Audio Interface Parameters: THD+N

While looking for the right audio interface we often have many doubts as to what exactly would perfectly suit our needs. Its not uncommon to find yourself bewildered looking at the technical parameters on the Tech Specs sheet.

However, comparing the Tech Specs sheet of different audio interfaces gives a clearer picture as to what we can really expect from the interface. Often the success of the previous product-line of a particular brand has a great influence on the new product release. The buyer is often attracted to the brand name more than what actually the product offers. Thus, its important to look beyond the brand image and dive into what numbers have to say about the product.

Today, there are multiple brands that offer audio interfaces in various price ranges. The product that suits you the best definitely has to do with the price range first. If price is not a concern or there are multiple options in your budget, then this article might help you decide which interface to go for.

Total Harmonic Distortion (THD)

According to Sweetwater, technically distortion is any deviation in the shape of an audio waveform between two points in a signal path. According to Texas Instruments, ideally, when a signal at a given frequency travels into an amplifier, it is given perfect gain which matches the frequency of the input signal. However, in a real situation, imperfections and non-linearities in device circuits distort the wave, mostly by adding harmonics (at integer multiples of the input signal).

The noise of a device is all the output energy that is not related to the input signal and can be caused by multiple sources including power supply hum, radio frequency interference, etc. THD+N is the sum of the total harmonic distortion and the noise present in the device circuit and is often expressed as a ratio comparing it to the input signal.

Since the THD+N value will always be smaller than the total signal (which also contains the input signal apart from THD+N), the percentage value would always be less than 100. However, it is more common to express the value in decibels. Thus, what it truly represents is how well an input signal is represented and gained inside an audio interface. The higher the value of THD+N, the more affected the input signal is. Now, these additions to the input signal can have a character and whether it is desirable or not is subjective. But the more sophisticated the circuit, the lesser the value of THD+N is.

So the next time you look into a catalogue of audio interfaces, you'll have an extra parameter to help you make that decision easy. I'll come up with other parameters that can help you with your decision in further articles.