Learning Your ADCs and Your DACs
Once upon a time, the electronic world was all analog; everything was smooth sailing, with infinite variation between one extreme and the other. Then along came digital. Everything became ones and zeros, on and off, yes or no; no variation, nothing in the middle. The cold world of digital, some called it. Somehow, these two worlds had to come together, they needed a translator, otherwise analog equipment and digital equipment could never talk to each other.
That’s where ADCs and DACs come in.
ADCs are Analog to Digital Converters; they take an analog signal, and convert it to a digital one. DACs, Digital to Analog Converters, do the opposite; they take a digital signal and convert it to an analog one. For our purposes, this is most important when talking about audio signals.
The Difference Between Analog and Digital Audio
Before choosing your equipment, it might be a good idea to understand what this stuff is and what it can do for you. So, let’s look at the difference between analog and digital, and see how it affects our sound.
The sound waves you hear are analog. Likewise, the signal that is sent to speakers, in order to produce sound waves is analog. If you send a pure digital signal to a speaker, it would be like connecting the speaker to a battery, with a push-button switch in between. Then take a person with a nervous twitch and have them hit the switch as many times as they can. The only thing that comes out of the speaker will come out sounding like static (not to mention that it will ruin the speaker). A pure digital signal is either on or off; it’s often referred to as ones and zeros.
In its simplest form, an analog signal is a sine wave. It varies from a high limit (positive voltage),
through a zero point, down to a low limit (negative voltage). This is what AC (alternating current) in a house is like. At the peaks of high and low, it measures 120 volts. It cycles between those peaks 60 times per second, otherwise known
as 60 hertz.
Computers do everything with digital signals, those pesky ones and zeros. So, if we want to use computers to help with
our signal processing, we have to change that sign wave from analog to digital. The way that is accomplished is to measure the voltage at equally spaced points through time, and convert each voltage level to a binary number (10011010 for example).
That’s what an ADC does. While you and I can’t understand that number, the computer does.
The number of times that a voltage measurement is made in a second is called the Sampling Rate.
Sampling rates can range anywhere from 6000 times per second, all the way up to 192,000 times per second. The higher the sampling rate that is used, the more accurately the information we are supplying to the computer.
Compact Disks (CDs), have set the standard, operating at 44,100 samples per second.
The greater the sampling rate, the more accurately the original waveform can be produced.
This affects the frequency response, or ability to accurately reproduce high and low notes, especially those super high ones. If you consider that the human ear can hear notes up to 20,000 hertz, a CD is only providing 2 samples for each waveform of that sound; not a whole lot. Higher sampling rates help us not only to distinguish the difference in note, but in the tonal quality of that note; in other words, helping distinguish between a piccolo and a really high soprano
The 44,100 sampling rate of CDs is largely based upon the Nyquist Theorem. To put this into simple language, Nyquist calculated that an analog signal can be perfectly reconstructed if the sampling rate is at least 2x the highest frequency in the waveform. While this isn’t perfect, for the sake of reproducing music, time has proven it to be adequate to our needs.
After the computer processes that information; possibly adding effects to it, such as echo, noise reduction filters, compression or reverb, the signal is converted back into analog. This is done by a DAC. Although the DAC is converting the signal back to analog, the process isn’t really perfect. The output of a DAC is somewhat like the diagram of the digital signal above, stepped waveforms instead of smooth ones.
Actually, since an audio waveform is much more complicated than our simple sine wave, it’ll look something more like this.
The waveform on the top is the entire 1.2 second digital waveform for a laser sound effect like you’d find in a Science Fiction movie. The waveform on the bottom is a chunk of that waveform about where the red spot is located, but lasting less than 1/100 of a second. The little spots on the waveform are the places where the digital signal has been sampled. The audio program that made these images automatically converted the digital “stepped” signal into an analog signal, smoothing out the waveform.
The other specification, besides sampling rate that greatly affects how easily we can understand the audio which has been digitized, is the Bit Depth. This refers to the number of bits within each bite of sound. Bit Depth affects the dynamics of the sound, how many times louder the loudest sound can be, when compared to the softest sound. The rule of thumb is that each bit of increase in the byte (bit depth) allows for 6 dB of dynamic range.
Since 3 dB is roughly doubling the volume of a sound, a bit depth of 16 bits, allows for a dynamic range of 96 dB, or limiting the loudest sound to being no more than 32 times as loud as the softest. While 32 times sounds like a lot, it all depends upon what you’re listening to. Hard Rock has a dynamic range of about 2 dB, since everything is played as loud as it will go. An orchestra, on the other hand, may have a dynamic range that’s even greater than that 96 dB in a typical concert.
There are two main differences that Bit Depth makes. The first is in either dropping out the lowest (volume) sounds or clipping the highest volume ones, thereby causing distortion. The second is in eliminating noise, such as static. A higher bit depth allows a better signal-to-noise ration, getting rid of unwanted noise, mostly static.
Putting all that Theory to Work
Okay, so what does all that technical mumbo-jumbo mean to you and your sound? Well, there’s actually a bunch of things that it means.
First of all, it means that if you have both analog and digital audio equipment, you’re going to need ADCs and/or DACs, so that they can communicate with each other.
Be sure you know which piece of equipment is digital and which is analog. If you’re connecting an analog output to a digital piece of equipment, you’ll need an ADC.
On the other hand, if you’re connecting a digital output to an analog piece of equipment, you’ll need a DAC. Don’t bother trying to hook them up backwards if you’ve got the wrong one, it won’t work.
Secondly, you really don’t want to buy cheap ADCs and DACs. Let’s say that you have a really awesome analog stereo system, and you want to hook up your new 50” plasma TV to it, so that you can have some great sound to go with your movies. Okay, so you’re a little short on cash and decide to go cheap on your DAC.
What you’ve succeeded in doing is to take all that awesome high dollar equipment and turn it into an el chepo special. Your audio quality will always be limited by the lowest quality component in your system, in this case, your DAC.
Now that you know what those specs mean, when you’re looking at ADCs and DACs, look at the specs. Make sure that you’re buying an ADC or DAC that has specs at least equal to the rest of your setup. If you don’t have a great setup, no problem; but if you’ve got a high dollar system, make sure you buy converters that will live up to it.
How are ADCs and DACs Used Anyway?
Generally speaking, DACs are much more common than ADCs. That’s because people use them to connect their digital televisions to their analog sound systems. While there are digital sound systems on the market, most of them are still analog. Even the digital ones convert their output to analog, with an internal DAC, before sending the sound off to the speakers.
If you still have an analog television (yes, some people still do) and want to connect a piece of digital equipment to it, such as a PS3, you’d also use a DAC.
ADCs are mostly used when somebody wants to connect an output of their analog stereo to a digital recorder. This might happen when recording video for editing. You might also end up in this situation if you want to connect another device to your stereo receiver, but the only input that isn’t currently full is a digital one.
Sending Audio Signals over Long Distances
Sending audio signals digitally has the advantage of reducing signal loss, distortion and noise. Because a digital signal is all ones and zeros, these common problems don’t
- Digital signals are typically 5 VDC (that’s volts direct current). Noise on the line is typically less than 100 mv (mili-volts), which is less than 1/10 of a volt, and the piece of equipment receiving the signal is going to interpret anything less than 2.5 volts as a zero, all that noise disappears.
- Voltage drop over distance is typically 5% per 100 feet. That means that a 5 VDC signal is going to drop 1/4 of a volt per hundred feet. Since the receiving digital equipment is going to interpret anything above 2.5 volts as a one, you can send a signal 900 feet without any effect from signal loss.
- Distortion comes from a variety of sources. However, it’s hard to distort a signal that’s either on or off. So, distortion in digital signals essentially doesn’t exist.