Power Mac needs reconfigured audio hardware

MacWeek, Jan 26, 1998 by Ric Ford

The Mac helped revolutionize audio production and has long been the computer of choice for audio work. Current Macs have high-fidelity audio input and output as standard features. With CD recorders and blank media now available at reasonable prices, plus a decent selection of software, this seems like an ideal time for low-budget audio production on the Mac. Unfortunately, basic hardware problems mar the Mac's audio image. Setting up a Power Mac 7500 for audio work brought these issues to light.

Down to business

Macromedia Inc.'s SoundEdit 16/Deck II package arrived with some interesting notes in the documentation: "There is a bug in the Power Macintosh sound hardware that has an effect on input level settings and monitoring."

To avoid the bug, Macromedia recommends a fixed input level, which also avoids the "low-quality on-board preamp" Apple used in the computer. Digidesign Inc.'s Pro Tools addresses the problem by offering just two recording levels for built-in hardware: on and off. Unfortunately, this means an external mixer may be needed just to control input levels for digital recording.

The second bug appeared as soon as I began capturing audio on a Power Mac 7500. The digital recording contained an obvious and unacceptable low-frequency hum, which was not present with the audio source (a DAT recorder) connected to any other devices. The hum appeared with other sources on the Power Mac, too.

An investigation eventually determined that the hum vanished when the audio source was connected to the Power Mac's mini-stereo input, rather than its RCA jacks. Thinking that this early Power Mac 7500 might have a hardware problem, I repeated the tests on a new Power Mac 8600/200. The same thing happened. Hum appeared on the RCA jacks and disappeared with the same source switched to the mini-stereo input.

Other experiences

Visitors to the MacInTouch Web site described widely divergent experiences. Some encountered no problems recording on Power Macs, while one fellow said he eventually resorted to soldering a ground wire from the Power Mac chassis to the shell of the RCA jack to eliminate the hum.

An 8500 owner reported the same hum on RCA jacks with none on the mini-stereo input. Two other people sent files demonstrating audio distortion on the RCA inputs that was not present on the mini-stereo input. Various tricks provided some relief for hum on others' Power Macs. Aside from grounding wires, running the Power Mac's video input to another device helped in two cases. Discarding an AC adapter and running the source on batteries eliminated the hum, too.

These problems should not occur with an audio card, such as Digidesign's AudioMedia series, but that is no excuse for cheap audio shortcuts in the world's foremost multimedia computer.

Comments on MacInTouch are welcome via e-mail to mit@macintouch.com. The MacInTouch home page is at http://www.macintouch.com.