Re: [PATCH 08/10] dvb_frontend: create a new ops to help returning signals in dB

[Mauro Carvalho Chehab]

Hi Steven,

Em Thu, 30 Jun 2016 09:48:30 -0400
Steven Toth  escreveu:

> > add a new ops that will allow tuners to better report the
> > dB level of its AGC logic to the demod drivers. As the maximum
> > gain may vary from tuner to tuner, we'll be reversing the
> > logic here: instead of reporting the gain, let's report the
> > attenuation. This way, converting from it to the legacy DVBv3
> > way is trivial. It is also easy to adjust the level of
> > the received signal to dBm, as it is just a matter of adding
> > an offset at the demod and/or at the bridge driver.  
> 
> Mauro,
> 
> Have you verified this work with a detailed spectrum analysis study?
> If so then please share. For example, by measuring the I/F out of
> various tuners in a mix of use cases, with and without the AGC being
> driven by any downstream demodulator? Also, taking into consideration
> any external LNA variance.
> 
> I'm concerned that a tuner AGC Gain is a meaningless measurement and
> in practice demodulators don't actually care, and tuners don't
> implement their gain reporting capabilities correctly at all.
> 
> This feels like a solution to a problem that doesn't exist.

The role idea here is to provide a rough estimation about the signal
strength, and not to enter into the measurement instrument business.

I know that the actual tuner AGC gain is not precise[1]:
- it may vary with the frequency;
- it may vary with the component variance;
- it may vary with modulation parameters;
- it may vary with the temperature.

Yet, the signal strength is used on some software to detect "weak"
signals and skip such transponders. There is such code, for example
in Kaffeine since 2009:
commit 73a5aae68ab46d761267043c9774289833b79dbc
Author: Christoph Pfister 
Date:   Sat Apr 25 13:41:28 2009 +

add autoscan support

+   if ((signal != 0) && (signal < 0x2000)) {
+   // signal too weak
+   carry = false;
+   }

The color on Kaffeine's gauge gradient bar also tries to express if
the signal level is OK or not, showing only red for weak signals,
and becoming green as the signal increases up to a certain level.

The strength is also one parameters to estimate the signal quality.
The libdvbv5 considers it, when reporting DTV_QUALITY parameter.

So, the hole idea here is *not* to provide a precise measure, but
to try to use about the same scale on the drivers, whenever
possible.

Regards,
Mauro

[1] I would give a 5-stars here for Siano: on the tests I did, the
signal strength reported by the siano devices is very precise.


Thanks,
Mauro
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Re: [PATCH 08/10] dvb_frontend: create a new ops to help returning signals in dB

[Steven Toth]

> add a new ops that will allow tuners to better report the
> dB level of its AGC logic to the demod drivers. As the maximum
> gain may vary from tuner to tuner, we'll be reversing the
> logic here: instead of reporting the gain, let's report the
> attenuation. This way, converting from it to the legacy DVBv3
> way is trivial. It is also easy to adjust the level of
> the received signal to dBm, as it is just a matter of adding
> an offset at the demod and/or at the bridge driver.

Mauro,

Have you verified this work with a detailed spectrum analysis study?
If so then please share. For example, by measuring the I/F out of
various tuners in a mix of use cases, with and without the AGC being
driven by any downstream demodulator? Also, taking into consideration
any external LNA variance.

I'm concerned that a tuner AGC Gain is a meaningless measurement and
in practice demodulators don't actually care, and tuners don't
implement their gain reporting capabilities correctly at all.

This feels like a solution to a problem that doesn't exist.

-- 
Steven Toth - Kernel Labs
http://www.kernellabs.com
+1.646.355.8490
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[PATCH 08/10] dvb_frontend: create a new ops to help returning signals in dB

[Mauro Carvalho Chehab]

add a new ops that will allow tuners to better report the
dB level of its AGC logic to the demod drivers. As the maximum
gain may vary from tuner to tuner, we'll be reversing the
logic here: instead of reporting the gain, let's report the
attenuation. This way, converting from it to the legacy DVBv3
way is trivial. It is also easy to adjust the level of
the received signal to dBm, as it is just a matter of adding
an offset at the demod and/or at the bridge driver.

Signed-off-by: Mauro Carvalho Chehab 
---
 drivers/media/dvb-core/dvb_frontend.h | 14 --
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/media/dvb-core/dvb_frontend.h 
b/drivers/media/dvb-core/dvb_frontend.h
index 9592573a0b41..e8a4d341f420 100644
--- a/drivers/media/dvb-core/dvb_frontend.h
+++ b/drivers/media/dvb-core/dvb_frontend.h
@@ -227,8 +227,17 @@ enum dvbfe_search {
  * should return 0.
  * @get_status:returns the frontend lock status
  * @get_rf_strength:   returns the RF signal strengh. Used mostly to support
- * analog TV and radio. Digital TV should report, instead,
- * via DVBv5 API (@dvb_frontend.dtv_property_cache;).
+ * analog TV and radio. This is deprecated, in favor of
+ * @get_rf_attenuation.
+ * @get_rf_attenuation:returns the RF signal attenuation, relative to 
the
+ * maximum supported gain, in 1/1000 dB steps. Please
+ * notice that 0 means that the tuner is using its maximum
+ * gain. So, a value of 1, for example, means a 10dB
+ * attenuation. This is ops is meant to be used by
+ * demodulator drivers to estimate the maximum signal
+ * strength. For that, it will add an offset, in order to
+ * expose the signal strength to userspace via dvbv5
+ * stats, in dBm.
  * @get_afc:   Used only by analog TV core. Reports the frequency
  * drift due to AFC.
  * @calc_regs: callback function used to pass register data settings
@@ -264,6 +273,7 @@ struct dvb_tuner_ops {
 #define TUNER_STATUS_STEREO 2
int (*get_status)(struct dvb_frontend *fe, u32 *status);
int (*get_rf_strength)(struct dvb_frontend *fe, u16 *strength);
+   s32 (*get_rf_attenuation)(struct dvb_frontend *fe);
int (*get_afc)(struct dvb_frontend *fe, s32 *afc);
 
/*
-- 
2.7.4

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