle pitch errors

Troubleshooting

Problem Solution
Match always False despite tone present Increase Shape Tolerance — the tone may have distortion. Also verify Minimum signal level isn't too high.
Match True on speech/music Decrease Shape Tolerance to require a cleaner sine wave. Values of 0.5 or lower reject complex audio.
Measured frequency fluctuates Increase Window length for more averaging, or the source tone may be unstable.
Consistency never reaches 100% Short dropouts or level variations may cause momentary failures. Increase Shape Tolerance or decrease Minimum signal level slightly.
Wrong frequency detected Check that both stereo channels contain the tone. The operator averages both channels.
No detection on very low frequencies Frequencies below 100 Hz need longer audio frames to detect accurately. This operator works best above 100 Hz.
False positives on silence Increase Minimum signal level to reject low-level noise. Try -24 dB or higher.

Tips for Best Results

Signal Quality

  • Use the cleanest signal path possible for test tones
  • Avoid heavy compression or limiting on tone signals

Tolerance Settings

Start with moderate tolerances and tighten as needed:

  • Frequency Tolerance: Begin at ±20 Hz, reduce if false positives occur
  • Shape Tolerance: Begin at 1.0, reduce for stricter verification

Stereo Considerations

The operator analyzes both left and right channels independently and requires both to match for a positive result. Ensure your test tone is present on both channels, or the signal is properly configured as dual-mono.

Continuous Monitoring

For long-running verification:

  • Use Window length of 10+ seconds for stable readings
  • Monitor Consistency (%) rather than instantaneous Match for trend analysis
  • Consider logging results for later review

Technical Notes

  • Audio is processed at 48 kHz sample rate
  • Zero-crossing detection counts positive-going crossings only
  • Crest factor for ideal sine wave: √2 ≈ 1.41421356
  • Both stereo channels must pass all checks for Match to be True
  • History is maintained using a sliding window queue weighted by frame duration
  • The operator passes audio through unchanged — it monitors but does not modify the signal