When I wrote an assembly language card reader suite a few years ago I recall that the clock stream before the start sentinel and after the end sentinel (or before the end sentinel and after the start sentinel if reading the card in the reverse order) was not necessarily garbage but a well defined pattern to stabilize the tape reader's decoding circuitry before actual data started!! I do not seem to recall if this "sync period" data was all 0's, 1's or some other sequence.

- mkaras