Listeners may want to record what they hear on internet streaming SDRs for many reasons. Maritime or aeronautical, military or emergency communications, or amateur radio operations are ever changing resources, fleeting by nature. These are environments where things happen once and do not tend to repeat, so there are advantages to being able to record and archive interesting traffic.
Indeed, sites such as LiveATC.net do archive a large volume of aero mobile radio traffic. Be aware that the audio data they store is reduced in quality, for mass storage, and it is centrally controlled. For top quality archiving and content control, consider making your own recordings.
Here is a brief summary of techniques and considerations for making the best quality recordings when monitoring radio signals on WebSDRs and OpenWebRX sites, which stream three key things: audio, signal spectrum, and signal strength. Review these listed items, then watch the videos embedded above to see them applied to actual radio recordings.
- Set up the recording infrastructure before it is needed. Software, storage devices, which SDRs to use, setting audio levels: have it all ready ahead of time.
- Use a screen capture application on the same system which holds the SDR interface. In other words, if you stream a WebSDR in Firefox on a laptop computer, download and install a screen recorder on that machine.
- In Windows, Use Virtual Audio Cable or Virtual Audio Wire to route audio directly to the recorder application.
- In Linux, Pulse Audio and Jack Audio Kit are used to route audio from the browser to the recorder application.
- Use up the best SDR settings: waterfall color scheme and range, AGC threshold and slope, waterfall zoom, noise blanker, and so forth.
- Record in as high quality as possible: higher bit depths and sampling rates. For RF, bit depths are not changable, but may be increased by oversampling and decimation. RF sampling rates should be as high as possible. For audio, use at least 16 bits, 44100 or 48000 samples per second.
- Set moderate gain levels. Reduce the RF gain / AGC action for strong signals; keep AGC active for weak signals. Audio with no narration should average between -10 dBFS to -5 dBFS. If you will narrate or do a voice-over, set the radio to average between -15dBFS to -10dBFS and your voice to between -10dBFS and -5 dBFS.
- Avoid the overuse of noise reduction or other processing. If signals are weak or noisy, consider judicious use of adaptive filters and manipulating the bandpass to improve reception. Do not mangle the signal; sometimes there is simply too much noise or interference.
- If signals are weak or noisy, is there another SDR site where you can access the signals with less noise or interference? Use the best possible reception site.
- Use a narrower bandpass for weaker signals, especially if there is adjacent channel interference.
- If multiple frequencies are in play or advanced modulation schemes are being used, consider taking a large chunk of spectrum as a quadrature (I/Q) recording. Then you may independently tune and demodulate individual signals in the future.
- Use a good editor to process signals after recording. Some DSP plugins can work wonders against noise.
- A high performance computer system is not absolutely necessary. Smart choices of equipment and software, plus patience, is more important.
- Record more than you think you need. Trimming a long recording is better than having a short recording and wishing more traffic had been captured.
The above listed items cover most, but not all things to consider for capturing interesting signals for archiving and playback for other people in the future. Though they take far more storage space, I/Q recordings are a great way to archive radio events, as future DSP advances may provide better fitering an demodulation. With a very wideband I/Q capture, it is possible to follow action on multiple frequencies, giving a petter picture of what was happening at the time of the recording.
While not strictly related to recording from OpenWebRX or WebSDR internet servers, think about data modes. Data modes (for example: ADS-B, ACARS, FAX, DRM, NAVTEX) are recordable as much as voice modes. Narrowband modes fit waterfall into standard audio recording methods. Wider modes are more suited to captured as I/Q data. Think about the potential gems in a whole day of raw transponder captures on 1090 MHz. Digital spectrum data can be mined and analyzed years into the future.
Good luck with your SDR recordings! Remember to keep them clean, process less instead of more, and always be ready for the next big event to happen when you least expect it.
Tags: WebSDR, OpenWebRX, SDR Recordings