Using the PowerSDR I/Q Recorder to Listen to Your Transmitted AM Signal - W1AEX


Unfortunately, the way the “Monitor” function is implemented in PowerSDR, you can’t listen to your transmitted AM signal in real-time as you can with SSB, however, it is possible to hear what your AM signal sounds like by using the “Recorder” function of PowerSDR. By setting the recorder to make a pre-processed I/Q recording, you can view your transmitted signal in the pandadpater and listen to your actual transmitted audio. This is very useful for checking your audio for distortion, clarity, and tonal balance. The setup for accomplishing this is very simple:

1.  From the menu choices along the top left of PowerSDR select Wave.
2.  This produces a settings box named Wave File Controls where you should select Options.
3.  In the Wave Record Options box select Pre-Processed Audio for the Receive setting to make an I/Q wave file.
4.  We won’t be re-transmitting what we record so ignore the setting for Transmit.
5.  Click the “X” in the upper right corner of the Wave Record Options box to close it

wave file controls

wave record

When those settings have been made, in the PowerSDR GUI select the AM transmit profile you want to sample then press the Record button in the Wave File Controls box. As the recorder is running, key the transmitter to make a test transmission. I would suggest that you wait about 5 seconds before you start speaking. The reason will become clear when you read the playback instructions below. At the conclusion of your test transmission, press the Record button again to stop the recorder.

To play back the recording you made, press the Add button in the Wave File Controls box and browse for the recording you just made. It will be time stamped so it should be very easy to locate. After selecting the correct file, check to make sure that you have PowerSDR set to receive in the SAM mode and with a bandwidth that matches your transmitted bandwidth. To avoid distortion on the playback of your recorded I/Q file I would suggest selecting an AGC setting of "Long" as other choices in that menu appear to produce a slight garbling effect with AM signals. I would also suggest setting the AGC-T slider to "0" or even "-5" to prevent distortion or an over-compressed sound on playback.

When everything is set, press Play in the Wave File Controls box. Because there is an 11 KHz offset when you transmit, your recorded signal will not be on the frequency you are tuned to. There are a couple of quick ways to center your recording in the receive passband:

1. Simply press the “0 Beat” button repeatedly until it locks in. Note that the reason for waiting a few seconds before speaking when you made the recording was to give you time to get the signal centered in the receive passband for playback.

2. Right click in the panadapter to bring up the "QSY Crosshairs" and then move the crosshairs over your big signal and left click on it. The signal will take off and show up about 11 kHz up the band. Just move the crosshairs onto the signal and left click again. At this point, hitting the zero beat button will center your recorded signal in the passband and you'll be good to go.

(Note: Ignore the smaller version of your signal that appears in the panadapter. It's a cute little replica of your signal but it's not an accurate sample of your transmitted signal.)

Adjust your Volume setting and take a listen to how you sound. What you'll hear is an excellent recording of what your signal sounds like when you are transmitting. What you see in the panadapter will give you an indication of the peaks and valleys in your transmitted frequency response as well as an indication of your transmitted bandwidth.

The link below is a video recording of my Flex playing back a recorded I/Q file of an AM test transmission. You’ll see at the beginning of the recording that it took 6 presses of the “0 Beat” button to get my transmitted signal into the passband and centered. At this point I have found the "Crosshair" method to be the quickest way to get the signal centered.

Although you can’t monitor your AM transmissions in real-time, the I/Q recording function built into PowerSDR can be used to give you an excellent “air check” of your AM signal. For real-time monitoring, a simple RF sampling monitor can be built with a handful of parts, but to be honest, the I/Q recordings reveal much more about your signal and can be a great help when adjusting gain settings, EQ, and compression settings.

After installing a new Marshall MXL-770 microphone at my Flex operating position, I relied on the I/Q recording method to help me find settings that produced natural sounding audio. One of the hardest things to determine was the amount of software compression in PowerSDR to utilize. The temptation to overdo it is great because lots of compression certainly does increase the over-all loudness factor, but when you have the ability to hear the end result of aggressive settings I expect most people will be inclined to dial it back down. The I/Q recording below allowed me to determine that compander settings between 5 and 9 brought up the audio level without too much audio degradation. The DX compressor was over the top for AM use at anything above a setting of 1 or 2, and even then, it degraded the clarity enough to keep me from using it. You can judge for yourself in the video below.

The I/Q recording feature is a great tool that is available for examining any of the voice modes that you care to analyze. Hey, it's built right in and the software is free so what have you got to lose?

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