The Nagra IV-SJ Portable Instrumentation Tape Recorder
This version of the Nagra IV comes up at vintage equipment fairs and auction sites from time to
time. I have even seen one at a camera fair in South London once. While an interesting and
probably much rarer version of the famous portable audio recorder, this Nagra is a highly
specialized instrumentation recorder, actually rather unsuited to straightforward high-
fidelity audio recording
. What sets this machine appart from the audio machine is that it has two
'direct' type wideband recording channels that have a frequency response of 2.5 Hz to 35 kHz,
and one frequency modulated channel with a frequency response of 0 Hz (DC) to 4 kHz. It also
has precision switched input attenuators with a resolution of 1dB (see the image and further
comments below), an accurately calibrated 'modulometer' with Peak and RMS scales, and in-
built physiological filters type A, B, C and D. In other words it is actually a precision
recording sound level meter to the 'CEI 179 norm' and not a audio recorder
, and with a
separate B&K measuring microphone, made this recording measuring instrument a popular tool
for noise and vibration analysis. Portable systems such as these were often to be found
measuring the noise generated by jet engines, aircraft at airports, or the rumble of trains in a
tunnel. I imagine that these days a simple laptop and a digital microphone interface have become
the much cheaper modern equivalent, and hence these unwanted specialized Nagras appear from
time to time. They were probably much more costly than the audio recorder, great to admire in a
collection, but are not much use for simple high quality audio recording.
For a while I did wonder quite what was this difference was between a 'direct' recording
instrumentation standard recorder and a conventional 'audio' recorder. This was because a
Racal Thermionic Store Four recorder I have does seem to make rather fine audio recordings.
However, apart from the different ANSI 'instrumentation' standards for record level,
track placing and width (though many of the tape speeds are the same), direct recording
takes very careful account of recorded phase and flatness of frequency response. But
more significantly, unlike the audio recorder's use of high frequency pre-emphasis
(boost) in record, and de-emphasis (cut) on playback, to improve the overall signal to
noise ratio at some cost to phase performance, ANSI 'direct' record electronics do not.
Leaving out de-emphasis and using special tape heads and electronics can produce
astonishingly flat and wideband record / replay performances, but at an unacceptable
cost to the overall signal to noise ratio for Hi-Fi use. In other words the recordings will be
too noisy for audio use.

Click on the above picture for images of the machine's interior.
(Click on the above image for a close up of the tape heads.)