Section II

Crash Course in Digital Audio

Whether you studio is digital or analog, there are a few things you need to know before really getting into any recording. When digital audio services are linked together, it is imperative that the user understand terms like word clock, sample rate, and bit rate. So, here’s a crash course in digital audio.

The first thing we need to explain is what this squiggly line that everyone always draws actually represents. For the physics buffs, this displays force over time. For the rest of the world, think of this line as the path the speaker diaphragm will take to reproduce the sound. If you put a ruler vertically across the wave, and drag the paper across underneath the ruler horizontally, the waveform line tells you where the speaker moves when making sound.

Digital audio converters are kind of like a movie projector on 15 cups of Starbucks coffee. In a movie, the moving picture is made up of a series of still images flashed by you at a rate of 24 to 30 frames a second. But remember, each picture is still an image. In audio, we are examining the waveform at a rate of 44,100 times per second ( 44.1 kHz) or 48,000 times per second ( 48 kHz). The number of times per second we are examining the sound wave is our sample rate.

Each time the waveform is examined, the position of the waveform is recorded with a binary number, called a word. The number of digits in the word is your bit rate. ( Each digit is one "bit".) For instance, 0100110100111001 is a 16 bit word, because it has 16 digits in it. 011010001011110100110010 is a 24 bit word, because it has 24 digits in it.

Sampling 1/1000th of a Second of Audio @ 48 kHz

When it comes time to convert the digital signal back into analog, the sample data must be reconstructed. The data itself is simply storing the location of the sound wave at a given interval.

Recovering Sampled Data

Once the data is recovered, the waveform is reconstructed. This is a big game of dot-to-dot, except the converters are typically armed with some fancy mathematical symbols that round out the edges. Once that’s done, you’ve got your sound back!

Reconstructing Waveform

So, the two main aspects of a digital signal are the sample rate, and the word length AKA bit rate. The word length of each device can vary because most digital devices can truncate extra digits out, or fill in blank spaces in with more zeros. But, the sample rate must remain the same when the connection is made digitally. In fact, the sample rate not only needs to be the same, but the sample rate needs to be synchronized between all the units.

The sample rate is synchronized by utilizing a word clock master. One device in the chain will be designated the master, and the others will follow. Some digital lines are internally clocked, meaning they carry word clock information within the line. Others will require separate word clock lines. In Section III, Recording Hookup Instructions, you will find diagrams for some common hook-ups for the TM-D1000, as well as instructions for setting the word clock status on your TM-D1000 mixer.

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