When we tout the excellent performance of Apogee’s digital audio convertors, we often site both the THD+Noise and Dynamic Range specifications of our products in comparison to other audio interfaces. But, why those two? What exactly do they mean?

Total Harmonic Distortion measures the difference in dB between a signal level (usually a 1kHz tone close to 0 dBfs) and the summed level of all distortion and noise generated by a circuit when converting that signal. The specification is expressed as a negative number – it means that the total distortion and noise is 110 dB lower than the signal. The lower the distortion and noise, the less audible it is.

Dynamic Range measures the difference between a circuit’s noise floor with NO signal being converted and the circuit’s POTENTIAL maximum level. It’s expressed as a positive number range, but again, the larger the number, the less audible the noise is.

What’s important about these specs is that they measure the fundamental quality of the circuit, but at opposite extremes – THD+N with a maximum signal input and Dynamic Range with no signal input. That’s why both specifications are given – they provide an evaluation of the converter’s performance across the entire range of operation.

Apogee Engineers closely monitor these two specs as the most important benchmarks when designing our conversion circuits – they give the most accurate sense of how well a circuit is performing.