On 16/03/2010, at 8:35 AM, vickipowys wrote:
> I might add that I don't hear this hiss in other recordings I have
> from Andrew (recorded with SASS), nor from my own SASS recordings. =
> Also Paul says that the hiss does not occur in later parts of the
> (long) recordings that they made, so it must be simply the fact that
> Andrew's mics were pointed slightly upwards towards the tree canopy
> and the SASS has picked up leaf noise when there has been a slight
> wind movement.
Hi Vicki and all,
I've gone back to listen to a segment I've chopped out for Rob and I'm now =
not so certain this is foliage noise. To get to the levels of the files we=
submitted to you for inclusion in the journal and which uploaded to the we=
bsite I think the SASS recordings have been boosted by around 12dB+. My HS=
PBA recording has had additional 6-9dB gain applied to match the SASS files=
. When I match and boost another quiet section a similar level of hiss is =
apparent and I am fairly sure this is just the shaped noise floor of the MK=
H mics becoming audible due to the amount of post-processing gain applied.=
Raimund Specht's noise floor testing illustrates the shaping used in the =
MKH mics perfectly - http://www.avisoft.com/test/noisefloors.pdf - the ris=
ing noise floor of the MKH60 above 10khz is clearly illustrated. You could=
consider this as the "dirty little secret" of the MKH series which is tail=
ored to give good A-weighted noise figures. Compare the a-weighting curve s=
hown on http://www.tonmeister.ca/main/textbook/node330.html with the risin=
g noise floor of the MKH-60 shown in the noisefloors.pdf and you can see pr=
ecisely why the MKH-60 has a good a-weighted noise spec. There is obviously=
less happening above 10khz in most nature recordings so we can often knock=
back 10Khz+ safely with a HF Shelf filter but I know I got a shock when I =
first heard an MKH-416 compared to a ME66 - the HF hiss was really very obv=
ious in quiet recordings.
Warning - this deviates into a bit of a diatribe on noise measurements...
Doing a bit of research on the weight curves I found that "A-weight" was no=
t designed to be used for measuring electronic equipment. The CCIR/ITU_R 46=
8 weighting is apparently more appropriate for this use: http://en.wikipedi=
a.org/wiki/ITU-R_468_noise_weighting
"It is important to realise that the 468 specification covers both weighted=
and 'unweighted' (using a 22 Hz to 22 kHz 18 dB/octave bandpass filter) me=
asurement and that both use a very special quasi-peak rectifier with carefu=
lly devised dynamics (A-weighting uses RMS detection for no particular reas=
on[citation needed]). Rather than having a simple 'integration time' this d=
etector requires implementation with two cascaded 'peak followers' each wit=
h different attack time-constants carefully chosen to control the response =
to both single and repeating tone-bursts of various durations. This ensures=
that measurements on impulsive noise take proper account of our reduced he=
aring sensitivity to short bursts. This quasi-peak measurement is also call=
ed psophometric weighting.
This was once more important because outside broadcasts were carried over '=
music circuits' that used telephone lines, with clicks from Strowger and ot=
her electromechanical telephone exchanges. It now finds fresh relevance in =
the measurement of noise on computer 'Audio Cards' which commonly suffer cl=
icks as drives start and stop."
The same situation really applies to digital field recorders that are often=
subject to low level electronic noise that is smoothed over by a-weighting=
and rms readings of EIN. There seems to be some promising developments in=
that Robin Whittle has proposed to begin writing a free software implement=
ation of a CCIR 468-4 filter during 2010.
http://realfield.com/anm/software/
"The program will be useful for applications including reliable measurement=
of:
=95 Pre-amp and ADC noise.
=95 Microphone noise, as equivalent input weighted SPL noise.
=95 Background noise in any electronic device."
I'll be very interested to see how this kind of filtering will reshape our =
ranking of recorders and external preamps :)
cheers
Paul
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