Operator`s manual

Page 32 PRINCIPLES OF OPERATION
C
YBER
A
MP
380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.
Audio Monitor
When monitoring data, experimenters need not be limited to their sense of sight. The data may also
be monitored with great sensitivity by ear. The data are input to an audio monitor and fed to a
loudspeaker or to a set of headphones.
The audio monitor input is chosen from the output of any of the eight signal-processing channels or
from an external input connected to a BNC on the rear panel.
Click Mode
The audio monitor in the CyberAmp has two modes of operation. In CLICK mode, signals in the
bandwidth from 20 Hz to the low-pass filter -3 dB frequency directly drive the speaker. The
bandwidth is further limited by the internal speaker, which has an audio bandwidth of about 200 Hz to
5 kHz.
This type of audio monitor is frequently used in EMG monitoring and in central nervous system
recording. Each spike is heard as an audible click, and the rate and volume of clicking is a good
indicator of the muscle or nerve activity. CLICK mode is sometimes referred to as
AM
(amplitude
modulated) mode in other instruments.
Hint. If the signal is small or slow, clicks may be difficult to hear. An unused CyberAmp channel can
be configured to improve the sound. AC couple the signal into the unused channel using high gain
and the low-pass filter set to 10 kHz. The signal will probably cause this channel to saturate, causing
distinct clicks each time the signal is clipped.
Tone Mode
In TONE mode, the CyberAmp 380 audio monitor is a tone generator with output pitch modulated by
the DC level of the input signal. The pitch increases with increasingly positive signals. This type of
audio monitor is often used in intracellular recording. It provides a very sensitive monitor of a cell's
membrane potential. TONE mode is sometimes referred to as FM (frequency modulated) mode in
other instruments.
The range of tones is from 10 Hz to 3 kHz. Even though sinusoidal frequencies below 20 Hz are
inaudible, the output of the audio monitor in TONE mode can easily be heard at these low frequencies
because the drive to the speaker is a square wave. At 10 Hz, you hear the high frequency click of the
square-wave transitions repeated 10 times per second.
Squelch and Offset
In Click mode, the background signal activity between signal pulses is often audible. It sounds like
static or noise, and can be quite annoying. The SQUELCH control can be used to suppress this
background noise. The operation is described in Figure 13. The audio signal is AC coupled to
remove any DC level. Signals smaller in magnitude than the SQUELCH level are suppressed.
Signals that exceed this level are amplified by the audio monitor. Note that the SQUELCH level is
applied symmetrically about zero volts.