[mythtv] Re: Re: [mythtv-commits] Re: Ticket #339: SimplifyTuning process

Mac Michaels wmichaels1 at earthlink.net
Fri Sep 23 17:55:59 UTC 2005

How is  the driver is supposed to return AGC RF/IF Loop 
Filter Accumulator Values? The return value holds one 
unsigned16 bit value. There are two signed 12 bit values 
for the AGC RF/IF Loop Filter Accumulator. Although Taylor 
may know how to compute signal strength, I can not pass him 
the data from the driver. So, how is signal strength 
calculated? I am not aware of the signal strength formula 
for the lgdt3302 and lgdt3303 chips. Please advise and I 
will update the driver.

With regards to the snr value. I have already written the 
code to compute the snr in db (without using floating 
point). It is #ifdef'd out in the driver. Folks complained 
that it was wrong to return a signed value in an unsigned 
variable. For example mythTV displays it as an unsigned 
value. Thus a negative number associated with poor signal 
conditions showed up as near 100% while a small positive 
number that indicates good signal conditions reports near 
0%. Instead I return the truncated raw Equalizer Mean 
Square Error value.

I can't find a FE_READ_SIGNAL_QUALITY function call in the 
DVB API Version 3.

On Thursday 22 September 2005 11:44 am, John Jurrius wrote:
> The Signal Strength data for the lgdt330x is intended to
> be calculated from AGC RF/IF Loop Filter Accumulator
> Values.  The SNR is calculated from the Equalizer Mean
> Square Error.  The routines for calculating both have
> already been provided to Taylor.  If you cannot return
> the SNR in dB perhaps you can do what is done in Windows
> driver for the HD5000 which is convert the SNR to a
> percentage of its value over a the usable range of the
> SNR which is then referred to as Signal Quality.
> John
> -----Original Message-----
> From: mythtv-dev-bounces at mythtv.org
> [mailto:mythtv-dev-bounces at mythtv.org]On Behalf Of Mac
> Michaels
> Sent: Thursday, September 22, 2005 9:08 AM
> To: mythtv-dev at mythtv.org
> Subject: Re: [mythtv] Re: Re: [mythtv-commits] Re: Ticket
> #339: SimplifyTuning process
> The lgdt330x chips do not provide signal strength data.
> It might be possible to compute signal strength from the
> automatic gain control (AGC) data, but the spec sheet
> does not provide a formula. As a result the signal
> strength is always set to 0.
> Data is available from which the signal to noise ratio
> (snr) can be calculated. The driver currently returns raw
> data since the unsigned 16 bit return value it not
> appropriate for returning the actual snr in db. Snr must
> be signed because it is negative when the signal is less
> than the noise and positive when the signal is greater
> than the noise.
> --Mac
> On Thursday 22 September 2005 09:53 am, David Engel wrote:
> > On Thu, Sep 22, 2005 at 10:42:16AM -0400, Taylor Jacob
> wrote:
> > > Many drivers just report arbitrary vaules that have
> > > no unit because the author did not have the
> > > datasheet, and this will not change.. Not even if all
> > > manufacturers of demodulators decide to support linux
> > > for some reason..
> >
> > FWIW, my Fusion HDTV5 always reports 0 for signal
> > level.
> >
> > David
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