Impmooc – Week 2 – Analog to Digital Conversion

Hi there! My name is Karyn Ellis. I am a professional songwriter / performer from Toronto, Canada.

This week I am going to talk about the Analog to Digital Conversion Process. In the following lesson, I will give you an overview of how the process works and talk about the two major parameters involved. I will finish up with a couple of things for you to keep in mind to avoid problems when converting from A to D.

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When we want to record sound digitally using a DAW, we first need to convert analogous sound waves (in the form of voltage variations transduced from sound pressure variations) into numerical information that the computer can process. This is called the SAMPLING PROCESS or QUANTIZATION.

analogtodigitalconversionadj2

From the Encyclopaedia Brittanica, 1999.

The conversion happens inside the audio interface via the ANALOG TO DIGITAL CONVERTER (abbreviated as ADC, A/D or A to D). The A to D creates a string of 0s and 1s — aka BINARY INFORMATION — by measuring the amplitude (volume) of the the incoming voltage signal at repeated intervals. The amplitude at each interval is assigned a value (quantized) and the values are mapped into a series of BITS. This creates a numerical “picture” of the wave shape.

samp_1ADJ

SETTING DIGITAL PARAMETERS OF YOUR PROJECT

Before starting any recording project in the DAW, you need to set the two parameters that will affect the A to D process and the quality outcome of your recording. They are BIT RATE and SAMPLE RATE.

The suggested settings in a professional studio is 24 bit / 48 Khz. We will look at the two parameters in more detail in the following sections.

Bit Rate

BIT RATE (also called WORD LENGTH, BIT DEPTH and RESOLUTION) sets the dynamic range of the recording.

    There are primarily two Bit Rates related to digital studio recording:

  • 16-bit: the standard for manufactured compact discs. This setting has a dynamic range of about 96 dB.

  • 24-bit: the standard for professional studio recording. This setting has a dynamic range of about 144 dB. Human hearing has a dynamic range of approximately 140 dB. 24-bit gets downsampled to 16-bit during the mastering stage of production.

What is the benefit of doing the recording at 24-bit?

The much wider dynamic range afforded by the higher Bit Rate gives you more headroom to play with. You can record at lower values in the amplitude and still get good recording level, reducing the chance of distortion.

Sampling Rate

The SAMPLING RATE (also called SAMPLE RATE or SAMPLING FREQUENCY) is the number of discrete measurements or “samples” taken per second of the amplitude along the analog wave. The sample rate is described in Hertz, and it sets the frequency range of the recording.

The higher the sampling rate, the higher the frequency that can be represented in the project.

    Two main Sampling Rates related to digital studio recording are:

  • 44,100 hz: This is the sampling rate for CDs, which can accurately represent all frequencies we can hear as humans.

  • 48,000 hz: This is the sampling rate used in professional studios. This sampling rate can represent frequencies far beyond the human hearing range, but since it is the standard for video using this setting means your project will be compatible with a variety of applications.

Nyquist Frequency

The highest frequency that can be represented by a particular Sampling Rate is known as the NYQUIST FREQUENCY. That frequency equals half the Sample Rate.

  • At 44,100 hz sample rate, the Nyquist Frequency is 22,050 hz. Human hearing goes to approximately 20,000 hz.

  • At 48,000 hz sample rate, the Nyquist Frequency is 24,000 hz.

Alias / Anti-Aliasing Filter

The Nyquist Frequency is important because any frequency above that number cannot be accurately represented at the corresponding sample rate. Since the sampling process assigns values at discrete intervals, anything that happens between those points is conjectured by the computer. Consequently – as the following diagram demonstrates – if the frequency is faster than half the sampling rate, there could more than one frequency with the same sampling points. This is called ALIASING and causes sound artifacts.

500px-AliasingSines

To avoid aliasing and the sound artifacts created, the input to an A to D must be low-pass filtered to remove frequencies above the Nyquist Frequency — those frequencies that are faster than half the sampling rate. This filter is called an anti-aliasing filter.

Mismatching Sample Rates

Another potential issue to keep in mind is when sample rates are mismatched between recording audio and playing it back. Similar to playing a 33-1/3 LP vinyl album at 45 rpm or vice versa, this mismatch results in tuning / timing issues.

For example, if you recorded a 500 hz sine wave at 48,000 hz for a length of 4 seconds and then played it back at double the sample rate (96,000 hz – another pro studio standard), it would sound like a 1000 hz sine wave playing for 2 seconds – double frequency for half the time.

The difference between recording at 48 Khz and playing back at 44.1 Khz is not nearly as drastic as the example above, but the recording will sound slightly out of tune and will be slightly longer.

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REFLECTION on the PROCESS:

Thank for taking the time to read and critique my lesson. Any feedback you have would be great.

I really enjoyed researching this assignment, but when it came to putting things down into something that is readable I found it quite difficult to organize my thoughts around this topic. I am finding it is hard to explain some of this stuff, even though I feel like I know it. And so I hope it makes sense to you as you read through it. I’ve kind of lost perspective on it.

We didn’t cover Aliasing frequencies in the course material, but the topic was so interesting when I came across it during my research and search for images I had to include it here. Though I feel like I’ve barely scratched the surface of it.

Also, I decided to leave out the stuff about how to find the number of values you can represent in a word length and all that “bit” stuff. Quite frankly, it was wrecking my flow. It just didn’t seem like the best use of space in my lesson.

Like last week, I’m curious whether this succeeds as a “lesson” in its written format or whether it seems to be more of an article. Also, I wonder how readable it is – whether there is enough formatting and whether the images as useful.

Thanks again! Hope you enjoyed!
Best, Karyn

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