sendiq: raw I/Q samples

Transmits I/Q samples from a file. Anything you can produce as I/Q, from GNU Radio, csdr or your own code, goes on the air this way. With a shared memory token, another program can change the frequency and power while it runs. Code: sendiq.go.

Fields

The frequency field is freq here, not frequency, and it is in Hz.

JSON	Go field	Type	Required	Rules
inputFile	InputFile	string	yes	Path to an existing file, or - (see below)
freq	Freq	float64	yes	Hz, 5000 to 1500000000
sampleRate	SampleRate	*int	no	10000 to 2000000; the program defaults to 48000
harmonic	Harmonic	*int	no	1 or more; the program defaults to 1
iqType	IQType	*string	no	i16, u8, float or double (gorpitx.IQType* constants); the program defaults to i16
power	Power	*float64	no	Drive level 0 to 7, clamped into that range rather than refused; the program defaults to 0.1
sharedMemToken	SharedMemToken	*int	no	Non-zero shared memory key for run-time control
loopMode	LoopMode	bool	no	true adds -l and restarts the file at its end

The package exports the program’s defaults as DefaultSampleRate, DefaultHarmonic, DefaultIQType and DefaultPower. gorpitx does not send them; it leaves a flag out when you leave its field out.

sampleRate above 200000 is reduced by sendiq itself: it divides by the smallest factor from 2 to 11 that brings the rate under 200000 and decimates the samples by that factor.

power is printed with two decimals, so 9 becomes -p 7.00.

inputFile: "-" passes -i -, which makes sendiq read its own stdin. This module gives the process no stdin data, so in practice - reads nothing. Write the samples to a file or a named pipe and pass that path instead.

Example

{
  "inputFile": "/home/pi/signal.iq",
  "freq": 434000000,
  "sampleRate": 96000,
  "power": 2.5,
  "sharedMemToken": 12345,
  "loopMode": true
}

Runs:

stdbuf -oL "$GORPITX_PATH/sendiq" -i /home/pi/signal.iq -f 434000000 \
  -s 96000 -p 2.50 -m 12345 -l

With loopMode, the program never exits by itself. Give Exec a timeout or call Stop.

Run-time control over shared memory

With sharedMemToken set, sendiq creates a System V shared memory segment with that key and checks it while it transmits. It also forces the I/Q type to float, whatever iqType says.

The segment holds this C struct from rpitx’s sendiq.cpp:

struct shared_memory_struct {
    bool  updated;
    int   command;
    float data;
    char  common_data[1024];
};

Write a command and set updated to true. The commands:

Command	Effect	data
1111	Switch to I/Q mode	unused
2222	Switch to frequency and amplitude mode	unused
3333	Set the drive level	level, 0 to 7
4444	Change the frequency	frequency in Hz

From Python with the sysv_ipc package, for example:

import struct

import sysv_ipc

shm = sysv_ipc.SharedMemory(12345)

# updated=True, command=4444, data=144.5 MHz
shm.write(struct.pack("?if", True, 4444, 144500000.0))

?if uses native alignment, which matches the C struct on the Pi. A 32-bit float holds about seven significant digits, so a frequency sent this way lands on the nearest value a float can hold, within a few tens of Hz at 144 MHz.