Well, here are my first measurements. There are some puzzlers here.
All are measured with no mic rolloff, level controls turned up all the way,=
with a 150 ohm resistor in the mic input. Resulting recordings were
FFT-filtered to a flat 20-20k passband, and the noise analyzed to calculate=
the average rms power in dB relative to the full-scale power, using a
sine-wave standard.
I'll space-format a table of the results. Hopes it comes out okay.
Recorder dB-FS gain A-EIN-FS FS EIN
Marantz 670 -62 48 -110 -8 -118
SD722 -78 70 -148
Sony NHF800 -72 75* -147
Jukebox 3 -56 48 -104
dB-FS is the 20-20k unweighted noise level in dB relative to full scale.
gain is the gain of the preamp, if known.
A-EIN-FS is the Apparent EIN relative to the Full-Scale level.
FS is the full scale level in dBu (0 dBu=3D0.775v).
EIN is the measured EIN value in dBu.
*I used the value of 75 dB for a different Sony MD recorder for this figure=
.
Sony doesn't say what it really is, so I've used the value Rob mentioned in=
another post (http://groups.yahoo.com/group/naturerecordists/message/21105)=
.
So, I hoped you noticed the problem with the SD722 and the MD. Both are wel=
l
below the theoretical minimum EIN of -136 dBu for a 150 ohm resistor. My
only conclusion is that either my technique sucks, or both must have a full=
scale level that is well in the positive dBu range. To make the SD722 match=
the published specs for this test ("-128 dBu max (-130 dBV), 150 ohm source=
,
20 Hz-20 kHz BW flat filter, gain fully up") the FS level must be +20 dBu.=
Frankly I doubt this, and am uncomfortable with the results.
Thoughts?
At first I thought it was a problem in the way Audition measured levels.
There everything is called "power", but upon reflection I think everything=
we call "levels" really are powers because the old reliable dB=3D20log(v1/v=
2)
is a power form of the dB calculation (based on P=3DV^2/R). The -78 dB for =
the
722 is much lower than it was before the bandwidth was cropped.
By the way, the Jukebox3 has a very funny noise signal: it's really a 90 Hz=
sawtooth waveform with some noise on top of it. Got to be the disk drive.
Bruce Wilson KF7K
http://science.uvsc.edu/wilson
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