sdrRewind and Black Cat ALE

Many DXers record large swaths of the radio spectrum, and then go back to analyze the recordings, looking for signals of interest. Much of the time, they play the recordings back through their SDR software. This works, but is a slow process, no better than monitoring in real time.

Modern software can dramatically speed up the process. In this article, I’ll show how sdrRewind and Black Cat ALE can team up and speed up the process of finding and decoding ALE (Automatic Link Establishment) transmissions.

Black Cat ALE is a full featured multi-channel ALE decoder for Windows and macOS. It decodes ALE transmissions from either audio fed into a sound card input (live decoding) or from WAVE audio files. Download a copy here:

sdrRewind may take a little more explanation. Rather than just play back an SDR recording file, it allows you to select any of your SDR I/Q recording files, and display a waterfall of the entire file at once, as one large waterfall, with a temporal resolution of one second per line. This is more than adequate to see the various transmissions contained in the recording. Select a signal of interest by dragging a rectangle around it with your mouse, and sdrRewind will demodulate and play back the audio, either to your speakers or a virtual audio device feeding a decoder. It can also demodulate to WAVE files, which can then be fed into your decoding software.

It’s also possible to define a set of frequencies and process several SDR I/Q files at once, generating a collection of WAVE files which can then be fed into the decoding software. In the case of Black Cat ALE, it can be configured to monitor a directory looking for new WAVE files, and automatically process them. So even if the demodulation and decoding process will take some time, you can set it up, then walk away and do something more productive while your computer is busy processing the data. Then come back when it is done and view the results.

Download a copy of sdrRewind here:

Black Cat ALE Configuration:

Select Set Directory To Monitor For New Files from the File menu, and choose the directory in which sdrRewind will store demodulated WAVE files. (Create one if you need to)

Select Monitor File Directory from the File menu. Black Cat ALE will start looking in this directory for new WAVE files. The name of these files must end in “.wav” or “.WAV”. It will ignore any files that already exist in this directory.

sdrRewind Configuration:

Set the directory for your SDR recording files, using Set Recording Directory in the File menu

Open Settings in the Edit menu, go to the Demod Directories tab, and create one or more entries for where demodulated WAVE files should be stored, including at least the directory Black Cat ALE will be monitoring. Create each entry by right clicking on the list and select Add Entry. Then right click on that entry and select Set Path and select the directory to use. Repeat as necessary. Close Settings. Go to Select Demod File Directory in the File menu and select the directory where Black Cat ALE will be monitoring for new WAVE files.

Select one of your SDR I/Q recording files from the list of files in the list on the left side of the main window. After a moment, a waterfall for the entire file will appear. Adjust the min and max dB sliders as necessary for good contrast.

Set the mode to USB.

Find an ALE signal in the waterfall and drag around it with the mouse cursor (can’t find any? Go to another I/Q file). You’ll want to make sure the lower frequency is an integer kHz value (or 0.5 kHz for those ALE channels), so edit the frequency as needed. Zero Frequency kHz in the Edit menu can quickly do this for you. Don’t forget to make sure the upper frequency is high enough to cover the entire ALE spectrum, about 3 kHz. Click the Timestamped button. sdrRewind will demodulate the signal and write it to the specified directory. When Black Cat ALE sees the file, it will open and decode it, printing out the results.

Sometimes you want to decode ALE signals from one or more specific frequencies, over an entire set of SDR I/Q recording files. sdrRewind can help with this as well.

Select Demodulate Multiple Files from the Edit menu and a new window appears.

On the left hand side is a list of your recording files, as in the main window. Select a file and basic information about that file will be displayed: the center frequency, sample rate, bandwidth, starting date and time, and length in seconds.

Demodulation settings are displayed immediately to the right of this, again as in the main window. Configure this for the frequency of interest, then select one or more I/Q files and click the Start button. Each I/Q file will be demodulated and written to a separate timestamped WAVE audio file. The entire I/Q file will be demodulated, from start to finish.

Do not make any changes to any controls in this window will files are being processed.

If you wish to demodulate several frequencies from each file, instead use the list to the right:

Right click in it and select Add Entry. A new row will be added. Set the low and high frequency limits of the IF passband, as well as (optionally) the pass band tuning (PBT). Change the mode by right clicking on it, and select a different mode from the popup menu. The same AGC settings will be used for all entries.

When you are finished, click the Start All button. Each I/Q file will again be processed, this time for each of the frequencies in the list. Click abort to stop processing additional files, however the file currently being processed will need to finish.

The Clear button can be used to quickly remove all entries from the list.

An UNID Pirate Station on 1710 kHz

Several pirate stations use 1710 kHz, but reception here is difficult, due to the Hudson County NJ TIS station, which puts in a strong signal. It’s an annoying pest, and seems to just play the same 3 or 4 pre-recorded messages over and over. I doubt anyone in Hudson County actually listens to it.

Last night, I started to hear some music on 1710 around 2350 UTC (6 February 2018), so I decided to stick around and listen. I also started the SDR recording. Glad I did!

The station was fading in and out, so my reception was alternating with the TIS.

Here’s what I heard. Some songs were ID’d with Shazam, so they could be iffy, plus there could be another station in there:

2352 Eagles “What Do I Do With My Heart”.
2356 “Green Hornet Theme”
0002 Neil Diamond “Stones”
0005 “You Only Live Twice”
0042 Oliva Newton John “Let Me Be There”
0044 Country song?
0047 Johnny Mathis “Wonderful! Wonderful!”
0052 The New Seekers “Look What They’ve Done To My Song Ma”
0055 Jose Feliciano “High Heel Sneakers”
0057 Jewel “Standing Still”
0106 Donna Summer “Macarthur Park”
0109 Elvis “Don’t Be Cruel”
0112 “Wiggle Wobble”
0114 Conway Twitty “Hello Darlin”
0117 Colbie Caillat “Realize”
0121 Howard Jones “No One Is To Blame”
0125 “Heart Of Gold”
0128 “Little Bit O’ Soul”
0130 “As Time Goes By”
0133 “Ahab The Arab”
0148 “How Do You Do It?”
0150 “Nobody But Me”
0203 “Bittersweet”
0210 “Cinnamon Girl”
0217 “It Might Be You”
0221 “Baby, I’m Yours”
0223 “Sara”
0227 “Freaky Behavior”
0234 QRT I think.

I ran the SDR recording files through my Carrier Sleuth app, and produced this high resolution waterfall of 1710 kHz. Click on the image to view it full sized.

The pirate is the carrier around 1710.009 kHz that goes QRT around 0234 UTC. The carrier around 1710.0025 kHz is the Hudson County TIS. I think one of the carriers is another TIS in PA, I heard a mention of an address in PA at one point. Probably KID-761, Bedford, PA, the Flight 93 Memorial.

Look at all the other carriers on 1710! One is probably the Springfield MA TIS, others may be pirates? I am not sure how many other TIS stations are authorized on 1710.

It’s very interesting how there is another carrier around 1709.995 kHz that went QRT the same time as the pirate. It is weaker, and does not have the same wiggles as the pirate carrier, so I do not think it is a locally produced image. I am not sure what it is, or if the sign off time is coincidental.

If the operator of the 1710 pirate sees this post, and would like to send me a QSL / eQSL, it would be greatly appreciated!

KitchenAid Mixer QRM

I discovered a new QRM / RFI source today, my wife’s new KitchenAid 7-Quart Pro Line Stand Mixer. Here’s a waterfall screenshot after it turned on, you can see the roughly 15 kHz spaced bands of interference. These use a DC motor, presumably that is the cause of the RFI, vs mixers with a regular AC motor.

Fortunately she doesn’t use it that often, and she’s testing out a new low carb dough recipe, so I can live with it. Speaking of low carb, here’s our low carb pizza recipe.

More adventures in filtering the power supply for an AFE-822 SDR

I frequency monitor and record the 285-325 kHz DGPS band, looking for DX beacons. Recently, I noticed a noise source centered around 315 kHz, almost 10 kHz wide, on my AFE 822 SDR with a 500 ft beverage antenna:

I tried hunting around the house with a portable radio, looking for it, but could never find it. I then checked on my netSDR, with a 670 ft sky loop antenna, and it was not visible there. Very curious. I then tried the beverage antenna, and could still not observe it. But it was there with the AFE822, with either antenna. This made me suspect noise was entering the AFE-822 through the power supply. I was use the USB input for power, and previously wrote about my attempts to reduce the noise from the power supply. This noise source was new since then, possible due to something else added to the shack.

I decided to put together a filtered DC power supply, using linear wall transformer, and adding filtering via capacitors and an inductor.

The circuit itself is fairly simple:

The output of the transformer I used is about 10 volts under load. I chose a 5 ohm power resistor to place in series, which dropped 2.5 volts, so the resulting DC power supplied to the AFE 822 is 7.5 volts. The value of this resistor depends on the output voltage from the DC supply. The AFE-822 draws 0.5 amps, Ohms Law can be used to calculate the desired resistance. The AFE822 has a voltage regulator inside it (it appears to be an LM7805 variant, possibly low drop out), so it can tolerate a wide range, the AFE 822 website specifies 7 to 10 volts.

The inductor is from the junk box, I don’t know what the value is. While I’m telling myself it helps to filter, I might try to find a known, larger value. The 1000 uF electrolytic capacitors provide low frequency filtering, the 0.047 uF ceramic caps provide RF filtering.

The filter circuit was constructed dead bug style on the lid of a small metal can:

Here it is mounted on the can:

And now the spectrum, with the new power supply. Certainly an improvement:

Yet Another !&*%$! Noise Source

The past few days, I have noticed higher than usual noise levels, generally on the lower frequencies, and particularly on the longwave band, including the 285-325 kHz DGPS band, where I run nightly SDR recordings, to later process the data and decode and detect DX DGPS stations using my Amalgamated DGPS app.

Thinking back to what new electronics devices have been added to the house, two came to mind, a new cable modem, and a new ethernet switch. The switch is up here in the shack, so it seemed to be a likely candidate. The switch is a D-Link DES-1008E 8-Port 10/100 Unmanaged Desktop Switch. It uses a mini USB port for power, using either the included AC adapter, or power from a USB port. When I installed it, I decided to not use the AC adapter, but rather a USB port on my UPS, figuring it was better to not add yet another potentially noisy switching power supply to the mix.

The test was easy, I just unplugged the power to the switch. Sure enough, the noise vanished. Great, the switch is a RFI generator. Or is it? As another test, I plugged it into a port on a USB hub. No noise. Hmm… so it seems that the noise is indeed from the USB port on the UPS. I did not notice any increase in the noise floor when I got the UPS a few months ago, but It’s something I should look into again, just to be sure. The UPS is a CyberPower CP1350PFCLCD.

Here’s a waterfall from the SDR, showing the DGPS band, 280-330 kHz. You can see where I changed the power to the switch from the UPS USB port to the USB hub, the bottom part of the waterfall is when the switch was still powered by the UPS (click to enlarge it):

I still have a noise source just above 305 kHz to hunt down.


I decided to see what I could do to improve things, and reduce the noise floor.

Here is the baseline, after no longer powering the switch from the UPS:

First, I relocated the AFE822 away from the computer and rats nest of assorted cables behind it, powered from an HTC USB charger:

The squiggly noise around 305 kHz vanished!

I then switched to an Apple USB charger / power supply, as their products tend to be a bit better made:

Another improvement, the overall noise floor is a bit less now.

But can we do better? I then switched to an older USB hub for power to the AFE822, that I thought might be better filtered:

I then changed to a linear supply plugged directly into the AFE822. I don’t notice any obvious improvement? Maybe it even looks like a little more noise? Difficult to tell. You can see a DGPS station popped up on 304 kHz while I was switching things around, between the last two tests, it was likely Mequon, WI.

Spying on your neighbor’s grill thermometer – Monitoring the 433.92 MHz ISM Band with an RTL Dongle

Remote weather stations, some car key fobs (although many in the US use 315 MHz), wireless grill thermometers, and many other devices use the 433.92 MHz ISM (Industrial, Scientific and Medical) band. Chances are good that if it is a wireless sensor, it uses this band.

Here is a waterfall showing transmissions observed here, using one of the inexpensive USB RTL DVB-TV Dongles:

The entire waterfall occupies 139 seconds.

You can observe several periodic transmissions. I have a remote weather station and a remote thermometer, so that accounts for two of them.

If you have an RTL tuner dongle, take a look and see what 433 MHz transmissions are occurring near you.

Chinese Firedrake Jammer

Firedrake is the unofficial name of a shortwave broadcast featuring loud oriental orchestral music. It exists solely to jam other signals, such as broadcasts by Sound of Hope, which broadcasts programming from Taiwan that is often critical of Chinese government policies and human rights abuse. The broadcasts only contain music; no form of on-air identification has ever been reported. Apparently, the source of the Firedrake shortwave transmissions is a China National Radio satellite feed.

While international regulations prohibit jamming, this has never stopped China (or the Soviet Union, Cuba, Iran, etc) from doing so. Rather than use a traditional jamming sound, apparently China believes that transmitting music 24 hours a day on dozens of frequencies doesn’t qualify as jamming. Or that no one will notice.

Here is a recording of Firedrake from March 29, 2012 on 14970 kHz.

Below is a relatively current list of known Firedrake transmissions:

Frequency     UTC Time
6280          2200-2400
7105          2200-2300
7280          1100-1300
7310          1300-1400
7310          2300-2400
7525          2300-2400
7565          2200-2400
7615          2200-2400
7970          0000-2400
9200          0000-2400
9450          1400-1600
9540          0900-1100
9635          2200-2300
10300         0000-2400
10965         0000-2400
10970         0000-2400
11500         0000-2400
11550         1200-1300
11760         0900-1100
11820         1330-1400
11980         2000-1700
12130         1500-1630
12160         1130-1200
12175         1300-1330
12175         1600-1700
12230         0000-2400
12300         0000-2400
12600         0000-2400
12670         0000-2400
12980         0000-2400
13060         0000-2400
13130         0000-2400
13270         0000-2400
13500         0000-2400
13850         0000-2400
13920         0000-2400
13970         0000-2400
14400         0000-2400
14700         0000-2400
14900         0000-2400
14970         0000-2400
15070         0000-2400
15500         0000-2400
15745         1230-1300
15750         1300-1330
15750         1400-1500
15800         0000-2400
15900         0000-2400
15970         0000-2400
16100         0000-2400
16700         0000-2400
16980         0000-2400
17100         0000-2400
17250         0000-2400
17450         0000-2400
17560         1400-1430
17920         0000-2400
18180         0000-2400

How Wide Can You Go (And Does the FCC Let You Spew QRM Over HF)

Here’s a waterfall I just made at 2155 UTC today, March 28, 2012, of WWCR Nashville TV on 6875 kHz, as captured by my netSDR running SdrDx software:


The sidebands extend all the way out to 6850 and 6900 kHz. That’s +/- 25 kHz wide. I inserted up to 30 dB of attenuation on the input signal, and the wide sidebands didn’t go away, so I don’t think this is an overloading issue.

Does the FCC have limits on the channel width SWBC stations can occupy? Is this really necessary?


Here’s a waterfall from 2327 UTC, showing both WWCR on 6875 and WYFR on 6915. Both are of similar signal strength, but only WWCR shows the very wide signal. Double click on the image to open it full size:


Over modulation?

FWIW, you can see that with both of these stations on, there isn’t a lot of space left for pirates on 43 meters. At 6925, you run into possible interference from WYFR on 6915. WWCR takes out at least 50 kHz, from 6850 to 6900. There’s several UTEs scattered around as well.

An Afternoon in the 1230 kHz Graveyard

Below is a waterfall of 1230 kHz, captured with the netSDR. The recording starts just before 1700 UTC (at the bottom of the image) and runs until about 0030 UTC (top of the image), click on the image to expand it:

The total frequency width of the graph is 100 Hz, that is it extends +/-50 Hz from 1230 kHz. Now that I have the Rubidium Reference on the netSDR, I don’t have an issue with the radio itself drifting over time.

1230 kHz is a “graveyard” medium wave frequency in the US. There are six graveyard channels, 1230, 1240, 1340, 1400, 1450, and 1490 kHz. These channels were set aside as local channels by the North American Radio Broadcasting Agreement, which went into effect in 1941. The term graveyard comes from the weird mix of sounds often heard at night, as dozens of stations mix together. Graveyard stations are restricted to 1000 watts maximum, and some use well under that at night, sometimes under 100 watts.

As you can see by the graph, even at 1700 UTC (local noon) there are dozens of carriers present. Locally I have WRBS 33 miles away and WKBO 40 miles away. Within around 100 miles, there’s quite a few stations.

As it gets later and the D layer starts to go away, new stations appear, and the existing stations get stronger. At about 2200 UTC (5 PM local time) the background noise becomes more obvious as well.

Two of new carriers have an interesting sawtooth pattern to the carrier frequency.

The FCC requires a +/-20 Hz frequency accuracy for medium wave broadcast station carriers. It looks as though most if not all of the stations maintain that, it is impossible to say for sure what some of the outliers are, they could be MW stations or they could be a semi local QRM.

What’s All This SDR Stuff, Anyhow?

The Software Defined Radio (SDR) has become very popular in the radio hobby scene over the last few years. Many hobbyists own one, certainly most have heard of them. But what is an SDR, and why might you want one, over a traditional radio?

First, a very brief explanation of how the traditional superhetrodyne radio works. This is the type of radio you have, if you don’t have an SDR (and you don’t have a crystal radio).

Here’s a block diagram of a typical superhetrodyne receiver:

superhetrodyne block diagram

The antenna is connected to a RF amplifier, which amplifies the very weak signals picked up by the antenna. Some high end radios put bandpass filters between the antenna and RF amplifier, to block strong out of band signals which could cause mixing products and images.

Next, the signals are passed to a mixer, which also gets fed a single frequency from the local oscillator. A mixer is a non linear device that causes sum and difference frequencies to be produced. I won’t go into the theory of exactly how it works. The local oscillator frequency is controlled by the tuning knob on the radio. It is offset by a fixed amount from the displayed frequency. That amount is called the IF frequency. For example, the IF of an radio may be 455 kHz. Time for an example…

Say you’re tuned to 6925 kHz. The local oscillator generates a frequency of 6470 kHz, which is 455 kHz below 6925 kHz. The mixer mixes the 6470 kHz signal with the incoming RF from the antenna. So the RF from a station transmitting on 6925 kHz gets mixed with 6470 kHz, producing a sum (6925+6470=13395 kHz) and difference (6925-6470=455 kHz) signal. The IF Filter after the mixer only passes frequencies around 455 kHz, it blocks others. So only the difference frequencies of interest, from the 6925 kHz station, get paseed. This signal is then amplified again, fed to a demodulator to convert the RF into audio frequencies, and fed to an audio amplifier, and then the speaker. The IF filter is what sets the selectivity of the radio, the bandwidth. Some radios have multiple IF filters that can be switched in, say for wide audio (maybe 6 Khz), and narrow (maybe 2.7 kHz). Perhaps even a very narrow (500 Hz) filter for CW.

This is a very basic example. Most higher end HF radios actually have several IF stages, with two or three being most common. The Icom R-71A, a fairly high end radio for its time (the 1980s) had four IF stages. Additional IF stages allow for better filtering of the signal, since it is not possible to build real physical filters with arbitrary capabilities. There’s a limit to how much filtering you can do at each stage.

Now, onto the SDR. I’ll be describing a Direct Digital Sampling (DDS) style SDR. The other style is the Quadrature Sampling Detector (QSD), such as the “SoftRock” SDR. The QSD SDR typically mixes the incoming RF to baseband, where it is then fed to the computer via a sound card interface for processing. The main advantage of the QSD SDR is price, it is a lot cheaper due to fewer components. The sacrifice is performance and features. You can’t get more than about 192 kHz bandwidth with a sound card, and you suffer from signal degradation caused by the sound card hardware. Some try to compensate for this by buying high end sound card interfaces, but at that point you’re approaching the price point of a DDS SDR in total hardware cost anyway.

Here is a block diagram of the SDR-IQ, courtesy of RF Space, you can click on it to see an enlarged image.
sdr-iq block diagram

The RF input (from the antenna) goes in at the left end, much of the front end is the same as a traditional radio. There’s an attenuator, protection against transients/static, and switchable bandpass filters and an amplifier. Finally the RF is fed into an A/D converter clocked at 66.666 MHz. An A/D (Analog to Digital) Converter is a device that continuously measures a voltage, and sends those readings to software for processing. Think of it as a voltmeter. The RF signals are lots of sine waves, all jumbled together. At a very fast rate, over 66 million times per second in this case, the A/D converter is measuring the voltage on the antenna. You’ve got similar A/D converters on the sound card input to your computer. The difference is that a sound card samples at a much lower rate, typically 44.1 kHz. So the A/D in an SDR is sampling about a thousand times faster. It is not too much of a stretch to say that the front end of an SDR is very similar to sticking an antenna into your sound card input. In fact, for many years now, longwave radio enthusiasts have used sound cards, especially those that can sample at higher rates such as 192 kHz, as SDRs, for monitoring VLF signals.

The output of the A/D converter, which at this point is not RF but rather a sequence of voltage readings, is fed to the AD6620, which is where the actual DSP (Digital Signal Processing) is done. The AD6620 is a dedicated chip for this purpose. Other SDRs, such as the netSDR, use a device called a FPGA (Field Programmable Gate Array), which, as the name implies, can be programmed for different uses. It has a huge number of digital logic gates, flip flops, and other devices, which can be interconnected as required. You just need to download new programming instructions. The AD6620 or FPGA does the part of the “software” part of the SDR, the other part being done in your computer.

The DSP portion of the SDR (which is software) does the mixing, filtering, and demodulation that is done in analog hardware in a traditional radio. If you looked at a block diagram of the DSP functions, they would be basically the same as in a traditional radio. The big advantage is that you can change the various parameters on the fly, such as IF filter width and shape, AGC constants, etc. Automatic notch filters become possible, identifying and rejecting interference. You can also realize tight filters that are essentially impossible with actual hardware. With analog circuitry, you introduce noise, distortion, and signal loss with each successive stage. With DSP, once you’ve digitized your input signal, you can perform as many operations as you wish, and they are all “perfect”. You’re only limited by the processing power of your DSP hardware.

Since it is not possible feed a 66 MHz sampled signal into a computer (and the computer may not have the processing power to handle it), the SDR software filters out a portion of the 0-30 MHz that is picked up by the A/D by mixing and filtering, and sends a reduced bandwidth signal to the computer. Often this is in the 50 to 200 kHz range, although more recent SDRs allow wider bandwidths. The netSDR, for example, supports a 1.6 MHz bandwidth.

With a 200 kHz bandwidth, the SDR could send sampled RF to the computer representing 6800 to 7000 kHz. Then additional DSP software in the computer can further process this information, filtering out and demodulating one particular radio station. Some software allows multiple stations to be demodulated at the same time. For example, the Spectravue software by RF Space allows two frequencies to be demodulated at the same time, one fed to the left channel of the sound card, and one to the right. So you could listen to 6925 and 6955 kHz at the same time.

Another obvious benefit of an SDR is that you can view a real time waterfall display of an entire band. Below is a waterfall of 43 meters at 2200 UTC (click on it to enlarge):
43 meter band waterfall

You can see all of the stations operating at one glance. If a station goes on the air, you can spot it within seconds.

Finally, an SDR allows you to record the sampled RF to disk files. You can then play it back. Rather than just recording a single frequency, as you can with a traditional radio, you can record an entire band. You can then go back and demodulate any signals you wish to. I’ll often record 6800 to 7000 kHz overnight, then go back to look for any broadcasts of interest.

For brevity, I avoided going into the details of exactly how the DSP software works, that may be the topic of a future post.

And yes, I borrowed the “What’s All this… Stuff, Anyhow” title from the late great Bob Pease, an engineer at National Semiconductor, who wrote a fabulous series of columns under that title at EDN magazine for many years.