RJ> Any one have an idea of the maximum practical recording level on a
RJ> machine like this? Could it be used at 6 db over 250 nWb/m?
RJ> Thanks, Bob_J

This kind of question comes up frequently. It can be answered by
measurements, if anyone has some simple lab equipment, and the time &
ambition to do the measurements. It would be a great service to the
recording community if someone would do it, and post the results, for
the popular recorders.

One could do all sorts of fancy measurements vs frequency and vs
percent of distortion; and measurements to separate out where the
distortion is occurring. But I think the following simple measurement
would give the basic picture, and be a lot better than the present
ignorance.

I am assuming "one skilled in the art" is doing the measurements,
since I have not by any means mentioned all of the possible booby
traps. Also, this is only one of several ways that the measurement
could be performed. (Sorry if this sounds like a patent
specification!)


Set up the reproducer with a Calibration Tape to some known reference
fluxivity, and for flat response, correct azimuth, etc. Using an
external voltmeter, set the repro gain so that the reference fluxivity
produces a -20 dB level (that's 125 mV for the usual pro machines that
put out 1.25 V for 0 dB on the vu meter). (This is so you can test the
repro pre amp and the recording amp for overload, while not
overloading the line-out amplifier.)

First test the reproducing side:

Feed a 1 kHz signal from an oscillator, thru a resistor (perhaps 50
kohms) into with the repro head (that is, by hooking it across the
repro head output terminals). Look at the waveform at the line out
terminals. Turn up the input voltage until the output waveform
distorts (or clips); reduce the input voltage until that waveform is
clean again. Measure the output voltage, and calculate the input
fluxivity that overloads the pre-amp.

Example: If a 250 nWm/m fluxivity gives 125 mV output; and the input
signal that is "just clean" gives 2 V output, then the maximum input
signal before overload is (2 V/125 mV) * 250 nWb/m = 4000 nWb/m.

In this case, since the highest-output tapes put out a peak fluxivity
of about 2500 nWb/m; and the measured overload was a peak value of
5600 nWb/m (that is, 4000 times root 2); you could easily use any tape
without overloading this preamp.


 For the recording side:

 Choose the kind of blank tape you want measure -- a high-bias,
high-output tape would be good for an "extreme" test; otherwise use
whatever tape you're interested in. Set up the recorder's bias,
recording gain, etc, as usual.

 Many recorders have a resistor in series with the return leg of the
 recording head for monitoring bias current. That can be used to
 monitor the recording current, without having to do any
 soldering-iron work. Put a 'scope across that resistor to monitor the
 recording head current waveshape. (You may also want to put a passive
 low-pass filter in series with the scope feed to cut the bias; a 3-dB
 point around 10 kHz would give about 20 dB attenuation at a 100 kHz
 bias frequency, and that would probably be enough; and would not
 seriously reduce the distortion of the 1 kHz signal.)

 Feed a 1 kHz audio signal into the recorder input, and  look at the
waveshape of the recording current. Measure that input voltage level
for the normal recorded fluxivity. Turn up the oscillator output
voltage until the wave distorts noticeably. Turn back down until it's
just clean. Measure the input voltage level again. The difference in
the levels is the "overhead margin" of the recording amplifier using
this tape and recorded fluxivity. It should be at least 15 dB; better
20- to 25-dB.

Another useful test while you're doing this: Look at the reproduced
waveform when recording this maximum undistorted recording current.
The RECORDED waveform should be distorted -- perhaps more-or-less a
square wave. This shows that the tape overloads before the recording
amp overloads. In fact, if you now reduce the input voltage until the
recorded signal looks like a sine wave again, you can measure how much
overload margin there is between the tape clipping and the recording
amp clipping. That should be at least 6 dB -- 10 dB is better.

In this case, it is clear that that the recorder is "good for"
recording this tape. If the recording amp doesn't have at least 6 dB
margin when the tape clips, it is NOT good for recording that tape.

Jay McKnight

 --
 Best regards,
 MRL
mailto:mrltapes@flash.net