Signal Transparency and THD Performance

Overview

The LF Audio SSA (Smart Signal Analyzer) is a passive, inline RCA accessory that sits between your signal source (DSP, head unit, or radio) and your amplifiers. It reads signal data in real time — providing statistics, clip detection, and integration with the WVC for automatic volume response — without modifying or degrading the audio signal passing through it.

This article documents the measured THD (Total Harmonic Distortion) and frequency response performance of the SSA, confirming its transparency in the signal chain.

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How the SSA Fits in Your Signal Chain

The SSA is wired inline on your RCA path:

DSP / Head Unit / Radio
        ↓  (RCA out)
   SSA Input
   SSA Output
        ↓  (RCA out)
  Amplifier / ZVL / Multi-Amplifier Synchronizer

The SSA reads the signal passing through it and reports data to the WVC system. If a clipped signal is detected, the WVC can automatically mute or reduce volume. The device is designed to be fully passive and transparent — it observes the signal without altering it.

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Design Intent: Transparency

The SSA was specifically engineered to avoid two common failure modes of inline signal accessories:

1. Added distortion — Some inline devices introduce measurable THD due to active components or poor impedance matching.
2. Low-frequency attenuation — Devices with RC (resistor-capacitor) safety networks can roll off bass frequencies, particularly below 100 Hz. The SSA was designed without attenuation-causing RC networks in the signal path, resulting in a flat frequency response from 20 Hz to 20 kHz.

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THD Test Methodology

Testing was performed using a Quant Asylum QA403 audio analyzer at two signal levels:

• Low voltage (~12 dBV output) — representative of typical DSP/head unit output levels
• High voltage (~36 dBV output) — representative of high-output PA amplifier or high-voltage DSP signal chains

Baseline Establishment

Before inserting the SSA, a direct loopback measurement (QA403 output → QA403 input via RCA cable only) was taken to establish the measurement floor of the test equipment:

Metric	Baseline (no SSA)
RMS Level	~3.9 V
THD+N	~−110 dB
THDN %	~0.003%
SNR	~111 dB

Low-Voltage Result

With the SSA inserted in the signal path (QA403 output → SSA input → SSA output → QA403 input), all measured values were within the error threshold of the test equipment — indistinguishable from the baseline loopback measurement.

High-Voltage Result

At elevated signal levels (~36 dBV, approximately 8 V RMS peak), the SSA engages an internal voltage divider relay to handle the higher input range. At this level, the measurement floor rises slightly (to approximately 0.009–0.018% THDN), which is a characteristic of the relay-switched attenuator circuit rather than active distortion. The SSA output remained transparent relative to the direct loopback at the same signal level.

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Frequency Response Test

A full-band chirp sweep (20 Hz – 20 kHz) was performed through the SSA and compared against the direct loopback baseline.

Result: The frequency response through the SSA was indistinguishable from the baseline across the full audio band. No low-frequency rolloff was observed. Any minor deviation at the top of the frequency range matched the baseline identically, confirming it is a characteristic of the QA403 test equipment rather than the SSA.

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Summary

Test	Result
THD+N (low voltage)	No measurable difference vs. baseline
THD+N (high voltage)	No measurable difference vs. baseline
Frequency response (20 Hz – 20 kHz)	Flat; no attenuation at any frequency
Signal level impact	None — RMS level unchanged through SSA
SNR impact	None — within test equipment error threshold

The SSA has no measurable negative impact on signal quality at any tested voltage level. It can be inserted into any RCA signal chain without concern for added distortion, noise, or frequency response changes.

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