The signal strength value returned is not quite correct, it decreases
when I increase the gain of my antenna, and vice versa. It also
doesn't span over the whole 0x0000-0xffff range. Compute a value which
at least increases when signal strength increases, and spans the whole
allowed range.
In practice I get 67% with my antenna fully amplified and 51% with
no amplification. This is close enough to what I get on my other
DVB-T adapter with the same antenna.
Signed-off-by: Jean Delvare <khali@xxxxxxxxxxxx>
Cc: Steven Toth <stoth@xxxxxxxxxxxxxx>
---
This was written without a datasheet so this is essentially
guess work.
drivers/media/dvb/frontends/cx22702.c | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)
--- linux-3.3-rc3.orig/drivers/media/dvb/frontends/cx22702.c 2012-02-11 08:28:15.000000000 +0100
+++ linux-3.3-rc3/drivers/media/dvb/frontends/cx22702.c 2012-02-12 14:45:33.361921465 +0100
@@ -502,10 +502,26 @@ static int cx22702_read_signal_strength(
u16 *signal_strength)
{
struct cx22702_state *state = fe->demodulator_priv;
+ u8 reg23;
- u16 rs_ber;
- rs_ber = cx22702_readreg(state, 0x23);
- *signal_strength = (rs_ber << 8) | rs_ber;
+ /*
+ * Experience suggests that the strength signal register works as
+ * follows:
+ * - In the absence of signal, value is 0xff.
+ * - In the presence of a weak signal, bit 7 is set, not sure what
+ * the lower 7 bits mean.
+ * - In the presence of a strong signal, the register holds a 7-bit
+ * value (bit 7 is cleared), with greater values standing for
+ * weaker signals.
+ */
+ reg23 = cx22702_readreg(state, 0x23);
+ if (reg23 & 0x80) {
+ *signal_strength = 0;
+ } else {
+ reg23 = ~reg23 & 0x7f;
+ /* Scale to 16 bit */
+ *signal_strength = (reg23 << 9) | (reg23 << 2) | (reg23 >> 5);
+ }
return 0;
}
--
Jean Delvare
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