Long ago, in a studio far, far away, audio engineers labored with fragile plastic ribbons covered with a mysterious rust-colored substance. Their task: to coax audio recordings onto and off of reels filled with these ribbons. While there was a great deal of science involved, a generous dose of black magic was required to achieve a quality recording. Not to mention the time and money required archiving these delicate recordings for future generations.

Today, things are much easier. Video producers have the ability to record near-perfect performances all day with just the click of a mouse. Editing is easy and mistakes are just an undo away. Archiving with optical discs is the norm and the digital media revolution makes it both possible and practical to keep multiple copies of our work scattered all over the globe. Unfortunately, with all this ease-of-use comes confusion about the process of recording and processing digital audio. In this month's Sound Advice, we'll take a closer look at all those ones and zeros.

History and Math
Back in the late 1920s, a man named Harry Nyquist developed a theorem that describes how to accurately convert an analog signal into a data stream. While the theoretical framework could easily fill this space on its own, the basic equation is pretty simple. To recreate our signal correctly, Nyquist said, we must take a sample of the audio with a period that is at least twice the frequency (pitch) of the audio. Wait. Sample? Frequency? What does that all mean?

Let's use the universal audio CD as an example. The digital audio on a compact disc is sampled at a periodic rate of 44,100 times per second (with 16-bit accuracy: we'll get to that in a moment). If we divide the sampling rate by two, we see that our music CDs can theoretically record audio frequencies of around 22,000 cycles per second (hertz), which is higher than the average range of human hearing. That's a theoretically maximum, however, and it would be nice if we had a little more wiggle room. If it helps, you might think of the sample rate in terms of resolution. More resolution means more detail, in this case, more detail in the higher frequencies.

The number of digital bits we assign to each sample also influences the quality of the recording. In the digital world, there are only two possible values for any memory location: zero or one. You can also think of them as Off and On. Each of our 16 bits can have one of those two values. A little calculation (216) shows there are 65,536 possible combinations in our 16-bit word or sample. That's just one sample. That's a lot of ones and zeros! If it helps, you might think about the audio bit- depth in terms of the number of potential colors for a particular sample.

Of course, as soon as CD audio hit the streets, there were those in the audio community who believed the quality wasn't high enough and began to look to future standards. Although our audio CDs are still 16-bit, 44.1kHz, many cutting edge studios record with sampling rates of 192kHz with 24-bit accuracy. 24 bits allow for over 16 million variations in the audio sample. Most of the desktop audio recording packages will record 48kHz at 24 bits. These numbers will continue to rise as computer technology advances.

Split Personality
So what digital audio system does Mini DV use? Well, actually, it uses two different systems: a high-quality two-channel system and a lower-quality 4-channel system. The high-quality version uses 16-bit accuracy and a sampling rate of 48,000 times per second. This is actually better than CD quality and one of the reasons clever video producers use their camcorders as portable audio recorders as well. When you record audio with these settings, rest assured the quality of the recording will only be limited by the audio sources, not the recording medium. It isn't a coincidence that DVD PCM audio is also 16-bit, 48kHz.

The secondary audio system is 12-bit with a sampling rate of 32,000 samples per second. This is not bad, by any means, but it is not as high as it could be (see the Sidebar for an important tip). So, why did they go to the trouble of designing a completely different, if inferior, audio system?

The 12-bit system has two independent pairs of stereo audio channels: 4 channels in all. OK, great, but what can you do with it? Unless you're recording voice-overs in the field, not much. This feature is a holdover from the analog video days. Those producing video on 3/4-inch tape, back in ancient times, often recorded natural audio on one channel and the voice-over on another channel. During the design phase of the DV format, it seemed like a good idea to allow two stereo pairs of tracks for this purpose. Of course today, most camcorder owners in the Americas don't even know about the option, let alone how to use it.

Squeeze Me
When producing a DV project, it's best to use the 16-bit, 48kHz standard for audio, since it's higher quality and is a natural for DVD distribution. Many editing applications ask you to choose the audio format for your project when you start the program. Most of the time, you can just select an NTSC DV template and the audio settings will be automatic.

While most professional editing applications allow you to mix and match audio formats, including MP3s and WAV files of various rates, it's best to leave the audio conversion to your dedicated audio software. We've seen bugs around this issue in the past: incorrect sample rate conversion will turn your distinguished narration into Alvin and the Chipmunks. I've personally seen two of the major applications choke and die during real-time previews of multi-source audio.

If you have either of these increasingly rare problems, using a dedicated audio utility to properly convert your audio to the correct format before bringing it into your video editor will solve the problem.

I know we've done a little more math than usual in this month's column, but the goal is to give you a better understanding of the digital audio process. Armed with this knowledge, you're on your way to higher quality audio and a streamlined production process.