SDRC: New Bitmap Display helps to raise DX!

Bitmap Display showing 24 hours from 0 to 25MHz from a recording of 23NOV2021.

Simon Brown, G4ELI, author of free SDRC software to control (and much more …) most of the SDRs walking on earth, again surprised the community: he added a stunning fast “Bitmap Display” to get a literally overlook onto the content of a recording. The screenshot at the top shows a 25 MHz recording over 24 hours, made with Winradio’s Sigma SDR (16 bit), produced from a near-9TB file within only few seconds. It clearly shows how propagation follows the sun. Medium wave signals thin out after sunrise (06:56UTC here on 23NOV2021) to fade in just before sunset (15:16UTC). You also see the still active broadcasting bands, and, alas, also some interference from PVs at the higher end of the spectrum. You also see the power of s state-of-the-art SDR like this Winradio Sigma, at a professional wide-band active vertical dipole antenne MD-300DX.

See, for comparison, the range of 24MHz/24 hours on a summer day, namely 08JUN2021 (SR 04:00/SS 19:39 UTC), with Elad FDM-S3:

During a short June’s night, the lower frequencies are only sparsely populated And on the higher frequencies you see something of a “summer’s depression”, where in the late autumn’s screenshot they get some boos from the “winter anomaly”, but fading in later and fading out much earlier.

This “Bitmap Display” is called via the tab “Rec/Playback“, then menu “Navigator“. It works on recorded HF files with a fixed width of 4096 data points. So, with a recording of 25MHz width you get a freqeuncy resolution of roughly 6kHz. This makes it ideal for AM broadcast under 30MHz, as well as for all wider modes above 30MHz, let it be the full FM band to identify even short openings, the airbands to check most active channels etc. The time resolution can be set between on second and 60 seconds, see screenshot below.

The time resolution can be set in seven steps.

This “Bitmap Display” adds to the alread known “Grid Display” which still is on board, see the two screenshots below.

Toggle between “Bitmap” and “Grid” display …
“Grid Display”: set ot 06:00 UTC.

Both displays set the recording to the matching time by just a mouseclick. The frequency, however, has to be set separately in the “Receive” Panel. You can switch beween this two windows with a tab at the left bottom, see the following two screenshots.

Toggle between Receive and Playback with Bitmap/Grid.
The reception frequency is shown on the “Bitmap Display” as a white dashed double arrow, pointing to this frequency on the scale at the bottom of the display (here: 9420kHz has been tuned).

The ingenious double function “Click and display time and frequency” is still reserved for the File Analyser module, which is somewhat more complex to operate.

More than just a consolation for this, however, is the loop function: here you set the times for the start and end of the loop by numerical input or simply by mouse click – and off you go! See the both screenshots below:

Start and end of the playback loop can be set either numerically, or …
… by a right mouse click which will duly transfers the time for starting and finishing into the numerical display shown above. This has the advantage to match start/end time visually to the footprint of a signal.

One very fine feature is zooming into the “Bitmap Display”. Even though this software zoom does not change the resolution, this function is an important tool for checking the occupancy of a broadcast band, for example, and for jumping specifically to the start of a broadcast.
Frequency-wise – by position and bandwidth – the slider below the running Spectrogram (“Waterfall”) of the main window is responsible for this. This can be moved as well as changed in its width, so that the corresponding area is displayed. The following two screenshots are more helpful than any quick guide.

The slider has three handles which a mouse click transform into a double arrow to change lower end, upper and end centre. For a better time resolution, this has been changed here from 60 seconds to 1 second per pixel. With the scrollbar on the right you may scroll through the whole “Bitmap Display”.
Here the zoom has been set to the 25 meter broadcast band (slider), and the time resolution set to 1 second/pixel. With the scroll bar, I scrolled the “Bitmap Display” to around 13:00 UTC, and I clicked to 13:26 UTCon 12’040kHz.

It is also possible to tune to a specific frequency when only the “Playback” window is open, and not the “Receive” window. This workaround-like procedure is done by the function/window “Frequency Database” which has to be filled with at least one set of channels. I use the voluminous ILG for this.
With this or another database already loaded, click View -> Frequency Database. Your SDRC window should look like the screenshot below:

How tuning is accomplished by the “Frequency Database”.

Then set all the demodulation controls to match the type of signal you want to recevie, i.e., AM and 5kHz etc. for broadcast. In the next step, simply double-click to the frequency entry in your “Frequency Database”. The “Receive” frequency changes (as you might hear). If your displays had been zoomed and the new frequency is out of focus, a simple trick brings the new channel to full glory: click to “Centre”, see screenshot below.

A click onto the “Centre” icon, and the zoomed “Bitmap Display” window etc. is changed to that channel.

Thanks, Simon, for another great feature of your software!


What are the main differences between the “Bitmap Display” and the “File Analyser”?
* The “Bitmap Display” is by far faster to build up a spectrogram. It also features the whole bandwidth of a recording.
* The “File Analyser” is more flexible in frequency resolution, offers “see, click, tune” when a spectrogram has been built up, and features flexible CSV export of data – up to the whole spectrogram. But it takes much looonger to build up.

Can the “Bitmap Display” also being used to raise short-living utility signals like ALE?
* It depends. Limiting factor is the frequency resolution. With some experience, I can clearly make out ALE signals in an 1MHz wide recording, 1 second time resolution.

Do you have a wishlist? Thanks for asking, but it is an only small one:
* It would be nice if there were several options for (higher) frequency resolution. OK, it will slow down processing, but …
* As I like to process spectrograms, a CSV export would be welcome (as with the File Analyser).
* Undoubtedly, to change not only time, but also frequency would be the ice on the cake.



  • See, click, tune isn’t available in the BMP display and it can’t click, drag and zoom? That’s disappointing so still catching up with the excellent Vasili File Player plugin for SDR Sharp.

  • Hi, Rob – „see“ is the main feature of the BMP. „Click“ is available for choosing the time. „Drag and zoom“ is both neatly done by the slider. Click onto a database entry tunes the receiver. And, fallback, the „File Analyser“ tool stays tuned with much more features. All those things are explained with many examples and screenshots in my blog. Am I really that bad in explaining? SDR Sharp, btw, excels in many applications. But it so far fails in visualizing etc. what I call big data: files of 30MHz width over 24 hours, weighing >9TB. 73 Nils, DK8OK

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