Signal-to-Noise and Distortion Ratio (SINAD) is a crucial metric in audio systems that quantifies the combined effect of noise and distortion on a signal. It is typically expressed in decibels (dB) and provides a clear indication of the quality of an amplifier, digital-to-analog converter (DAC), or receiver.
What is SINAD?
SINAD is defined as the ratio of the total signal power to the sum of noise and distortion power:
A higher SINAD value means the system reproduces signals with less added noise and distortion, resulting in better audio fidelity or RF performance. In high-end audio equipment, a SINAD above 100 dB is considered excellent, while values below 70 dB may indicate noticeable degradation.
How to Measure SINAD
SINAD is measured using a signal analyzer or an Audio Precision (AP) test setup by performing the following steps:
- Inject a Pure Sine Wave – A test tone, usually 1 kHz, is fed into the device under test (DUT).
- Capture the Output Signal – The output is analyzed using a spectrum analyzer or an FFT-based audio analyzer.
- Calculate Signal Power – The power of the fundamental frequency is measured.
- Measure Noise and Harmonics – All non-fundamental frequency components (harmonic distortion and noise) are summed.
- Compute SINAD – The ratio of signal power to the total noise and distortion power is calculated in dB.
SINAD vs. Total Harmonic Distortion (THD)
While SINAD and THD are related, they measure different aspects of a system's performance:
- THD (Total Harmonic Distortion) quantifies how much of the output signal consists of harmonics relative to the fundamental frequency. It ignores noise and focuses solely on distortion.
- SINAD, on the other hand, includes both distortion and noise, making it a more comprehensive measure of signal fidelity.
A low THD can indicate good linearity, but if the noise floor is high, the SINAD might still be poor. This makes SINAD a more practical metric for evaluating real-world audio and RF performance.
Conclusion
SINAD is a key indicator of the clarity and fidelity of an audio or RF system. It combines the effects of noise and distortion into a single measurement, making it highly useful for comparing amplifiers, DACs, and other audio devices. While THD focuses purely on distortion, SINAD provides a more holistic view of signal quality.
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