Spectrum DSP M2 User Manual page 32

available: 1750 Hz and 2135 Hz. The tone burst is activated by pressing-and-holding
button MENU while transmitting.
FM RX Bandwidth – This selects the detection bandwidth when in FM mode:
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Selection of "audio bandwidth" is disabled as it is irrelevant in FM mode and this setting
is unlikely to be changed very often. The selections are:
◦ 7.2 kHz – This is suitable only for "Narrow" operation and even this will result in a
bit of added distortion as the filter about as narrow as it may be to pass audio.
Using this setting the weak-signal sensitivity is the highest amongst the FM filter
bandwidth settings since this narrowest filter also intercepts less noise under weak-
signal conditions.
◦ 10 kHz – This is the "default" bandwidth and is suitable for "Narrow" bandwidth and
while it will work for "Wide" bandwidth, slight distortion may occur on voice peaks.
◦ 12 kHz – This is wider than necessary for "narrow" bandwidth and recommended
for "Wide" bandwidth.
FM Deviation – Two "modes" of FM are available: "Narrow" with +/- 2.5 kHz peak
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deviation (w/1kHz modulation) and ""Wide" with +/- 5 kHz peak deviation, the former
being commonly used on HF and the latter being that which is used on VHF bands
except for those instances where the "narrow" (+/- 2.5 kHz) mode is specifically used.
These two modes are more or less interoperable, with the following provisions:
◦ Operation of "Wide" (+/-5kHz) on "Narrow" frequencies will result in "loud" audio,
possible "squelch clamping" and splattering onto adjacent "narrow-spaced"
channels.
◦ Operation of "Narrow" (+/-2.5kHz) on "Wide" frequencies will result in chronically
"low" audio, reduced "copyability" under weak-signal (noisy) conditions and
everyone telling you to talk louder!
AGC and other receiver-related items:
AGC Mode - The selections are SLOW, MEDium, FAST, CUSTOM and MANUAL.
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These related to the "decay" speed (e.g. "hang") of the receive AGC. When in
MANUAL mode the AGC is disabled and the audio gain is set to maximum – see "RF
Gain", below. WARNING: Reduce volume level before setting this to MANUAL!
RF Gain - This is the same as the "RFG" (RF Gain) control from the main menu and in
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this context it is used in conjunction with the MANUAL AGC mode.
Cust AGC (+=Slower) - When AGC Mode is set to CUSTOM this sets the decay rate
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with a higher setting setting a slower decay. A setting of "12" is equal to the "MED"
AGC setting. Values lower than 3 are displayed in RED to warn the user that the
decay rate of the AGC is likely to be extremely fast, that the resulting audio is likely to
be unpleasant and that a bit overshoot/undershoot is possible on the tail end of a
signal. This parameter is displayed in orange if CUSTOM AGC mode is not selected.
RX Codec Gain - Normally set to AUTO, this determines whether or not the A/D input
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gain on the Codec is automatically controlled based on the input signal levels. If the
input levels start to approach full-scale, the gain of the coded is automatically reduced,
but if these level have not been attained for a while, the gain is gradually increased
again. If this is set to anything other than AUTO there is the risk of significantly
reducing the dynamic range (e.g. performance) of the receiver. When not in AUTO
mode, the settings range from 8, which is "maximum" gain and the highest
susceptibility to overload to 0 which is the lowest receiver sensitivity. Settings other
than AUTO are indicated in RED to warn the user of likely receiver degradation.
Spectrum DSP M2 User Manual V1.1
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