From:
>
> Hi Julie,
>
> I would like to explain a different perspective. I know that the majority of
> the Nature Recordist group emotionally favors the MiniDisk system and I
> don't want to re-launch the previous passionate discussion on that subject
> ;-) However, there are also a few arguments that suggest to use a recorder
> that is able to record without compression (e.g. the Marantz solid state
> recorder)
I've waited and waited, it appears there are no hordes of wild eyed
minidisc supporters. Just one old geezer trying to keep the facts
straight. Wasting his time. I'd suggest you go sit with the novice
recordists while I give a different perspective on your different
perspective.
> It is true that the potential ATRAC compression artifacts are often less
> dramatic than expected. However, the ATRAC artifacts heavily depend on the
> frequency content of the recordings. Large signal bandwidths (e.g. noisy
> sounds or high-pitched whistles with harmonics) may push the ATRAC
> compression algorithm to it's limits. This is often not a problem because
> there is usually only little energy above about 8 kHz if you are recording
> at larger distances from the sound sources (higher frequency components will
> be attenuated while traveling over large distances). Things will change when
> you get closer to your subject or if you use a parabolic reflector.
> Consequently, the energy of the harmonics will rise. Unfortunately, the
> ATRAC algorithm does not only distort these high-frequency components - it
> then also degrades the more important lower frequency components. As you
> might imagine, the opportunities for approaching a subject very close are
> usually rare. So, I would be very disappointed, if my recorder failed to
> capture those close-distance sounds appropriately.
So, if I get it right the recording of one bird calling, if recorded
close, will overwhelm the capabilities of ATRAC? Amusing
Then one can assume it won't have enough capability to record any more
species calling simultaneously. Now how is it that I and quite a few
other recordists do this with high quality frequently? With frogs I've
had a maximum of 8 species calling at a site, with sometimes thousands
of individuals. Quite a few of these species use multifrequency calls,
not just harmonics, but combination calls with multiple frequencies
simultaneously. Yet the sonograms of all that is just as clear with
minidisc as it is with DAT, maybe clearer in some cases. And I've not
even touched on the bird and insect species that are in those recordings
simultaneous with the frogs. The Georgia breeding bird study was very
happy to mine out night calling birds from my Georgia Herp Atlas
recordings. As a result I've county records for birds that I was not
even trying to record. Just incidental behind all the mobs of calling frogs.
The point is to say that ATRAC can be overwhelmed by a single call at
any distance is a joke. As those of us with extensive experience with
minidisc in real recording know.
An example of such
> artifacts can be seen on the following spectrogram of a MiniDisk recording
> (made with a pro-level MD Recorder):
>
> http://www.avisoft-saslab.com/compression/MDtest2MD.gif
>
> The following spectrogram shows the same (undistorted) signal that has been
> simultaneously recorded onto a DAT recorder (a CF recorder running without
> compression would produce the same result):
>
> http://www.avisoft-saslab.com/compression/MDtest2DAT.gif
>
> One of the artifacts is a temporarily higher noise floor that can be much
> higher than the self-noise level of the microphone or the recorder
> electronics.
Everyone reading this, if anyone is, set up these two gifs so you can do
a blink comparison. In netscape just put them on two tabs in the browser
and click back and forth between the tabs. Look at the waveform at the
top, not the sonogram, as you do this. Note how much fatter that
waveform is in the MD version. It was recorded at a higher signal
strength than the DAT version. Note, particularly in the second call how
the time period that contains the anomaly on the sonogram has peaks that
are hitting the 0dB cutoff, particularly at the negative end. Since this
waveform display is a extreme compression of all the waves involved
there are most certainly many more points than the averaging shows. Note
also that the peaks are higher on the negative side than positive.
Now what do we make of this? First off, it's not a equal test but a
biased one. In fact the MD was fed a high enough signal to clip at the
loudest points. In addition the offset toward negative indicates the
input signal contained a DC bias.
What will this do. Well, the A/D clipped. Then that flawed digital
signal was fed to ATRAC, which tried to faithfully reproduce those
flaws. A clip introduces random changes in the instantaneous changes in
sound levels, ie the slope of the waveform. It is this which results in
producing random signals all across the frequency spectrum as it's the
slope that gives the frequency. It really did not matter that the
damaged digital went through ATRAC, although it was changed by it. The
D/A at the other end would have produced messed up sound even without ATRAC.
Now, remember that DC offset? Well, it's also a nice way to introduce
extra noise in a recording. Digital equipment is designed for signals
that are symmetric around the zero line, and will produce noise from the
DC offset. It's why good sound editors have the ability to recalculate
the samples to remove the offset.
The conclusion I get from these samples is that the recordist committed
a novice mistake. The gain going into a recorder is something we set.
It's very common for novices to set that gain too high. Because they
don't hear overt clipping they think it's ok. But, this fine type
clipping does exactly as Raimund said, it sounds like a self noise
problem from mic or pre. And it's not a error of the compression.
We have had this discussion many times, the metering on recorders is
still that designed for the more forgiving analog tape. It's not good
enough to accurately run a digital recorder. It gives us a hint, but we
have to learn the rest by using the recorder and checking what happens.
With time and experience with a recorder we will be able to generally
avoid clipping by setting a offset. I typically use about 15 dB of pad,
in other words the high indications on the meter are down about 15 dB
from top. Some may consider this excessive, but even at this level I do
still get clipping sometimes.
> The overall quality of a recording system is always limited by the weakest
> component within the entire signal chain. In certain recording situations
> (at close distances), the ATRAC compression system might become the weakest
> part (instead of the microphone or the preamplifier).
>
> This is of course my personal opinion...
I had thought better of your technical abilities. You lowered my
estimate of your ability with this novice error. Try to become familiar
with the equipment you are testing before testing.
Please, everyone note, I'm not saying ATRAC compression does not change
the digital stream. In fact I'm saying very little about it because the
case here does not pertain to it. Every single recorder there is does
not fully reproduce the signal it's given and that includes minidisc.
I'm only saying that on balance a quality minidisc will do a excellent
job of recording. In fact, in some ways, by more intelligent allocation
of memory ATRAC has advantages over uncompressed. That comes from years
of experience using one and I'm not the first one to have that opinion.
And, if you get closer, lower the gain appropriately to avoid clipping.
It is actually the point of getting closer, to lower the gain so less of
the background noise is picked up. It's not a argument against ATRAC
that you can force a minidisc to clip, you can force any digital
recorder to clip. It has a long history in anti-minidisc, it's Cornell's
main argument to this day. And is as wrong from them as here.
I'm a little ashamed of the group that none of the technically adept in
the group commented on this obvious error. Of course maybe everyone
assumed it was so obvious it did not need to be pointed out.
Walt
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