*libcsdr* is a set of simple DSP routines for Software Defined Radio.
It is mostly useful for AM/FM/SSB demodulation and spectrum display.
Feel free to use it in your projects.
Most of the code is available under the permissive BSD license, with some optional parts under GPL. For additional details, see <ahref="licensing">licensing</a>.
- The package comes with a command-line tool `csdr`, which lets you build DSP processing chains by shell pipes.
- The code of *libcsdr* was intended to be easy to follow.
- *libcsdr* was designed to use auto-vectorization available in *gcc*. It means that it can achieve some speedup by taking advantage of SIMD command sets available in today's CPUs (e.g. SSE on x86 and NEON on ARM).
How to compile
--------------
The project was only tested on Linux. It has the following dependencies: `libfftw3-dev`
make
sudo make install
If you compile on ARM, please edit the Makefile and tailor `PARAMS_NEON` for your CPU.
To run the examples, you will also need <ahref="http://sdr.osmocom.org/trac/wiki/rtl-sdr">rtl_sdr</a> from Osmocom, and the following packages (at least on Debian): `mplayer octave gnuplot gnuplot-x11`
Credits
-------
The library was written by Andras Retzler, HA7ILM <<randras@sdr.hu>>.
I would like to say special thanks to Péter Horváth, HA5CQA and János Selmeczi, HA5FT for their continous help and support.
- Baseband I/Q signal is coming from an RTL-SDR USB dongle, with a center frequency of `-f 104300000` Hz, a sampling rate of `-s 240000` samples per second.
- The `rtl_sdr` tool outputs an unsigned 8-bit I/Q signal (one byte of I sample and one byte of Q coming after each other), but `libcsdr` DSP routines internally use floating point data type, so we convert the data stream of `unsigned char` to `float` by `csdr convert_u8_f`.
- We want to listen one radio station at the frequency `-f 89500000` Hz (89.5 MHz).
- No other radio station is within the sampled bandwidth, so we send the signal directly to the demodulator. (This is an easy, but not perfect solution as the anti-aliasing filter at RTL-SDR DDC is too short.)
- After FM demodulation we decimate the signal by a factor of 5 to match the rate of the audio card (240000 / 5 = 48000).
- A de-emphasis filter is used, because pre-emphasis is applied at the transmitter to compensate noise at higher frequencies. The time constant for de-emphasis for FM broadcasting in Europe is 50 microseconds (hence the `50e-6`).
- Also, `mplayer` cannot play floating point audio, so we convert our signal to a stream of 16-bit integers.
- We want to listen to one radio station, but input signal contains multiple stations, and its bandwidth is too large for sending it directly to the FM demodulator.
- We shift the signal to the center frequency of the station we want to receive: `-0.17*2400000*0.5 = -204000`, so basically we will listen to the radio station centered at 89504000 Hz.
- We decimate the signal by a factor of 10. The rolloff for the FIR filter used for decimation will be 10% of total bandwidth (as of parameter 0.05 is 10% of 0.5). Hamming window will be used for windowed FIR filter design.
I/Q source ------------> FIR decimation ------------> FM demod -> frac. decimation ---------> deemphasis -> sound card
*Note:* there is an example shell script that does this for you (without the unnecessary shift operation). If you just want to listen to FM radio, type:
csdr-fm 89.5 20
The first parameter is the frequency in MHz, and the second optional parameter is the RTL-SDR tuner gain in dB.
- ...and then plot its frequency response with octave.
- It will design a filter that lets only the positive frequencies pass (low cut is 0, high cut is 0.5 - these are relative to the sampling rate).
- If `--octave` and everything that follows is removed from the command, you get only the taps. E. g. the raw output of `firdes_lowpass_f` can be easily copied to C code.
- It is a modified Weaver-demodulator. The complex FIR filter removes the lower sideband and lets only the upper pass (USB). If you want to demodulate LSB, change `bandpass_fir_fft_cc 0 0.05` to `bandpass_fir_fft_cc -0.05 0`.
- We calculate the Fast Fourier Transform by `csdr fft_cc` on the first 1024 samples of every block of 1200000 complex samples coming after each other. (We calculate FFT from 1024 samples and then skip 1200000-1024=1198976 samples. This way we will calculate FFT two times every second.)
- The window used for FFT is the Hamming window, and the output consists of commands that can be directly interpreted by GNU Octave which plots us the spectrum.
Usage
-----
Some basic concepts on using *libcsdr*:
### Data types
Function name endings found in *libcsdr* mean the input and output data types of the particular function. (This is similar to GNU Radio naming conventions).
Data types are noted as it follows:
-`f` is `float` (single percision)
-`c` is `complexf` (two single precision floating point values in a struct)
-`u8` is `unsigned char` of 1 byte/8 bits (e. g. the output of `rtl_sdr` is of `u8`)
-`i16` is `signed short` of 2 bytes/16 bits (e. g. sound card input is usually `i16`)
Functions usually end as:
-`_ff` float input, float output
-`_cf` complex input, float output
-`_cc` complex input, complex output
Regarding *csdr*, it can convert a real/complex stream from one data format to another, to interface it with other SDR tools and the sound card.
The following commands are available:
-`csdr convert_u8_f`
-`csdr convert_f_u8`
-`csdr convert_i16_f`
-`csdr convert_f_i16`
How to interpret: `csdr convert_<src>_<dst>`
You can use these commands on complex streams, too, as they are only interleaved values (I,Q,I,Q,I,Q... coming after each other).
#### csdr commands
`csdr` should be considered as a reference implementation on using `libcsdr`. For additional details on how to use the library, check `csdr.c` and `libcsdr.c`.
Regarding `csdr`, the first command-line parameter is the name of a function, others are the parameters for the given function. Compulsory parameters are noted as `<parameter>`, optional parameters are noted as `[parameter]`.
Optional parameters have safe defaults, for more info look at the code.
`rate` is a floating point number between -0.5 and 0.5.
`rate` is relative to the sampling rate.
Internally, a sine and cosine wave is generated to perform this function, and this function uses `math.h` for this purpose, which is quite accurate, but not always very fast.
Internally, this function uses trigonometric addition formulas to generate sine and cosine, which is a bit faster. (About 4 times on the machine I have tested it on.)
It is an AM demodulator that uses `sqrt`. On some architectures `sqrt` can be directly calculated by dedicated CPU instructions, but on others it may be slower.
Low-pass FIR filter design function to output real taps, with a `cutoff_rate` proportional to the sampling frequency, using the windowed sinc filter design method.
`cutoff_rate` can be between 0 and 0.5.
`length` is the number of filter taps to output, and should be odd.
The longer the filter kernel is, the shorter the transition bandwidth is, but the more CPU time it takes to process the filter.
The transition bandwidth (proportional to the sampling rate) can be calculated as: `transition_bw = 4 / length`.
Some functions (below) require the `transition_bw` to be given instead of filter `length`. Try to find the best compromise between speed and accuracy by changing this parameter.
`window` is the window function used to compensate finite filter length. Its typical values are: `HAMMING`, `BLACKMAN`, `BOXCAR`. For the actual list of values, run: `cpp libcsdr.c | grep window\ ==`
The `--octave` parameter lets you directly view the filter response in `octave`. For more information, look at the [Usage by example] section.
-`hang_time` is the number of samples to wait before strating to increase the gain after a peak.
-`reference` is the reference level for the AGC. It tries to keep the amplitude of the output signal close to that.
-`attack_rate` is the rate of decreasing the signal level if it gets higher than it used to be before.
-`decay_rate` is the rate of increasing the signal level if it gets lower than it used to be before.
- AGC won't increase the gain over `max_gain`.
-`attack_wait` is the number of sampels to wait before starting to decrease the gain, because sometimes very short peaks happen, and we don't want them to spoil the reception by substantially decreasing the gain of the AGC.
-`filter_alpha` is the parameter of the loop filter.
Its default parameters work best for an audio signal sampled at 48000 Hz.
It is a faster AGC that linearly changes the gain, taking the highest amplitude peak in the buffer into consideration. Its output will never exceed `-reference ... reference`.
It performs an FFT on the first `fft_size` samples out of `out_of_every_n_samples`, thus skipping `out_of_every_n_samples - fft_size` samples in the input.
It can draw the spectrum by using `--octave`, for more information, look at the [Usage by example] section.
FFTW can be faster if we let it optimalize a while before starting the first transform, hence the `--benchmark` switch.
Calculates `10*log10(i^2+q^2)+add_db` for the input complex samples. It is useful for drawing power spectrum graphs.
#### Control via pipes
Some parameters can be changed while the `csdr` process is running. To achieve this, some `csdr` functions have special parameters. You have to supply a fifo previously created by the `mkfifo` command. Processing will only start after the first control command has been received by `csdr` over the FIFO.
By writing to the given FIFO file with the syntax below, you can control the shift rate:
<low_cut><high_cut>\n
E.g. you can send `-0.05 0.02\n`
#### Testbench
`csdr` was tested with GNU Radio Companion flowgraphs. These flowgraphs are available under the directory `grc_tests`, and they require the `gr-ha5kfu` set of blocks for GNU Radio.
