UK FreeView logical channel numbers

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Hi,

does anyone know where the logical channel numbers are
transmitted in FreeView? - ie BBC1 is 1, BBC News is 80, etc - I've been looking at the PAT and PMTs with dvbsnoop but can't see anything obvious.

- --
Simon Kilvington

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It's in the NIT, using a private descriptor - scan from dvb-apps
(http://linuxtv.org/hg/dvb-apps/file/9311c900f746/util/scan/scan.c)
knows about them, and they're fully specced in the DTG 'D-Book' (see
http://dtg.org.uk/publications/books.html).

HTH,

Simon Farnsworth


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thanks for the info

the reason I wanted to know was because when I did a scan with mythtv I had to give it each frequency by hand, then it only managed to find the channel numbers for the channels on one mux - though this may be because the nit on my transmitter seems to be split into two bits - one nit has the info for 5 muxs, the other nit has the info for the other mux

anyway, I wrote a small program that will get the channel numbers out of the nit(s) and dump some sql statements that you can use to update the channel numbers in an existing mythtv database - I've attached it here, compile it like this:

gcc -o get-lcns get-lcns.c

and run it like this (you'll probably need different dvbtune params):

dvbtune -f 722166667 -qam 16 -cr 3_4 && ./get-lcns > lcns

it will print a load of sql commands to stdout, so the > will put them in the "lcns" file. You can just pipe the output into mysql, like this:

cat lcns | mysql -umythtv -pmythtv mythconverg

to update your channel table

- --
Simon Kilvington

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/*
* dvbtune -f 722166667 -qam 16 -cr 3_4 && dvbsnoop 0x10 *
* NIT:
* PID=0x10
* TID=0x40
* tag=0x83
*/

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <stdbool.h>
#include <stdarg.h>
#include <errno.h>
#include <sys/ioctl.h>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <linux/dvb/dmx.h>

#define TIMEOUT 30

#define MAX_TABLE_LEN 4096

/* Network Information Table PID and TID */
#define PID_NIT 0x0010
#define TID_NIT 0x40

#define TAG_LCN 0x83

bool read_nit(char *, unsigned int, unsigned char *);
bool read_table(char *, uint16_t, uint8_t, unsigned int, unsigned char *); void fatal(char *, ...);

int main(void)

{
	unsigned char nit[MAX_TABLE_LEN];
	unsigned char *data;
	unsigned int section_len;
	unsigned int nit_len;
	unsigned int descriptors_loop_len;
	unsigned int transport_stream_id;
	unsigned int ntransport_ids;
	unsigned int transport_ids[6];
	unsigned int i;

	/*
	 * Malvern transmitter (repeater for Sutton Coldfield) broadcasts 2 NITs
	 * one has the LCNs for 5 MUXs
	 * the other has the LCNs for the other MUX
	 */
	ntransport_ids = 0;
	while(ntransport_ids < 6)
	{
		bzero(nit, MAX_TABLE_LEN);
		if(!read_nit("/dev/dvb/adapter0/demux0", TIMEOUT, nit))
			fatal("Unable to read NIT");

		/* generic header */
		data = nit;
		section_len = ((data[1] & 0x0f) << 8) | data[2];
		data += 8;
		/* header + CRC */
		nit_len = section_len - (5 + 4);

		/* skip the first descriptors loop */
		descriptors_loop_len = ((data[0] & 0x0f) << 8) | data[1];
		data += descriptors_loop_len + 4;
		nit_len -= descriptors_loop_len + 4;

		while(nit_len > 6)
		{
			transport_stream_id = (data[0] << 8) | data[1];
			/* have we already seen it */
			for(i=0; i<ntransport_ids; i++)
				if(transport_stream_id == transport_ids[i])
					continue;
			transport_ids[ntransport_ids++] = transport_stream_id;
			fprintf(stderr, "transport_stream_id=0x%x\n", transport_stream_id);
			descriptors_loop_len = ((data[4] & 0x0f) << 8) | data[5];
			data += 6;
			nit_len -= descriptors_loop_len + 6;
			while(descriptors_loop_len > 0)
			{
				unsigned char desc_tag = data[0];
				unsigned char desc_len = data[1] + 2;
				data += 2;
				if(desc_tag == TAG_LCN)
				{
					for(i=0; i<desc_len; i+=4)
					{
						unsigned int sid, lcn;
						sid = (data[i] << 8) | data[i+1];
						lcn = ((data[i+2] & 0x03) << 8) | data[i+3];
						printf("UPDATE channel SET chanid=\"%u\", channum=\"%u\" WHERE serviceid=\"%u\";\n", lcn + 1000, lcn, sid);
					}
				}
				data += desc_len - 2;
				descriptors_loop_len -= desc_len;
			}
		}

	}

	return 0;
}

/*
* output buffer must be at least MAX_TABLE_LEN bytes
* returns false if it timesout
*/

bool
read_nit(char *demux, unsigned int timeout, unsigned char *out)

{
	/* read it from the DVB card */
	if(!read_table(demux, PID_NIT, TID_NIT, timeout, out))
	{
		fatal("Unable to read NIT");
		return false;
	}

	return true;
}

/*
* output buffer must be at least MAX_TABLE_LEN bytes
* returns false if it timesout
*/

bool
read_table(char *device, uint16_t pid, uint8_t tid, unsigned int secs, unsigned char *out)

{
	int fd_data;
	struct dmx_sct_filter_params sctFilterParams;
	fd_set readfds;
	struct timeval timeout;
	int n;

	if((fd_data = open(device, O_RDWR)) < 0)
	{
		fatal("open '%s': %s", device, strerror(errno));
		return false;
	}

	memset(&sctFilterParams, 0, sizeof(sctFilterParams));
	sctFilterParams.pid = pid;
	sctFilterParams.timeout = 0;
	sctFilterParams.flags = DMX_IMMEDIATE_START;
       	sctFilterParams.filter.filter[0] = tid;

sctFilterParams.filter.mask[0] = 0xff;

	if(ioctl(fd_data, DMX_SET_FILTER, &sctFilterParams) < 0)
	{
		fatal("ioctl DMX_SET_FILTER: %s", strerror(errno));
		close(fd_data);
		return false;
	}

	timeout.tv_sec = secs;
	timeout.tv_usec = 0;
	do
	{
		/* we check for out[0]==tid to see if we read the table */
		out[0] = ~tid;
		FD_ZERO(&readfds);
		FD_SET(fd_data, &readfds);
		if(select(fd_data + 1, &readfds, NULL, NULL, &timeout) < 0)
		{
			if(errno == EINTR)
				continue;
			fatal("read_table: select: %s", strerror(errno));
			close(fd_data);
			return false;
		}
		if(FD_ISSET(fd_data, &readfds))
		{
			if((n = read(fd_data, out, MAX_TABLE_LEN)) < 0)
			{
				fatal("read: %s", strerror(errno));
				close(fd_data);
				return false;
			}
		}
		else
		{
			fatal("Timeout reading %s", device);
			close(fd_data);
			return false;
		}
	}
	while(out[0] != tid);

	close(fd_data);

	return true;
}

void
fatal(char *message, ...)

{
	va_list ap;

	va_start(ap, message);
	vfprintf(stderr, message, ap);
	fprintf(stderr, "\n");
	va_end(ap);

	exit(EXIT_FAILURE);
}

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The NIT is normally transmitted as two separate tables - one covers just the mux you are tuned to, while the other NIT covers all muxes in the network, apart from the one you're tuned to.

Simon Farnsworth


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