And so it came to pass, on 08-14-96 09:36,
   that Herry Budiutama spake unto Matt Ion:
MI>> In essence, boosts the high end going to tape, and reduces
  >> it (and hiss along with it) on playback.  It's a little
 HB> This is the definition I got from my tape manual:
 HB> Dolby NR system:         A system that eliminates tape hiss noise
 HB> by boosting low-level signals in the high-frequency range during
 HB> recording, then lowering them during playback.
 HB> Does hiss consist of high-frequency noises?  By this definition if
 HB> the hiss contains any low-frequency sounds, it won't be eliminated.
"Hiss" is, by definition, high-frequency noise.  If it was low-frequency, it 
would be "rumble" or some other phenomenon.  Hiss is a phenomenon particular 
to analog tape recording; it would take too long to really explain, but 
suffice to say, low-frequency tape noise is not generally a problem.
MI>> HX stands for Headroom Extension.  It operates only during
MI>> recording; the
  >> record bias is adjusted automatically depending on the signal
  >> content.  If there's a lot of high-frequency energy in the signal,
  >> the bias is reduced to avoid saturating the tape, to improve
  >> headroom.'
 HB> And what does headroom do?  
Every tape has a limit to how strong a signal can be recorded to it.  
Generally, a tape's noise floor will be at a certain minimum, and the hotter 
you can get the audio signal to tape, the higher it will be above the noise.  
"Headroom" is the amount by which you can safely exceed the optimum signal 
level on peaks.
If the signal you're recording is typically quiet, you'd normally want to 
boost the record level quite a bit; however, if it contains some very strong 
peaks, they can saturate the tape, causing distortion.  If you don't have 
enough headroom, you'd need to turn down overall level to bring the peaks 
under control, but at the expense of bringing the normal signal closer to the 
noise floor.
 HB> Why is saturating the tape bad? 
It's the same idea as clipping an amp -- saturation is the point at which the 
tape can accept no more signal.  Distortion is the result.
 HB> What is the purpose of the bias in the tape?
That takes a little more explaining, which I don't have time for at the 
moment.  Briefly, it's a high-frequency (44kHz-88kHz) signal that's added to 
the record signal to boost the overall level to overcome a certain anomaly 
that affects all magnetic recording at low levels (being an AC voltage, the 
audio signal does cross through a low-level zone every cycle).
 HB> Sorry if I sound dense, but I don't know how analog signal works. I
 HB> have no idea how music is stored on tape.
There are some good books you can check out that go into detail.  The only 
one that comes immediately to mind is Audio Recording Techniques by David 
Myles Huber...
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Q: Why don't blind people skydive?
A: It scares the heck outta their dogs!
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