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Signal to noise ratio

General information

S/N ratio and dynamic range really mean the same thing but a convention seems to have developed.

Often the S/N ratio (signal to noise ratio) is used for the ratio between the output level corresponding to a 1kHz digital full scale sine wave and the output level due to noise when the input is muted (digital zero). This is more accurately called the idle channel noise ratio and can be enhanced by using logic within the DAC to mute the output when a digital zero is being received.

The dynamic range is similar except the noise is measured in the presence of a 1kHz signal at -60dBFS. The 1kHz signal is eliminated from the noise measurement by a notch filter. This measurement is more representative of the accuracy of the DAC in the presence of real signals.

Noise measurements

There are two types of measurements with two types of weighting filters used for signal to noise measurements. A 20Hz to 20kHz band limiting filter is normally used for all measurements. Noise level is usually measured with an RMS-measuring meter.

In signal to noise and dynamic range measurements manufacturers will often use an 'A' weighting filter. If the noise floor is flat then this improves the figure by more than 2dB.

Standard

There are three main standards in the area in USA: IEC-61606, AES-17, and an EIAJ (Japanese) standard.

Completely different conventions are used in professional audio circles in the rest of the world. ITU-T (formerly CCITT) J.21 and ITU-R (formerly CCIR) 468, together specify a weighting curve that has up to 9 dB of gain at certain frequencies, and that a "quasi-peak" measuring meter should be used. This combination results in numbers that are up to 16 dB worse than with A-weighting and RMS on the same equipment.

So, it is vitally important to specify the test method used when discussing performance numbers.