Drop out gap

So your guitarist needs a hundred takes to get his solo correct. It shouldn’t
be problem as long as you can drop him in at the right points to fix dodgy notes.
The problem is that dropping in is easy enough, and analogue recorders have
carefully timed erase current ramping so that the changeover will be almost
perfect. Dropping out is another matter however. It is possible to drop in almost
at any point, as long as your timing is good, but dropping out always produces
a glitch in the recording so you have to drop out on a gap in the music. If
there isn’t a gap then you’re in trouble. On the ADAT, dropping in
and dropping out are both gap free. If you try it on a 1kHz sine wave (which
is the ultimate test), you’ll hear a short crossfade on entry and exit.
On music you are very unlikely to hear it at all.

Crosstalk

In any analogue equipment there will always be crosstalk, which is defined
as a signal leaking from a path where it is wanted to a path where it is unwanted.
This is a terrible nuisance, especially in multitrack recording when, let’s
say, a loud snare drum beat leaks onto the vocal track. The crosstalk will be
an inconvenience until you decide that the song doesn’t need that snare
drum after all - even when you erase it it will still be clearly audible on
the vocal track, and it becomes a major problem. Digital multitracks cannot
be totally free of crosstalk because there are still analogue signals within
the equipment, but there is no crossover from one track to another, which is
a major benefit (except that you will now become dissatisfied with the crosstalk
performance of your mixer!).

Record crosstalk

This is a different manifestation of the crosstalk phenomenon. During overdubbing
on a multitrack recorder you are asking one element of the head to play back
while another is recording. The recording element will be carrying a large current
while the playback element will be producing only a very tiny one. You will
notice the effects of record crosstalk when you record on the track adjacent
to the high high. As you record and monitor the signals from the multitrack
you will notice that the hihat has suddenly become much louder and much brighter.
If you solo the track you are recording on you will be able to hear it quite
clearly as the result of record crosstalk. It won’t be there when you play
back (apart from normal amount of track to track crosstalk you would expect),
but it’s a pain when you are recording. You may also notice that other
tracks seem to jump up and down in level corresponding to the rhythm of the
track while you are recording. This is a result of the record crosstalk sending
false signals to the noise reduction system. Once again it won’t happen
on playback - but on a digital multitrack it doesn’t happen at all.