[78-L] half track tapes

[DAVID BURNHAM]

It's certainly possible to play a half-track stereo tape like that, but I think you're getting seriously compromised audio.  Because the stereo half tracks don't line up exactly with the quarter track heads, the left channel head is not hitting the left track in the same place that the right channel head is hitting the right track.

In addition to the volume problem, you may also be getting audio that is going in and out of phase.  Also the pre-emphasis EQ is not the same for half track as it is for quarter track.

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'Pre-emphasis EQ'?  NAB curve is NAB curve (or IEC aka CCIR, etc.)
regardless of track widths.  There is also the previously unmentioned
phenomenon of 'fringe effect' whereby the low frequencies will be slightly
more predominant due to the recorded track widths being wider than the gaps
in the reproduce head.

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It's not so much to do with the recorded track widths as the fact that bass frequencies tend to bleed beyond the actual track - both in width and depth into the medium, treble frequencies are closer to the surface.  This is why, on quarter track tapes, even on a properly alligned machine, you will often hear some of the bass from the other side of the tape going backwards.  This is also why dirty heads cause a loss of high frequencies.

You will get more right channel than left channel when playing half track tapes on a quarter track machine because on a half track tape, there is a guard band at the edges of the tape, (where much of the left channel resides on quarter track tapes), as well as down the centre of the tape.

I can't see any way that playing a half track tape on a quarter track machine, or vice versa, will cause phase shifts between channels.  Certainly damaged tape will not ride evenly on the heads, causing levels to fade in and out but that's different.  Also, a head with the azimuth miss-alligned will cause phase shifts at high frequencies, but that phase relationship won't change.

db