IEC 61260-1: Real-Time Octave Band Analysis Requirements
TL;DR
Real-time analysis (RTA) requires simultaneous processing of all octave or 1/3 octave bands with continuous update. IEC 61260-1 does not specify update rates directly, but the companion IEC 61672-1 time weighting constants (125ms Fast, 1000ms Slow) define the effective temporal resolution. For meaningful real-time display, update rates of 10-25 Hz are typical (40-100ms per frame). All bands must be processed simultaneously — sequential scanning of a single filter across bands is not permitted for real-time applications. The detection stage follows each filter, typically exponential averaging matching the selected time weighting.
Simultaneous Processing
Real-time analysis requires all frequency bands to be computed from the same time segment of the input signal. This distinguishes RTA from swept-filter analysis where a single tunable filter scans across frequencies sequentially. RTA captures the instantaneous spectral content at every update.
Two implementation approaches achieve simultaneous processing:
- Parallel filter bank: A bank of IIR bandpass filters processes the signal simultaneously. Each filter feeds its own level detector.
- FFT-based: An FFT computes the complete spectrum, then band energies are summed across the bins within each band's edges. See digital implementation.
Update Rate
Typical RTA update rates:
| Update rate | Frame period | Application |
|---|---|---|
| 10 Hz | 100 ms | General monitoring, matches Fast weighting |
| 25 Hz | 40 ms | Transient analysis, live feedback detection |
| 50 Hz | 20 ms | Maximum temporal resolution (limited by low-freq band settling) |
The practical upper limit on update rate is set by the lowest frequency band: a 31.5 Hz octave band filter needs approximately 30 ms to settle, so update rates above ~30 Hz provide diminishing returns.
Detection and Time Weighting
After filtering, each band signal passes through a detection stage (squaring and averaging) that applies time weighting. The detector follows the same exponential time constants as IEC 61672-1: Fast (125 ms), Slow (1000 ms), or Impulse. The time-weighted level in each band is then converted to dB for display.
Display Considerations
RTA displays typically show:
- Bar chart: one bar per band, height proportional to band level in dB
- Smoothed line: interpolated between band centres for continuous appearance
- Peak hold: maximum value per band with slow decay
- A/C/Z weighting: applied to each band before display (or to broadband level)
The dynamic range of the display should match the instrument's linear operating range — typically 70-100 dB from noise floor to overload.
Latency Budget
For live sound applications, the total latency from sound arriving at the microphone to display update should be minimized. Typical budget: FFT buffer fill (21 ms for 1024 samples at 48 kHz) + FFT computation (<1 ms on modern hardware) + rendering (<16 ms for 60fps display) = approximately 40 ms total. This is acceptable for real-time monitoring and system tuning.
SonaVyx RTA
The RTA mode provides real-time 1/1 and 1/3 octave analysis with configurable update rate and A/C/Z weighting. The Rust WASM DSP engine processes each FFT frame in the Web Worker, and the results are rendered at display refresh rate. See response requirements for accuracy specifications and IEC 61672 integration for how RTA and SPL metering work together.
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Last updated: March 19, 2026