Description and Configuration of the Downward Expander

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Description and Configuration of the Downward Expander

Description and Configuration of the Downward Expander

  System Dependencies
  Minimum PowerSDR Version:     2.0.0 
  Applicable Hardware:     All FlexRadio Transceivers

Content provided by: FlexRadio Systems Engineering

A noise gate is a sound processing function that reduces background noise by muting a sound signal when it falls below a certain level and restoring it to full gain when the level increases again beyond a threshold setting. A noise gate must be used with care, because it may cut the signal off completely when you are talking quietly.  A noise gate can have a harsh sound due to the "all off /all on" nature of managing the microphone gain input into the transmitter.

With the release of PowerSDR 2.0, the  Noise Gate transmit audio processing control has been updated to be a more versatile control that behaves as a Downward Expander (DE). The button label on the PowerSDR console is now DEXP.

A downward expander is not a noise gate, per se.  A downward expander variably attenuates the mic gain by a certain percentage below the a gain threshold setting.  The function of a good downward expander is to increase the apparent dynamic range of the system by decreasing the gain during the relatively quiet times thereby moving the apparent noise floor downward. It does this by comparing the signal level to a threshold gain level.  When the signal level drops below this threshold, the downward expander decreases the system gain by some ratio.  A noise gate on the other hand, does not decrease the gain but switches it completely off below the threshold level.  A noise gate is a DE at it's most extreme setting where the slope (ratio) is infinite.

With the previous noise gate function, it worked best if there was not a lot of loud ambient background noise or the intensity of the background noise was mostly constant.  If the background sound levels were close to your voice level or the intensity of the background noise was not constant, you could not achieve "good gating" between words or worse, your threshold setting would be so sensitive that you would experience a "pumping" effect of the gate opening and closing in rapid succession while you were talking.

The DE provides what is known as a "soft knee" where the transition from closed to open isn't so abrupt (all off to all on) so that the "pumping" condition is no longer a problem.

So How Do You Use a Downward Expander?
The DE has two controls; the Threshold (dB) gain setting and the Attenuate (%) setting.  Both of these have to be set properly for the DEXP to work correctly.  These controls are found in the PowerSDR Setup form on the Transmit tab.  See the example below.

PowerSDR Setup: Transmit tab

For those who have used hardware based DEs previously, this one works a bit differently but the end result is the same.  The PowerSDR DE implementation uses a fixed slope or ratio for determining the gain reduction after threshold point.  The reduction in mic gain below the threshold point is controlled by attenuating the total mic input gain by a set percentage.  This percentage is determined by the Attenuate (%) setting.
  • If the Attenuate percentage is set for 100% you have the classic GATE function (all off/all on). 
  • Any Attenuate percentage value between 99 and 1 results in the a downward expander (DE) function.
  • If the attenuate percentage is set for 0% there is no gate or DE function at all.

Configuring the Downward Expander in PowerSDR
The following procedure will describe how you optimally configure the DEXP feature in PowerSDR.  It is best to use headphones so you can easily hear the change in gain ("soft knee" transition) and not feedback through the speakers.

Open the Setup form and click on the TRANSMIT tab so you have both the Gain and Attenuate % DEXP controls available for adjusting.

Prerequisite: Set your mic gain and EQ settings BEFORE you adjust the Downward Expander, as these settings will have an effect on the operation of the Downward Expander.

  1. Set you DRIVE to 0 on the PowerSDR console so you are not transmitting RF
  2. Make sure DEXP and MON are OFF
  3. Set the Attenuate % to 100
  4. Click on the MOX button to transmit.  >>Don't talk<<.   You want to set the Downward Expander gain to the ambient noise level in your shack.
  5. You will see a gain indicator showing signal in the DEXP box on the PowerSDR console. Move the DEXP Threshold slider control to the right until the red part of the gain indicator turns green and then a little more for good measure. (approximately -40 dB to -36 dB, but this is dependent on you microphone)
  6. Click on the DEXP button to engage the downward expander
  7. Click on the MON button.
  8. Click on the MOX button. You should hear nothing in the headphones because the DEXP is functioning as a GATE since the Attenuate % is 100 (and you will see no signal on the Panadapter either).
  9. Now adjust the Attenuate % value to a lower number one unit at a time until you will hear a little bit of "hiss" in the headphones. You should be able to hear it with an Attenuate % in the low 90s.  Once you hear the background "hiss", adjust the Attenuate % downward a little bit further to provide more of a "soft knee" transition from soft to loud sounds. This is usually 5-10% points below the point where you first started hearing the "hiss".
  10. At this point, talking into to mic should produce a very smooth transition between when DEXP is enabled (mic input is less than the DEXP threshold - when you are not talking) and when the DEXP is not enabled (mic input is greater than the DEXP threshold - when you are talking).
  11. Now all you should have to do to "fine tune" the DEXP is to adjust the DEXP Threshold to where you like when you are not talking. Since the DEXP is not acting as a noise gate, you do not have to set the DEXP Threshold point exactly at the ambient gain transition point (where the gain display turns from red to green).  As an example, if your DEXP Threshold transition point is -50 dB, set the DEXP Threshold value about 10 dB higher (-40 dB)  because the "soft knee" of the DEXP allows you to set it higher, providing for a more natural sound to your voice as the transmit gain ramps up to max value (when you are talking normally).
  12. Reset you PA Drive back to its normal setting.

As noted previously, any Attenuate % value less than 100 *attenuates* the mic input (gain) by the attenuation factor (percentage) providing a downward expander function.  Therefore you will see signal on the panadapter when the DEXP is working and MOX is enabled.  As the Attenuate % decreases, the mic output gain increases as shown on the Panadapter.

Also, listening on a second receiver will not work very well to set the DEXP, as you can not hear the "soft knee" transition point due to atmospheric noise. You must setup the DEXP using the MON function when transmitting.  Once setup, you can listed to your transmitted audio and should notice the soft knee transition between words, sentences and phrases where is is little modulation when you speak.

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Last Modified:Wednesday, April 28, 2010
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