Audio Recording

What is a sample rate converter in audio applications? It is used when converting sampling rates in digital domain. For example, say you are recording audio in 24 bits 96Khz. 96Khz is the sampling frequency/rates.

If you plan to down sample it to 24 bits 44.1 KHz then you need an audio sample rate converter. This is a very important mastering and mixing tool because sound engineers often work at a high fidelity environment and then down sample when finally exporting it for mass production ( for example CD audio projects which is using 16 bit 44.1 KHz).

Audio sample rate converter is not a dithering tool, although most converters allow also reducing the bit depth of the recording. Bear in mind that if you down sample a 24 bits 96 KHz into a standard red book audio CD at 16 bit 44.1 KHz, you need both a dithering tool and an audio sample rate converter.

The dithering tool is used to downgrade the 24 bits to 16 bit audio and then filling it with dithering noise such that normal listeners won’t notice (while not changing the dynamic range, so it still sounds like a 24 bit audio). The sampling rate converter is used in changing the sampling rate frequency from 96 KHz to 44.1 KHz for example.

One of the best audio wav sample rate converter is Voxengo R8Brain. Not only it does perform very well but it’s free and easy to use. You can download it here: http://www.voxengo.com/product/r8brain/

So how do you know that an audio sample rate converter is indeed performing well?

In the study of digital signal processing, an imperfect audio sample rate converter will add undesirable artifacts to the converted audio wave that affects the resulting audio quality. The good thing about this SRC (sample rate converters) is that they can be tested for quality purposes. If you visit this website: http://src.infinitewave.ca/ , you will be able to read the frequency response curve of the converted wave using different sample rate converters and check if there is an artifact/aliasing introduced in the process.

For example, this is not a good sample rate converter frequency sweep response curve:

It is because; there is too much aliasing frequency (artifacts generated) in the sweep signal from 0 to 48,000 Hz). If you can see; the artifacts(aliasing signals) are the background cobweb like colors. Some of these artifacts are around -60dB which can still be considered as a noise.

Below is the sample rate converter of Voxengo R8Brain free (sweep frequency signal mode):

As you have noticed, the only present signal is the original converted signal and there is no aliasing or artifacts introduced in the sample rate conversion process. You can see that the background is pure black (no cobwebs) indicating the entire absence of aliasing frequencies.

You noticed that after 22000 Hz, Voxengo R8brain low pass filter successfully attenuated the frequencies above this level because they cannot be represented in a 44.1 KHz format. Unlike the previous graph where you can still hear artifacts introduced after 22 KHz, that is one of the audio rate converter imperfections.

More of the test details and explanations are available here: http://src.infinitewave.ca/help.html