RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
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8-3
8.1.1 Voice and Audio Activated Message Encoding
Voice and Audio activated recording maximizes RAM storage efficiency. By setting control bit VOXREC (1A:3) before
enabling the encoder, the host automatically eliminates beginning of message silence or background noise by delaying
encoding until status bit VOX (17:3) is set.
The encoder sets status bit VOX when the average energy of the message being encoded exceeds the VOX Turn-On
Threshold (see parameter 55 in Section 4.2).
8.1.2 End of Message Detection
Accurate end of message detection improves RAM storage efficiency. The end of the message is reached when the status
bit VOX (17:3) remains reset for a period of time monitored by the host. All or part of the encoded data stored during this
interval may be discarded.
The encoder resets status bit VOX when the average energy of the input is less than the VOX Turn-Off Threshold
(parameter 56 in Section 4.2) for a period of time greater than the voice and audio encoder’s energy averaging filter time
constant. This time constant is programmable (see parameters 57 and 58 in Section 4.2).
8.1.3 Encoder Automatic Gain Control (AGC)
The encoder AGC achieves uniform playback energy levels by controlling the average energy of the voice or audio input.
The host may choose to disable the AGC using control bit AGCDIS (1A:2). Disabling the AGC is not recommended unless
nonuniform playback energy levels are desired. For each design the AGC parameter values must be adjusted to establish
the preferred playback level if different from the default selection. Selected parameter values are to be written to modem
RAM using RAM Access (see Section 4) after each voice or audio encoder mode configuration. Two encoder AGC’s are
provided: Energy AGC and Classifier AGC. AGC selection is performed using control bit AGCSEL (14:6).
The
Energy AGC is fully compatible with the and RFX144V12/ RFX96V12 and RFX144V12-S/ RFX96V12-S voice encoder
AGC, and may be used with either the voice encoder or audio encoder. Control bit DCVOX (14:5) has been added to the
Energy AGC to provide background noise suppression. With DCVOX reset the AGC gain is applied to the input only when
status bit VOX (17:3) is set. The four
Energy AGC parameters are the Energy Reference Level, Slew Rate, Gain Adaptation
Threshold, and Maximum Gain (parameters 59, 60, 61, and 62 in Section 4.2).
The
Classifier AGC, optimized for voice input, relies upon a voice classifier to decide when to adapt the gain using an
advanced adaptation algorithm. The
Classifier AGC, with control bit ENUPDT (14:5) set, compares the average input energy
against an energy threshold to decide when to adapt the gain. The voice classifier dependent gain adaptation is preferred
with voice input The energy threshold dependent gain adaptation is preferred with audio input. The three
Classifier AGC
parameters are the Energy Reference Level, Gain Adaptation Threshold, and the Maximum Gain (see parameters 65, 61,
and 64 in Section 4.2).
A diagram of the AGC operation is shown in Figure 8-2. Flowcharts for selecting the AGC parameters values are shown in
Figure 8-3 and Figure 8-4. Figure 8-5 shows the operating envelope for the AGC parameters.
Summary of Contents for RFX144V24-S23
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