Signal-to-Noise Ratio (SNR) was first used to evaluate the intelligibility of analog radio voice communications, and expressed the difference between the nominal signal level and the noise at the radio receiver. In excellent conditions, the voice might be 40 dB above the noise; in the worst conditions, the signal would be “lost in the noise” and unintelligible.
Strictly speaking, then, the “signal” level is arbitrary, although it is usually taken to be the “nominal” program level. SNR is actually two measurements: first, of the signal level; and second, of the noise level with the signal turned off. These two measurements are expressed as a ratio, almost always in decibels.
Like most noise measurements, the SNR results are often bandpass limited using high and low pass filters, or a weighting filter. Filter use should be stated in the results.
The APx500 Signal-to-Noise Ratio Measurement makes the two measurements and computes the ratio in one operation.
Dynamic Range is an expression of the ratio of the largest signal a device can pass to the device’s noise floor. “Largest signal” usually refers to a signal at a specified degree of distortion, often 1%. Signal-to-Noise Ratio and Dynamic Range are essentially the same measurement, except that the signal in SNR is arbitrary (and should be stated in the results), and the signal in Dynamic Range is at the maximum (details of which should also be stated in the results).
This measurement was first defined in a standard in Section 9.3 of AES17, under the heading “noise in the presence of signal.” A similar measurement called “dynamic range” is mentioned in IEC61606.
We recommend using this AES17 dynamic range measurement. It is intended specifically for ADC (analog-to-digital converter) and DAC (digital-to-analog converter) dynamic range and “noise in the presence of signal” measurements, as described in Section 9.3 of AES17. A similar method is defined in IEC61606.
This method differs from standard signal-to-noise and dynamic range measurements in that it uses a –60 dBFS stimulus during the noise measurement. This method is used for two reasons:
· In both ADCs and DACs, “idle tones” can be produced within the converter in the absence of applied signal. In the method here, a low-level tone is applied to the converter to avoid production of idle channel noise. The low-level tone is removed by a notch filter before measurement.
· In some DACs, the output of the device is switched off when there is no signal, providing an unrealistically quiet measurement. The low-level tone (again, notched out before measurement) defeats this muting mechanism.
At –60 dBFS, the tone is so low that any distortion products created are below the noise floor.
We recommend using the Signal-to-Noise measurement configured for dynamic range when testing other devices. The low-level signal is not required for non-converter devices.