ZETRON 4000 Series, Installation Manual

Page: 181 / 418

181
Dual Channel Card Audio Levels
Dual Channel Card Audio Levels
Theory of Operation
To adjust the audio levels it is necessary to understand the relationship between the levels
of various types of audio signals used in the system. The voice signals will all pass though
an automatic gain control (AGC). This normalizes the level of speech for a broad range of
input levels. Sine waves such as alert tones that also pass through the AGC are
normalized, but to a different rms level. If the sine wave level is at -6 dB rms then the
speech level will be about -12 dB rms. If the channel being setup uses tone control there
will also be some other transmit tone levels to consider. These are high level guard tone
(HLGT), function tones, and low level guard tone (LLGT). The figure above shows the
relationship between all these signal levels as they leave the dual channel card in a S4000
system. In the example figure it is assumed that the radio input level to cause 67% channel
deviation is -6 dB and that the dual channel card Tx adjustment has been set so the alert
tone level is -6 dB.
The transmit audio level output to the radio is adjustable over a broad range, for
transmitting a sine wave this is from a maximum of +14 dBm and a minimum of silence.
The high level guard tone is 5.5 dB louder than a console generated alert tone. Most voice
peaks are less than 4 to 6 dB above the alert tone peaks. Function tones are 4.5 dB below
the alert tone level. If the alert tone transmit level is set to 67% radio channel deviation the
dashed lines in the example figure will represent the 100% radio deviation level.