On Apr 5, 2006, at 9:24 PM, Bruce Wilson wrote:
> So, I hoped you noticed the problem with the SD722 and the MD. Both
> are well
> 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?
The filtering would have quite a lot to do with the reduced noise
power. Take a look at the spectrum of the noise without filtering. The
majority of the power is below 20 Hz, which is typical for thermal
noise. The 722's frequency response is flat down to 10 Hz, and it's
still pretty responsive down to 3 Hz according to the published specs.
My own crude measurements would seem to support that. What puzzles me
is that the noise is pretty white (and very low) through almost all of
the audible frequency range, and only follows the usual pink thermal
noise pattern below about 20 Hz. That makes it awesome for most of what
I record.
When recording a quiet ambience, I roll off below 40 Hz, or sometimes
higher. Any quiet sounds below that frequency would be buried in noise
anyway. The roll-off enables me to use more gain as well as reduce the
rumble.
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