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[media] dib8000: make a better estimation for dBm

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Use multiple linear segments to better interpolate the dBm
for the signal strength.

The table that converts from linear strength to dB was
empirically determinated with the help of a signal generator
(DTA-2111).

The entries from -35dBm to -22.5dBm were taken using just
the signal generator and the board.

For the entries from -36dBm to -51dBm, a 16 dB tap was used,
in order to extend its range.

Signals below to -51dBm are just linearly interpolated.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Patrick Boettcher <pboettcher@kernellabs.com>
This commit is contained in:
Mauro Carvalho Chehab 2013-12-16 21:45:19 -03:00
parent b4600d70c0
commit 42ff76bdb0
1 changed files with 92 additions and 19 deletions
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111
drivers/media/dvb-frontends/dib8000.c
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@ -3843,35 +3843,108 @@ static const struct per_layer_regs per_layer_regs[] = {
{ 556, 581, 583 },
};
struct linear_segments {
unsigned x;
signed y;
};
/*
* Table to estimate signal strength in dBm.
* This table was empirically determinated by measuring the signal
* strength generated by a DTA-2111 RF generator directly connected into
* a dib8076 device (a PixelView PV-D231U stick), using a good quality
* 3 meters RC6 cable and good RC6 connectors.
* The real value can actually be different on other devices, depending
* on several factors, like if LNA is enabled or not, if diversity is
* enabled, type of connectors, etc.
* Yet, it is better to use this measure in dB than a random non-linear
* percentage value, especially for antenna adjustments.
* On my tests, the precision of the measure using this table is about
* 0.5 dB, with sounds reasonable enough.
*/
static struct linear_segments strength_to_db_table[] = {
{ 55953, 108500 }, /* -22.5 dBm */
{ 55394, 108000 },
{ 53834, 107000 },
{ 52863, 106000 },
{ 52239, 105000 },
{ 52012, 104000 },
{ 51803, 103000 },
{ 51566, 102000 },
{ 51356, 101000 },
{ 51112, 100000 },
{ 50869, 99000 },
{ 50600, 98000 },
{ 50363, 97000 },
{ 50117, 96000 }, /* -35 dBm */
{ 49889, 95000 },
{ 49680, 94000 },
{ 49493, 93000 },
{ 49302, 92000 },
{ 48929, 91000 },
{ 48416, 90000 },
{ 48035, 89000 },
{ 47593, 88000 },
{ 47282, 87000 },
{ 46953, 86000 },
{ 46698, 85000 },
{ 45617, 84000 },
{ 44773, 83000 },
{ 43845, 82000 },
{ 43020, 81000 },
{ 42010, 80000 }, /* -51 dBm */
{ 0, 0 },
};
static u32 interpolate_value(u32 value, struct linear_segments *segments,
unsigned len)
{
u64 tmp64;
u32 dx;
s32 dy;
int i, ret;
if (value >= segments[0].x)
return segments[0].y;
if (value < segments[len-1].x)
return segments[len-1].y;
for (i = 1; i < len - 1; i++) {
/* If value is identical, no need to interpolate */
if (value == segments[i].x)
return segments[i].y;
if (value > segments[i].x)
break;
}
/* Linear interpolation between the two (x,y) points */
dy = segments[i - 1].y - segments[i].y;
dx = segments[i - 1].x - segments[i].x;
tmp64 = value - segments[i].x;
tmp64 *= dy;
do_div(tmp64, dx);
ret = segments[i].y + tmp64;
return ret;
}
static int dib8000_get_stats(struct dvb_frontend *fe, fe_status_t stat)
{
struct dib8000_state *state = fe->demodulator_priv;
struct dtv_frontend_properties *c = &state->fe[0]->dtv_property_cache;
int i, lock;
u64 tmp;
u32 snr, val;
s32 db;
u16 strength;
/* Get Signal strength */
dib8000_read_signal_strength(fe, &strength);
/*
* Estimate it in dBm
* This calculus was empirically determinated by measuring the signal
* strength generated by a DTA-2111 RF generator directly connected into
* a dib8076 device. The real value can actually be different on other
* devices, depending if LNA is enabled or not, if diversity is enabled,
* etc.
*/
if (strength == 65535) {
c->strength.stat[0].svalue = -22000;
} else {
tmp = strength * 25000L;
do_div(tmp, 11646);
c->strength.stat[0].svalue = tmp - 142569;
if (c->strength.stat[0].svalue > -22000)
c->strength.stat[0].svalue = -22000;
}
val = strength;
db = interpolate_value(val,
strength_to_db_table,
ARRAY_SIZE(strength_to_db_table)) - 131000;
c->strength.stat[0].svalue = db;
/* Check if 1 second was elapsed */
if (!time_after(jiffies, state->get_stats_time))