Things that make you go, hmmm… “I’ve been wondering why it’s not possible to use computer audio I/O to generate or receive a SMPTE signal? It sounds a bit silly, but a SMPTE signal is an audio signal, otherwise it can’t be recorded or played on tape machine. Why can’t sequencer software generate or read it through the audio I/O, and slave to the tape machine?”
The textbook answer to this question is because the sync signal needs to get into a data (serial or parallel) port of the computer in order to be able to properly communicate with the software. Consequently a device that converts audio SMPTE to MTC is needed. Additionally, with digital audio recording on the computer the sample rate of the digital audio must be controlled by something that can resolve to incoming time code.
In theory I suppose there would be a way to have an algorithm analyze one of the audio inputs and effectively do the SMPTE to MIDI conversion, but that data would then have to somehow be routed to the proper place in the computer for it to be able to control the software’s tempo and clock rate. While it is conceptually within the realm of reason to think this could be done the actual conduit (hardware and/or software) does not exist in any computer system I know of and I have no idea what it would take to build/write it. Given that this function is already built in to many very inexpensive MIDI interface boxes it probably isn’t worth investigating. After all, who really wants to give up one of their audio connections for this? Plus this solution would do nothing for most of the digital audio systems because quite often their D/A and A/D converters are in outboard boxes that have to be clocked from an outboard clock source that is resolved to the time code.
The future may bring changes, but for now things just aren’t set up to work that way.