All information in these pages is copyright (c) 1989-2003 by Roger Nichols.
All rights reserved. Permission for personal reference only, and may not
be reproduced by any method without written permission.
I'd like my overs easy, thank you!
by Roger Nichols
I remember when it was very easy to set the record and playback levels on
analog two track recorders. All of the analog machines, whether they were
mono, two track, 16 track or 40 track, had these little pieces of cardboard
with lines painted on them. Just in front of the cardboard was a little
rubber needle-like pointer that aimed in the general direction of the painted
lines. I know that the pointers were rubber because I never saw one that
wasn't bouncing all around while I was trying to adjust it. To set the proper
level with analog meters, all you had to do was turn a knob until the rubber
pointer hovered in the vicinity of the required reference mark on the cardboard
background. This reference point was 3% total harmonic distortion of the
audio signal stored on the tape. This point is called "Zero".
Any levels deviating from this reference were calibrated in dB. Plus one,
plus two and so on for levels above the reference and minus values in dB
for deviations below the reference. I never was quite clear as to whether
minus levels meant that you didn't have enough distortion in your recording.
If the reading on the meter wasn't just exactly what you thought it should
be, just tap the meter with your finger until the needle moved to the correct
position. Trying to be very accurate during these adjustments meant about
as much as deciding where to stick the garden hose in your swimming pool
so that the deep end won't get more water than the shallow end.
Then came digital recording. The references that we had to go by were thrown
out the window. The "Zero" reference on the digital meters means
just that. You have zero room left in the digital storage word. If you turn
the signal up any louder going in, it won't be any louder coming out, it
will be clipped, cut off, ugly, unusable. It is exactly like trying to pump
two gallons of gas into a one gallon can. It just don't fit!
Manufacturers of digital recording machines have put a little red light
at the top of each channel meter and labelled it "OVER". It doesn't
really tell you when you are over the limit, that is not possible on a meter
that is designed to indicate "OVERS" on playback. That would be
the same as being able to tell whether or not the one gallon gas can actually
had two gallons of gas poured into it. The "OVER" light is triggered
by what I call "The Great Carnak" circuit. It guesses whether
or not your signal is over the limit based on the history of previous samples.
That is, if you have three or four samples in a row that are recorded at
full level, then chances are that some part of the signal was a higher value
than could be stored in a 16bit word. Most professional digital machines
have this "OVER" light set to come on anywhere between 1 and 4
samples.
Where it really starts to get sticky in this "OVER" scheme is
with all of the secondary digital formats, such as F-1 and R-DAT machines.
Most of these machines do the metering in the analog domain. Some machines
rely on digital information to supply the trigger for the "OVER"
light, while others use analog peak indication as the source. The problem
seems to be where each manufacturer decides to turn on these indicators.
If you have ever digitally copied an R-DAT tape from one machine to another,
or played a tape back on a different brand than the one on which it was
recorded, then you know what I mean. The meters not only do not read the
same from machine to machine, a tape that you busted your rear end to record
perfectly on one machine will make the front panel "OVER" indicators
on another machine look like the Highway Patrol car in your rear view mirror.
The "OVER" on the Fostex D-20 R-DAT recorder comes on whenever
you have less than 2dB left. That is like saying that the 2 yard line is
close enough to the goal to count as a touchdown. The Panasonic 3500 indicates
"OVER" when you are less than .5dB from death. Very good. The
Luxman KD-117 R-DAT displays it's "OVER" at only .1dB before the
digital sky falls. You can see that something that something recorded legally
on the Luxman would show a plethora of "OVERS" on the Fostex.
HELP!
Sony makes a stand-alone digital meter called the DMU-30 that can accept
an AES/EBU (or SP-DIF) digital signal and display levels in the digital
domain. I plug this meter in to whatever digital machine I am using. I have
checked it with a computer generated audio signal and it is perfect. There
are switches inside the unit to set the number of full level samples that
will trigger an over indication. It's time to tell every digital audio accessory
manufacturer to make one of these gems. We need them. This little goody
makes life very easy. You know the old saying, "If GOD had meant Man
to fly, He would have bought us all Lear Jets." Well if He had meant
us to get the proper levels on digital tape, He would have bought us all
DMU-30s.
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