Setting Transmit Audio Gain in OpenHPSDR mRX PS - W1AEX

One of the great things about running the ANAN rigs with OpenHPSDR mRX PS is that the developers have made it possible to get up and running out of the box very quickly. It's nearly a plug-and-play operation and it's not unusual to tune across the band and hear a station that is on the air only a few minutes after unboxing their new ANAN. That being said, the developers have also provided users with numerous settings that offer lots of control over the quality of the signal they transmit on the bands. This quick setup guide is a simple step-by-step walk through of how to adjust some critical settings in the TX audio processing chain of OpenHPSDR mRX PS. One additional note to those who are coming from the Flex version of PowerSDR: Keep in mind that the entire TX audio chain in OpenHPSDR mRX is very different from what you worked with in PowerSDR. It is all new from front-to-back and the adjustment procedure in the software is made far simpler by look-ahead algorithms that prevent clipping and distortion products in the TX audio chain. In fact, you will find that it is impossible to drive the Leveler beyond 0.4 dB or the ALC beyond 0 dB. The comments below from Warren - NR0V (one of the lead developers of OpenHPSDR mRX) are an excellent description of what we should aim to do when setting the levels in the transmit audio chain:

"To drive the audio chain in PowerSDR mRX in the way the design intends, you should be frequently hitting 0dB on the ALC meter; frequently hitting 0dB on the Leveler if it is in use; and achieving 0dB or above on MIC if the Leveler is not in use. The speech processor we have in PowerSDR mRX PS is NOT the old compander that was used some time ago, it is a much different algorithm, as are the Leveler and ALC algorithms."

- For those operators who know their way around the setup menus in PowerSDR mRX simply follow the short list of 4 steps directly below. Click on the active links for a more detailed explanation if needed:

1. With the TX multimeter set for MIC, select your source (Mic in, Line in, or VAC) and then set the audio level so your voice peaks are regularly reaching close to 0 dB.
2. With the TX multimeter set for EQ, adjust the equalizer sliders for a flat response, then set the Preamp slider so your voice peaks are regularly reaching close to but not exceeding 0 dB.
3. With the TX multimeter set for LEVELER, adjust the Max Gain so voice peaks are regularly reaching 0 dB.
4. With the TX multimeter set for ALC, check to make sure your voice peaks are reaching 0 dB. If necessary, adjust the LEVELER so that your voice peaks regularly hit 0 dB. Note that this is essential for the Pure Signal algorithm to operate properly.

Those are the basic steps for setting up the initial levels in the TX audio chain. Go ahead and plug your headphones in, enable the MON function, adjust your TX Transmit Filter settings, and adjust your TX frequency response using the EQ sliders. If desired, enable the COMP setting and adjust it according to your preference. Re-check your levels again and when you are done, save all your settings into a new TX Profile.

- For those operators who are new to PowerSDR mRX, the detailed step-by-step instructions below should help you to get your levels set up optimally.

Steps to Follow for Adjusting the Gain Settings Throughout the PowerSDR mRX TX Audio Chain

1. Connect the ANAN to a 50 ohm dummy load and select one of the voice modes in PowerSDR mRX. For now, select the Default TX profile.

2. If you have the COMP button enabled on the front panel GUI of PowerSDR mRX, unselect it now.

3. In the upper left corner of PowerSDR mRX, click on Setup and when the tabbed interface comes up click on the Transmit tab to bring up the menu that is shown in the image below.

4. If you are using a microphone plugged into the front panel MIC jack then select Mic In for the source. If you are using an external audio chain into the back panel DB-25 line level input then select Line In for the source. If the 20dB Mic Boost option is checked I would suggest unchecking it for now. Keep in mind that if your microphone produces low output then you would want to come back to this setting and enable it to assure that enough gain is present to drive the audio chain adequately.

5. In the upper right corner of the PowerSDR mRX interface use the TX dropdown in the meter to select Mic as shown in the picture below. This allows you to view the level of the first audio stage of your ANAN.

It's not necessary to produce RF at this time so slide your Drive level down to "0" and then switch the rig to transmit. Speak into your microphone as you would when making a contact and observe your mic level in the TX Meter. Adjust this level so that it reaches close to 0 dB on voice peaks. Adjust the MIC slider to the right if you need more gain, or to the left if you need less gain. If you find that you are unable to set the gain high enough, go back and enable the 20dB Mic Boost setting in the Transmit tab and try again. When you are getting close to 0 dB on most voice peaks proceed to the next step.

6. Now, press the TX EQ button on the front panel of the PowerSDR mRX user interface and then set the TX meter to EQ. This allows you to view the level of your second audio stage. From the drop down menus along the top left of the PowerSDR mRX interface, select Equalizer and position it on your desktop so that you can access the Transmit Equalizer easily. I would suggest that you enable the 10 band equalizer function at this time if you have not already done so, and for now, leave the EQ sliders flat.

7. Note that the MIC gain and the EQ gain are actually two adjustable gain stages in series, with the output of the EQ stage being the one that is critical. At this particular point in the TX audio chain there is no limiting, so pushing the peak levels beyond 0 dB at this stage may cause ADC clipping which will result in a distorted TX signal. 
Observe the TX meter EQ level as you speak into the microphone and adjust the Preamp slider on the left side of the EQ interface so that your maximum peaks reach close to but do not exceed 0 dB. When the level is satisfactory, proceed to the next step.

8. Set the TX meter to Leveler. Select Setup again and open up the PowerSDR mRX tab for DSP and go to the AGC/ALC settings page.

9. As you transmit and speak into the mic, adjust the Leveler Max Gain (dB) setting upward or downward from the default setting of 5 so that your voice peaks on the TX meter are regularly reaching 0 dB. You will notice that one of the look-ahead-algorithms that is present in the TX audio chain makes it impossible for you to drive the Leveler beyond 0 dB. Think of it as a very effective soft limiter that prevents you from clipping or distorting but gives you an impressive amount of control over the loudness of your transmitted audio. For a little demonstration of the effectiveness of the Leveler visit this page to view a short video showing how the Max Gain settings from 0 to 20 sound to someone else on the receiving end. For now, choose a value where the TX meter starts to hit 0 dB with every voice peak. Move on to the next step when you are satisfied with this setting.

10. Set the TX meter to display ALC and while transmitting, check to make sure that you are regularly reaching 0 dB on voice peaks. If you are not, go back to the Leveler's Max Gain setting and increase the value until your voice peaks push the ALC to 0 dB regularly. You will note that the look-head algorithm at this stage makes it impossible to drive the ALC beyond 0 dB. You can rest assured that as long as the MIC + EQ level is set correctly, there are no worries about distortion as the look-ahead-algorithms in PowerSDR mRX will absolutely prevent your ANAN from clipping and splattering while transmitting. Additionally, when you engage the Pure Signal protocol, it is essential to drive the transmitter hard enough so that the pre-distortion algorithm samples the signal frequently as you are transmitting.

11. I would suggest at this point that you plug some headphones into your ANAN's headphone jack and then enable the MON button in the upper left corner of the PowerSDR mRX user interface. Bring up the software Equalizer again and as you transmit, adjust the equalization of your transmitted signal to suit your audio preference. When you are done with the equalization, check the EQ level with the TX meter to assure that you are reaching near 0 dB at this point of the chain and adjust the EQ Preamp slider as needed.

Note that when you have finished, your audio should sound smooth and clean. You can also create custom transmit filters in PowerSDR mRX to suit the kind of operating you wish to do. Go to Setup and then select the Transmit tab in PowerSDR mRX. In the upper left corner you will see the High and Low settings that you can adjust to create a suitable filter for your needs. In the example below, I set the low cutoff to 70 cycles and the high cutoff to 3070 cycles to create a filter that is exactly 3.0 kHz wide. Once you've adjusted your transmit filter, proceed to step 12 down below!

12. When you are all done with your adjustments, do not forget to save them into a TX profile! Each TX profile saves many of your operating parameters including the TX bandwidth filter settings, front panel Mic setting, EQ preamp level with either the 3 or 10 band adjustments, Leveler gain setting, front panel Compressor setting, and RF transmit Drive level. If you do not save your settings, they will be instantly lost when you switch TX profiles! Before you save your first transmit profile, set your Drive level to a setting that you would typically start out with when operating. I like to set all my profiles with the Drive level set to "35" because that works fine with either of my amplifiers and is low enough that no major catastrophes will occur if something in the station is not set correctly. I figure that once I'm ready to engage in a QSO I can manually slide the Drive level to whatever power level is required for the occasion.

To save a profile, in the Transmit tab as shown in the image above, select the "Save" button and this will bring up a dialog box that will allow you to save your new profile with a unique name. I usually select a name that identifies the mode and bandwidth for which the profile was created, but you can use whatever system works best for you. It would also be very wise at this point to make sure that the two Auto Save TX Profile options (shown in the image above) are UNCHECKED so that your TX profiles are not inadvertently altered as you are operating! As you can see in the image below, I have lots of different transmit profiles to suit operation for different bands, conditions, and modes so that my transmitted signal fits in with whatever is being done by others on the frequency of operation.

Summary: Once you have gone through these settings a few times you'll discover many other settings that are worth investigating. For example, in the pictures up above showing the Transmit tab you will see a check box for CESSB Overshoot Control. When you engage the COMP button (underneath the MIC button in the PowerSDR mRX front panel GUI) this feature comes alive and does a beautiful job of increasing your average RF output without creating excessively large RF output peaks. I keep this feature alive with all my transmit profiles by enabling the COMP button and setting the slider to a modest level of 1 dB and then saving it to each TX profile. Advancing the COMP level to higher levels will increase the processing effect if you wish. Another feature is the downward expander which is enabled with the DEXP button. I have no experience with this setting but Scott - WU2O has mentioned that changing the attenuation factor to 100% makes it a hard gate. He prefers a setting of 90% and notes that 80% is too soft a gate for his use. It's very worthwhile to experiment with different settings while monitoring your own signal with headphones and the MON button engaged.

There are many more settings to investigate but they go beyond the scope of this simple walk through. Hope this helps you get started!

Many thanks to Scott - WU2O for his many content and clarity suggestions for this page!