Digital Recording
by Jerry Vigil
Last month, in part one of this article, we set out to somewhat simplify the digital domain for those still unsure of their ability to grasp it gracefully. We focused on the digital control of equipment, one of the two roles digital technology performs in our studios. We compared digital effects processors to the personal computer and established the many similarities between the two. This month, we take a look at the other role of digital technology in radio production -- digital recording.
Let's begin by dissecting this lofty expression, digital recording. The operative word here is digital. This is the word that kindles the anxiety many of us feel towards this new technology. For the most part, this anxiety is unwarranted. Digital recording is still recording as we've learned it. However, because this type of recording is new, we have chosen to affix the adjective digital to specify the method of recording. In time, we'll drop the adjective. Think back for a moment. Before digital recording, there was analog recording; but producers didn't go around saying, "Hey, I work in an analog studio producing analog commercials on my analog recorders!" On the other hand, today's digital producer is saying, "Hey, I'm in a digital studio, mastering digital commercials on my digital recorders!" It's cool to say the word digital, cool to the point that the word is even part of some stations' logos! In a few short years though, the word digital is going to be as hip as the word stereo. The point is that there is no hidden beast in the kingdom of digital recording. We're still recording audio, a little differently, and we describe it as digital recording because it's not the standard -- at least not yet. When today's grade schoolers get their first jobs in radio, things will be the other way around. They won't hear the word digital as much as the word analog. Digital will become the norm and analog the exception needing to be defined as such.
Let's pick on this word analog for a moment. Why do we use this word? What is it that is analogous or similar to something else? When audio is recorded onto magnetic tape, the particles on the tape become aligned in patterns which are analogous to the electrical impulses of the audio waveform that put them there. Digital audio is different from analog audio in that the recorded audio is no longer analogous to the source audio. Digital audio is not a series of patterns on magnetic tape that "look" like the waveform of the source audio, but a series of numbers instead, ones and zeros to be precise. So we call it digital audio -- just words. There was probably a 50% chance of this technology being called numerical audio!
The conversion of this audio into numbers is at the heart of any digital recording device. This is done with your basic Analog-To-Digital (A/D) converter, nothing more than another computer. You don't need to know much about these converters, so we won't tell you how to build one; but a simple understanding of what they do is helpful in getting the full picture of digital recording. Let's begin with sound. Sound, as we know it, is what our minds perceive when air is vibrated at frequencies between 20 and 20,000 times per second. This vibration of the air caused by your voice, for instance, occurs at varying frequencies and amplitudes. It is these frequencies and amplitudes that digital audio deals with. Frequencies and amplitudes can be measured, and this measurement can be represented by numbers. Bingo! -- Where's our number cruncher, the computer? If we can analyze the different frequencies and amplitudes that make up a sound and store that analysis as a series of numbers in computer memory, we then have a digital recording of that sound. The A/D converters are the computers that do this job. Since you can't hear ones and zeros, the digits have to be converted back to analog audio before playback. This is done with the digital-to-analog (D/A) converter. This converter turns the digits back into electrical impulses that can be used to drive an amplifier or align particles on the magnetic tape of your analog recorder.
The rate at which our computer analyzes audio is another aspect of digital recording worth understanding. Quoting Todd Albertson from a recent MIDI Page article, "Music (sound) exists in time." Without time, there is no sound. There's no such thing as a sound zero seconds long! So, if we're going to analyze sound, we must concern ourselves with the passage of time as the sound occurs. The many frequencies that make up a sound can change very rapidly as time passes during the length of the sound and so can the amplitudes of those frequencies. Our computer must then analyze the audio at a given moment in time, record that data, then analyze the frequencies and amplitudes at the next moment and so on. If the analysis is made ten times a second, the result is a better analysis of the sound than an analysis taken only five times a second. More realistically, if the sound is analyzed tens of thousands of times per second, the result is a very good "recording" of the sound. It was decided that what our computer analyzed would be called samples of that sound, and the number of samples taken every second would be called the sampling frequency or the sampling rate.
Last month's MIDI Page examined sampling frequency and a formula for determining the frequency response of a digital recorder or sampler. Basically, the high end response of a digital recorder is 1/2 the sampling frequency. Therefore, a machine with a sampling frequency of 32,000Hz will give you a high frequency response of roughly 16,000Hz. This is about where FM broadcast rolls off high frequencies, so samplers that top out in this range are adequate for radio production. On the other hand, a musician recording music for mass production and consumer use will want the full 20kHz response available from recorders and samplers with sampling rates of 44.1kHz and the clarity of 16-bit processing. Samplers that feature variable sampling frequencies can save memory by lowering the sampling frequency for some recordings. For example, there is very little above the 10kHz range in the human voice; consequently, you can sample the voice at a sampling rate around 20kHz and virtually capture all the frequencies while saving memory in your sampler. This greatly extends sample time. Some samplers offer this variable sampling frequency by making the recording bandwidth variable. Adjusting the bandwidth down to 10kHz automatically resets the sampling frequency to 20kHz or thereabouts.
If we dig a little deeper into digital recording, we start hearing about bits -- 12-bit, 16-bit, 20-bit, etc. Not many people can boast of ever having actually seen a bit, but they're there; and they're either on or off, one or zero. These bits are found in groups, groups of 8, 12, 16, etc. We call these little clusters of bits bytes. You probably also know that a 16-bit sampler gives you a better quality recording than a 12-bit sampler, but do you know why? Maybe, maybe not. Do you need to know why? Not really. As a matter of fact, you can go right on to the next paragraph if you'd like; or you can stick around and try to make sense out of the rest of this one. Let's use 16-bit bytes for this interpretation. When you sample audio at 44,000 times per second, you're taking 44,000 samples. Each sample is stored in a byte. Now, think of each of the 16 bits as separate samples of each byte. Just as we get a better recording of the audio if we sample it several times per second, we get a better representation of each sample, or byte, if we cut those bytes into 16 pieces or bits. Any more bit knowledge than this will drive you batty (...or is that bitty?)
There are two primary advantages of digital recording in radio production. First, is the awesome power of computers to manipulate the audio once it's in the digital form (usually done with samplers). The second advantage is the non-existence of tape hiss and other analog garbage. When you record from a mic to a sampler or digital recorder (like the RDAT), what plays back is what was recorded, with nothing else added. There is no tape noise, no wow and flutter, and no print-through which are inherent with analog recording. Our computer is only reading the ones and zeros. It ignores everything else.
Manufacturers of digital recorders have spared us some agony by keeping our analog education in mind. They've used analog words to replace computer terminology. Instead of writing information to disk, tape, or computer memory, the function is called recording. Instead of reading information from disk, tape, or memory, the function is called playback. They've even incorporated the familiar rewind and fast forward buttons on tapeless recorders to move from the top of memory to the bottom of memory, or from the inside track of a disk to the outside track. So, when you get into digital recording, not much changes. You simply get improved quality and easier manipulation of the audio. Unfortunately, manufacturers didn't keep our analog budgets in mind, but they're working on that. When the technology is available at prices we're used to paying, analog will become history. RDAT cassettes will be the standard cassette deck in a car just like computer memory is now the standard "outgoing message" tape on most new telephone answering machines. Understanding the basics of digital recording will help remove any initial anxiety about this technology; and, as we asserted many times in last month's article, owners' manuals are a must read item for anyone stepping into the digital domain. Remember that the words digital domain are just words. Replace them with two synonyms and you get number world, and this world isn't that complex -- the only numbers in it are one and zero. Once you understand the digital concept, the technology actually seems simpler than its analog predecessor. In fact, the technology is simpler, and it is this simplicity that makes it so powerful.
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