Tuning into a Digital TV physical channel and starting decoding it requires changing a set of parameters, in order to control the tuner, the demodulator, the Linear Low-noise Amplifier (LNA) and to set the antenna subsystem via Satellite Equipment Control (SEC), on satellite systems. The actual parameters are specific to each particular digital TV standards, and may change as the digital TV specs evolves.
In the past, the strategy used was to have a union with the parameters needed to tune for DVB-S, DVB-C, DVB-T and ATSC delivery systems grouped there. The problem is that, as the second generation standards appeared, those structs were not big enough to contain the additional parameters. Also, the union didn’t have any space left to be expanded without breaking userspace. So, the decision was to deprecate the legacy union/struct based approach, in favor of a properties set approach.
Note
On Linux DVB API version 3, setting a frontend were done via struct dvb_frontend_parameters. This got replaced on version 5 (also called “S2API”, as this API were added originally_enabled to provide support for DVB-S2), because the old API has a very limited support to new standards and new hardware. This section describes the new and recommended way to set the frontend, with suppports all digital TV delivery systems.
Example: with the properties based approach, in order to set the tuner to a DVB-C channel at 651 kHz, modulated with 256-QAM, FEC 3/4 and symbol rate of 5.217 Mbauds, those properties should be sent to FE_SET_PROPERTY ioctl:
The code that would that would do the above is show in Example: Setting digital TV frontend properties.
#include <stdio.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/dvb/frontend.h>
static struct dtv_property props[] = {
{ .cmd = DTV_DELIVERY_SYSTEM, .u.data = SYS_DVBC_ANNEX_A },
{ .cmd = DTV_FREQUENCY, .u.data = 651000000 },
{ .cmd = DTV_MODULATION, .u.data = QAM_256 },
{ .cmd = DTV_INVERSION, .u.data = INVERSION_AUTO },
{ .cmd = DTV_SYMBOL_RATE, .u.data = 5217000 },
{ .cmd = DTV_INNER_FEC, .u.data = FEC_3_4 },
{ .cmd = DTV_TUNE }
};
static struct dtv_properties dtv_prop = {
.num = 6, .props = props
};
int main(void)
{
int fd = open("/dev/dvb/adapter0/frontend0", O_RDWR);
if (!fd) {
perror ("open");
return -1;
}
if (ioctl(fd, FE_SET_PROPERTY, &dtv_prop) == -1) {
perror("ioctl");
return -1;
}
printf("Frontend set\\n");
return 0;
}
Attention
While it is possible to directly call the Kernel code like the above example, it is strongly recommended to use libdvbv5, as it provides abstraction to work with the supported digital TV standards and provides methods for usual operations like program scanning and to read/write channel descriptor files.