Let’s get started recording SmartSDR IQ Data with Audacity. This lets you convert the 24 bit Flex data stream down to 16 bits for use with the File Source in GNURadio.
One of the great things with software defined radio is that you can record and save chunks of radio spectrum. You have two reasons for doing this. One is to record a whole bunch of signals and listen to them later. Typically, an SDR can provide a complete recording of a large bandwidth containing many signals. Some listeners even automate the process and make recordings unattended.
The second reason to record radio spectrum is to experiment with different signal processing techniques. For example, I have recently been using GNURadio to develop different AM detection techniques and try them out. This is easy when you can save the IQ data from your SDR. Afterwards, you just use the File Source in GNURadio to play back the data as a PCM WAV file.
This all works well with programs like SDR Console, when enables you to record the data from any of its supported hardware. Unfortunately, my Flex 6300 and its SmartSDR software do not contain any native ability to record IQ data. There are various API for exporting RF data from the Flex Signature radios. At this point in time, unfortunately, no one is providing software to easily access this data through the FlexLib support library or over its socket connection. I might have to write something later in the year.
However, my Flex 6300 does have the ability to export 96 kHz of IQ data through its DAX interface. DAX is a system of virtual cables that come with the Flex Signature Radios. These drivers show up as media devices on your PC which let you import or export audio or IQ data easily. For example, I use the DAX connections with my digital modes software.
Technically, there is no difference between stereo audio and IQ data. It’s all digital data at various sampling rates. Unfortunately, the SmartSDR outputs IQ data in a 24 bit format. I can save this to a WAV file, but my experimenter platform GNURadio can only import 8 or 16 bit data files.
Recording SmartSDR IQ Data – Audacity to the Rescue
Fortunately, we can use Audacity for recording SmartSDR IQ data in a compatible format. Audacity is an open-source solution for audio recording and editing. Here are the really simple steps to record a chunk of FlexRadio IQ data for later experiments.
- Install and run Audacity.
- Center your Flex 6300 on the 96 kHz chunk you want to record. (Higher end Flex will give you 192 kHz.) Enable DAX IQ RX1.
- In Audacity, select DAX IQ RX1 as your input device and set your project sampling rate to 96000.
- Record some minutes of IQ data in Audacity.
- Select all that you recorded and then Export as 16 bit PCM WAV file with an appropriate name.
Now, you can import your Flex IQ data into into GNURadio and use it there. Also, you can use other tools to explore your recording such as SDR Console Data Analyzer.