Signal Properties Filter Block

 

General description

This block provides a set of general settings, which can be used to modify the incoming signal. DC offset and gain can be set to a certain level. The automatic gain control can be activated. A DC offset can be automatically detected and removed. The saturation for D/A converters can be set active. The noise squelch may also be activated.

 

 

Parameters

DC offset

This is the DC offset (average) set by the user. The scale in the 8 bit mode is –128…127 and in the 16 bit mode –32768…32767.

 

Automatic DC Removal

The properties filter can remove the DC offset automatically. This ensures that the average value of the signal is zero.

 

Gain

The signal power can be adjusted by using the Gain field. The gain can be positive or negative. Negative gain means attenuation. Gain values are given as decibels i.e. 6 dB (voltage) gain doubles the amplitude of the output signal.

 

Automatic Gain Control

This switch activates the AGC. The maximum amplitude in the output is given by the optimum level parameter. So, for example, if the optimum level value is –3 dB the maximum output values are 3 dB below the maximum amplitude, which can be presented in either 8 or 16 bit values.

 

Opt Level

This is the optimum level value for the AGC. The optimal level shows how many decibels the output power is below the maximum power of the output device. The maximum output power depends on the output device format i.e. 8 or 16 bits per sample.

 

Average time

This is the time used to estimate the signal levels for the AGC. If this time is too short, the output level might change too rapidly. If the time is too long, it may cause signal overflows (this means that the signal amplitude exceeds the soundcard capability, or in other words, outside the ranges –128…127 in 8 bit mode or –32768…32767 in 16 bit mode).

 

Saturation Mode

This switch is used to activate the saturation mode. Saturation prevents large signal changes in the sound card’s D/A converters. If the saturation mode is active, then all signal samples, which are over MAX_VAL are set to MAX_VAL or those less than MIN_VAL are set to MIN_VAL. In 8-bit mode MAX_VAL is 127 and MIN_VAL –128. In 16-bit mode MAX_VAL is 32768 and MIN_VAL is –32767. This is effectively a software amplitude limiter or clipper.

 

Squelch

This switch activates Noise Squelch. If the Noise Squelch is activated and pure white noise is detected, the set output is set to a quiet state. When a signal (not just the white noise) is detected, the squelch will re-connect the signal to the output.