I have digitized numerous standard audio tape cassettes using GarageBand, with an analog cable connected directly from the tape cassette deck to the analog audio input on a Macbook Pro.
After I started doing it that way, I heard Leo Laporte say to several callers on The Tech Guy show not to use a computer's analog audio input for this purpose, because of the possibility of the computer's internal circuitry adding noise, which would end up getting digitized along with the source audio. Of course, analog tapes are inherently noisy (with tape hiss, etc.), and I do not believe in trying to fix noise on the source during the digitizing stage. But I became very concerned about whether I was digitizing any additional noise, and therefore always monitored the digitizing process very carefully with a pair of headphones and my ears.
Other than one occasion when a bad surge protector introduced additional noise, I always felt very confident that I was not allowing additional noise to get digitized using this process. Nevertheless, I agree with Leo and Scott's advice to instead use an outboard USB audio interface, to be safe rather than sorry. In fact, recently, I changed my protocol to using such a device, and I am loving the results.
In GarageBand, I digitize to a new basic track. I have learned to keep the waveforms "healthy"; in other words, set the input gain control to a level so the waveforms don't appear with flat tops.
After digitizing, I right click on the GarageBand project icon (with the application closed), select Show Package Contents, open the hidden Media folder, and copy the AIFF files. (Or even move the AIFF files out of the folder, and subsequently delete the GarageBand project.) Then I can rename the AIFF files to anything I want, and keep them organized with the Finder. Later, I started using Rogue Amoeba's Fission for lossless editing of the AIFF files, when I want to do simple trimming, merging, splitting, etc. I use GarageBand if I want to compress the AIFF files to MP3 files.
