Re: Details about DVB frontend API

["David T. L. Wong" <davidtlwong@gmail.com>]

Jean Delvare wrote:
> Hi folks,
> 
> I am looking for details regarding the DVB frontend API. I've read
> linux-dvb-api-1.0.0.pdf, it roughly explains what the FE_READ_BER,
> FE_READ_SNR, FE_READ_SIGNAL_STRENGTH and FE_READ_UNCORRECTED_BLOCKS
> commands return, however it does not give any information about how the
> returned values should be interpreted (or, seen from the other end, how
> the frontend kernel drivers should encode these values.) If there
> documentation available that would explain this?
> 
> For example, the signal strength. All I know so far is that this is a
> 16-bit value. But then what? Do greater values represent stronger
> signal or weaker signal? Are 0x0000 and 0xffff special values? Is the
> returned value meaningful even when FE_HAS_SIGNAL is 0? When
> FE_HAS_LOCK is 0? Is the scale linear, or do some values have
> well-defined meanings, or is it arbitrary and each driver can have its
> own scale? What are the typical use cases by user-space application for
> this value?
> 
> That's the kind of details I'd like to know, not only for the signal
> strength, but also for the SNR, BER and UB. Without this information,
> it seems a little difficult to have consistent frontend drivers.
> 
> Thanks,

Hi all,

   I am a bit late in this discussion.

   I just want to raise out a problem of the current architecture of FE 
+ tuner.

   Indeed, the actual "Signal Strength" can only be get from tuner. 
Tuner has amplifier internally and AGC. So demod can never know the 
accurate signal strength. Demod only roughly knows signal-to-noise ratio.

   Correct me if I am wrong that I found FE == Demod in current code.
Thus, asking FE to report the signal strength is not appropriate.

   To achieve reporting actual signal strength, in commercial 
proprietary code, it is a combination of readings from tuner + demod. 
Which in turn,
should sit in card/dongle specific code.

Regards,
David T.L. Wong