Instruction manual

ACU-1000 Operations Manual
2-94 Interoperability Now
algorithm functions: threshold, hangtime, and delay (See definitions below). VOX and
VMR use the same programming commands to set hangtime and threshold. Note: When
VOX is selected, the DSP-2 will default to an audio delay setting of 60 msec and a
hangtime setting of 775 msec. Other times may then be set.
• VMR- The VMR algorithm is designed to detect speech in a wide range of input audio
SNRs. Three parameters determine the performance of the VMR algorithm: threshold,
hangtime, and delay. Note: VMR and VOX use the same programming commands to set
hangtime and threshold. About default settings: when VMR is selected, the DSP-2 will
default to an audio delay setting of 220 msec and a hangtime setting of 775 msec. It’s
possible to rest these parameters to different values, but audio delay cannot be set below
220 msec, and hangtime cannot be set below 775 msec. These minimum settings are
required to ensure proper VMR operation
Threshold: The VOX threshold is signal amplitude related: the higher the threshold, the louder
the input must be to trip the VOX and open the squelch. However, the VMR threshold is not
amplitude related; instead, it specifies how stringent the VMR algorithm is when deciding
whether a signal contains speech or noise. Because of the statistical nature of speech and noise,
the VMR algorithm is not perfect and a performance tradeoff occurs at different threshold
settings: at Low threshold, the unit is least likely to fail to detect speech, but most likely to false
on noise. When the Threshold is set to the High setting, the unit is least likely to false on noise,
but will fail to detect some speech. The correct setting will depend on aspects of the incoming
signal and the requirements of the system. A lower threshold should be used if the input noise
is not excessive, such as from an AM or HF SSB radio. A higher threshold is necessary for use
with an open-squelch FM radio, where full noise is present when no signal is present. The
standard factory setting of Med2 should be suitable for most situations and signal types.
The threshold configuration programming command varies the threshold for both VOX and
VMR. One settings option disables VOX/VMR entirely, so no level of input audio will cause
the VOX or VMR to be tripped. This setting is useful only for system testing.
Hangtime: Hangtime keeps the audio path enabled for an adjustable duration after the moment
when speech is no longer detected, preventing the audio from being muted between syllables or
during pauses in speech.
This configuration item sets the hangtime for both VOX and VMR:
Delay The DSP-2 can add an adjustable delay to the module’s input audio, output audio, or
both.
2.17.2.6.1 Receive (Input) Audio Delay
When speech first appears at the audio input, some time passes before it can be detected.
The adjustable audio delay prevents the loss of the audio that is received before the
detection takes place and audio gates can be opened to send this audio on to other
modules. The amount of delay needed depends on the type of COR in use, as the different
COR methods require different processing times. When hardwire COR is used, the default
delay is 20 msec, because an external COR signal normally arrives before its associated
audio, so only minimum delay is needed. The VMR has a minimum speech detection time
of about 100 msec, so its default delay is 220 msec, which allows time for speech to be