Also, in my recordings, I'm getting a higher freq. 'echo' from the
bird songs I'm recording (multiple copies of the spectrogram above the
actual frequency recording is).
Desiree,
As John and Mitch says, clipping can produce extra harmonics,
especially at three and five times the original frequency, but a call
with rich overtones is full of harmonics anyway, as with ravens.
The other source of a double trace is with birds which have a double
syrinx and sometimes produce two tones at once. Sometimes these tones
are wide enough apart to give separate spectogram traces. I love to
slow down recordings like these by four or eight times to hear their
full beauty. Even a sparrow chirrup can have a lot of structure.
Clipping is easy to avoid simply by recording at a low level and the
joy of digital recording is that you can pull the level up afterwards
with no loss in quality. I set my recording levels according to the
noise levels rather than the peak levels, with mic noise about 10dB
higher than the system noise. 12 bits give you a signal ratio of 72dB
which is large, and you have another 4 bits or 24 dB spare. You can
always hear overload and rarely hear digital quantising noise.
Technical note: there is no such thing as 24 bit recording in
practice. The lower 8 bits are all mush in any analogue situation like
real life.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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