Digitized Amiga Sound

In the September 1985 issue of COMPUTE!, you mentioned that the Amiga computer will be able to digitize music. If I were to plug the output from a stereo system or a radio into the Amiga, could it record the music and play it back exactly like the original? How many different voices does the Amiga computer have?

Robert Patterson

The answer to your first question is a qualified yes. The Amiga can play back digitized sound, but you can't plug your stereo output directly into an Amiga and expect to record music without additional hardware and a program to control it. The output from conventional sound equipment is an analog signal, whereas the Amiga, like other computers, deals only with digital information (binary 1's and 0's). Before doing anything else, you'll need to pass the analog signal through an analog-to-digital (A-to-D) converter to put it in a form the computer can use. That sounds more forbidding than it really is: The components for A-to-D converters are cheap and readily available, and it probably won't be long before you see reasonably priced plug-in digitizers for the Amiga.

Assuming you can convert the incoming signal to digital form, the computer must then sample the signal at a rapid rate—usually thousands of times per second. At each sampling interval, it stores a numeric value which represents the sound input at that point in time. The more frequently you sample the sound, the higher the quality of reproduction—and the more memory is required. The Amiga's 68000 microprocessor runs fast enough to sample incoming signals at an extremely high rate—rivaling the quality of compact disc sound—but even 512K of RAM isn't enough to record significant amounts of high-quality music. Remember that a compact disc can store only up to 75 minutes of music with its capacity of 550 megabytes (563,200K). At that sampling rate, a 512K Amiga could barely record four seconds of music. Of course, by lowering the sampling rate (and accepting somewhat lower quality), that duration can be extended.

At the end of the digital sampling process, the computer has thousands of sample values stored in memory, which can be saved to disk for future use or output directly through a sound channel. To output the digitized sound, you simply reverse the process, reading the stored data from memory, converting it from digital to analog form, and sending the resulting signal to a conventional amplifier at the same rate it was sampled. The Amiga already contains circuitry that can perform the D-to-A conversion at the output end of the process, so sending digitized sound out doesn't require any extra hardware at all.

To answer your second question, the Amiga has four independently programmable sound channels (voices). However, it's difficult to compare them to sound channels on other computers because they're considerably more flexible than tone generators. Most computers are limited to producing one or several basic waveforms, but the Amiga lets you define your own waveforms. And since one channel can modulate (affect) another, it's possible to create extremely sophisticated sounds. Any single channel can simulate a complex waveform, so individual channels can make sounds which would require several channels on other computers. Two of the four channels are assigned to each of the Amiga's stereo outputs, so realistic stereo effects are fairly easy to achieve. The Amiga version of "Switchbox," found elsewhere in this issue, creates stereo effects by switching sounds back and forth between the two stereo outputs.