> In a message dated 6/20/03 10:48:55 PM Pacific Daylight Time,
>>There's still a lot of recording advice floating
>>around that's straight from analog recording and ruins digital
>>recordings. We need to root that out.
> Dear Walt,
> We (you) have touched on several of these before. But, it would be helpful if
> you could reiterate then in one message for those of us who need help!
I'm sorry I've not had the time on this. Keep tripping over something
missing on my system to dig out of backups, download, etc. And I'm also
busy stuffing my new Palm Tungsten C. Up around 200 frog photos in it so
far, and plenty to go.
The biggest focus is on how we set level readings on a recorder (or any
other place in the chain where we can set them). Standard analog
practice was to set the levels so that they only occasionally hit the
0dB mark, or sometimes even more aggressive than that. When the signal
clipped in analog, it would mostly just limit volume or dynamic range,
but still be pretty list enable. And analog had a lot less dynamic range
than digital, so it was necessary to do this to get acceptable dynamic
Digital, on the other hand is not at all forgiving of anything over the
top, even a single wave cycle. So setting the recorder the same way as
analog resulted in lots of clipped off wavetops. When these are fed
through the output to convert back to analog to play, the output
circuitry sees all kinds of changes in the waveform that it interprets
as other frequencies. And generally loud. Thus you get all sorts of
output sound that was not there. On top of whatever error the input side
did with the overlimit waves.
So, most people learn to back off the level indicated to avoid hitting
the 0dB level, often using the peak functions to try and find the level
of the peaks. This can be done and still have plenty of dynamic range in
digital because you have more dynamic range to work with, a extra 30dB
or more, compared to analog.
Unfortunately, you can run into the limits of metering. A metering
system is averaging, for nearly all even the peak indications are a
average. The true highest wave may be still higher. And even one wave
clipped can mess up the sound. This is where experience comes in. You
have to use the recorder in different situations enough to know just how
far off the indication is, and then set a pad to cover that, as well as
any unexpected loud sound that you might want.
It's this second sort of problem that it's common for people to not
catch. A few wavetops may not sound all that bad clipped, but they do
change the sound. If you are not sure this is happening with your
recording, get it in waveform display and expand it enough to see the
individual waves. Then scroll through the sound and look for clipped
waves. Naturally the most likely place is during the loudest parts.
One should also note that not all equipment clips at a indicated 0dB,
you have to figure it out for your equipment. Some have a built in pad,
and a combination system may have still a different pad. Stick in a
preamp and you may clip when the preamp is not near indicating full. You
have to look at every stage in your system leading up to where it's
converted from analog to digital.
As I've noted, it's not always possible to eliminate all clipping. You
may need to get good recordings of all species present, and the only way
to get the quiet ones is to tolerate clipping the high intensity ones.
This is a common problem in surveying mixed frog chorus sites. As a
example, frogs like cricket frogs put out very high energy, short
"clicks". Recording what else is there will very often clip the cricket
frog sounds. (and those "clicks" are very bad for not being indicated
properly on the meter) It's not that clipping should never happen, but
it's that you should decide when it does.
Now, this is just the most common problem moving from digital to analog.
There are other changes down the recording or processing chain. But
start at the input, learn to get that right first. Then fuss over the
others. We can get into them another time.