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
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?
