SDR Spectrum Recordings | The SWLing Post

Alas, ever since I started using Software Defined Receivers (SDRs) last year, I’ve found that I fill up hard drives faster than I can buy them. As you may have noted, I like to make spectrum recordings–especially during the night-time hours, as I slumber. The following morning, upon waking, I’ll “tune” through, say, the 31 meter band as if it were live. What makes it even more amazing for me, is that I can fast-forward through time and scan for DX stations even more quickly. Great fun–highly addictive. And did I say, space-consuming?

On my WinRadio Excalibur, I find that I use about 4 gigabytes of hard drive space for a one-hour-long spectrum recording, 100 kHz wide. Of course, if I were to record a 2,000 kHz (2 MHz) chunk of spectrum, it would chew through 4 GB in, roughly, 3.5 minutes.

Fortunately, I rarely ever record spectrum that wide. I find that the maximum width I ever record is 1.25 MHz, which I reserve for occasions once in a blue moon. Most of the time, I stick to 100 kHz-160 kHz widths.

After I record a chunk of spectrum, I usually listen to it, create an AF recording of anything of interest, then delete it from my drive. You’d think this would effectively keep my hard drive cleared out, ready to receive the next installment? Not so. Well, at least, not in my undisciplined SDR beginnings.

The flaw in my logic

Quite often, I make spectrum recordings while traveling, and do so remotely (using TeamViewer to control my PC). In the past eight months, I’ve done a lot of traveling. When I return from a trip, I find that I’ve often amassed a sizable collection of spectrum recordings. Upon returning from travel I also find (not surprisingly) that I’m typically busier than normal, catching up with email, phone calls, and delayed appointments. Thus, I never quite get around to reviewing–and therefore deleting–these files. Most of the spectrum recordings taking up space on my internal drives are those I’ve recorded remotely.

Last year, I thought I’d solve the space problem on my ailing laptop by purchasing a dedicated tower PC (Core i5) maxed-out with RAM and with a 1TB internal (7200 RPM) hard drive. This particular Gateway PC also has a bay that accepts cheap internal SATA drives; I simply insert an internal SATA hard drive in the ejectable bay, load the drivers, and it’s good to go. When I purchased an additional 2 TB SATA drive for spectrum recordings, I thought I would be set for years to come…Ah, how the mighty crumble…

As I write this today, I find I only have a total of 350 GB available on my PC. I’ve also filled an entire 1.5 TB external hard drive with recordings I plan to archive and share with a fellow SWLer.

The Tandy Color Computer 2 (or, "CoCo 2") was my first personal computer. (Image: Wikimedia Commons)

The Tandy Color Computer 2 (“CoCo 2”) was my first personal computer. (Image: Wikimedia Commons)

My, how times have changed

Reverse to the 1980s: When I was ten years old, I thought my Tandy Color Computer 2 was the best thing since sliced bread. Its 16 kB was surely plenty of memory for whatever I wanted to do, and the cassette tapes I used as a form of external hard drive gave me the certainty of a virtually limitless supply of memory.

Today, I doubt I could make an intelligible MP3 recording, even with aggressive compression, that would fit a 16 kB file size.

Facing the truth

The frank fact is that I’ve gotten much better at managing hard drive space, now that I’ve been doing spectrum recordings for more than a year. I shouldn’t need to buy additional hard drive space unless it’s specifically for archiving/sharing purposes. I just need to regularly face the music (or static)–dig through spectrum recordings made last year, and delete those I no longer need.

How do I manage space now? Here are my tricks for staying on the wagon, and saving both space and time:

Use the minimum amount of bandwidth possible while making recordings
If possible, have your SDR parse files into 2GB chunks. This makes it easier to delete sections of recording that are no longer needed without having to delete the entire recording. Happily, the WinRadio Excaliber allows for this.
Each time you create a new spectrum recording, have it saved into a specific directory with a label that will help you identify the contents. For example, “Saturday Night Pirates” or “31 M Tues AM.”
Use Notepad or any simple text application and create a log sheet for the spectrum recording; make notes, then save it in the same directory as your spectrum recording.
When saving MP3/WAV files, use a standard file-naming convention to help you quickly ID a recording (you’ll notice all of my recordings do this). Mine follow this pattern: “StationName-Fequency-Date-StartingTimeInUTC.mp3” –e.g., “RadioAustralia-9580kHZ-05Feb13-1000Z.mp3”
Delete unwanted spectrum recordings as soon as you decide they are not worth keeping. If you wait a few days, you may forget that they’re okay to delete.
I also use my Bonito RadioJet for narrow IF recordings (of, say, one station). It allows me to adjust filters and “tune,” but takes very little hard drive space. The same can be achieved by narrowing your SDR spectrum width to 20-48 kHz.

Are you an avid shortwave/medium wave audio archivist (aka, audio addict)? What are your tricks of the trade? Please comment!

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Though I’ve not yet achieved particularly advanced age, my history in radio certainly started with the analog. The vintage Zenith Transoceanic my great-aunt gave me when I was eight was a wonder to tune, and its ability to extract signals from across the planet captivated me. But there was a certain amount of guesswork in the tuning process. So, when I purchased my first digital portable in 1990, it seemed revolutionary: in a snap, I could punch in a frequency, and there I was (virtually speaking). No guessing required.

Front and back of the SSB LAN-SDR software-defined receiver

The next step in receiver evolution was, of course, Software-Defined Radios–those little boxes that you hook up to your computer that allow you incredible tuning flexibility and which permit amazing receiver performance.

So, you’ve never tried an SDR–? I know a fix for that.

In the course of an email conversation with Willi Paßmann, SDR support for SSB-Electronic, I learned that–simply by downloading a couple of files from their website–you can “test-drive” their high-end SSB LAN-SDR.

First, a brief primer…

Some SDRs–like the SSB LAN-SDR–actually allow you to record and to play back HF spectrum segments.

In a basic example: if I want to record pirate radio stations one evening, but am not sure where they might pop up on the spectrum, I can set my SDR to record, say, an 80 kHz swatch of bandwidth from 6,915 to 6,995 kHz, from, for example, 9:00 pm to midnight.

Later, I can play back and listen to the recording, with full demodulation and tuning capabilities. In other words, during playback, I can literally tune around in the spectrum, using all/any receiver functions of my SDR. It is as though I am listening and tuning, live, in real time, though it may be many hours or days later.

Those of you with SDRs will not be surprised by this remarkable feature, as most likely, you’ve already experimented with this incredible time-bending functionality.

Now, back to SSB-Electronic, and how to test-drive their LAN-SDR receiver. It’s easy, actually:

You download the software that runs the LAN-SDR
You download one (or both) available spectrum recordings

Once you install their software and import the recording, you can literally tune through and use all of the receiver’s features within the spectrum recording. You can listen to the noise floor, test the notch, adjustable filters, DSP, tuning rates–literally experience all the receiver functions in this process.

In my humble opinion, this is perhaps the most convenient and enjoyable way to try out a receiver.

Hopefully, other SDR manufacturers will follow SSB-Electronic’s lead and make their control software and spectrum recodings available online for download and testing.

Happy test-driving!

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