Mac Michaels wrote:
On Wednesday 12 April 2006 09:57 am, Robert Schlabbach
wrote:
From: "Rusty Scott" <rustys@xxxxxxxx>
1) That it be encouraged to report SNR in dB whenever
possible.
I'm afraid most hardware is incapable of producing such
measurements. Instead, I'd propose scaling all signal
measurements to _percentages_.
SNR (Signal to Noise Ratio) and Signal Strength are
different measurements of the RF signal.
Signal Strength is difficult to compute for a particular
card. It is the actual level (quantity) of the signal
appearing on the RF input connector of the tuner for a
particular TV Channel. There are both internal (RF
frontend, AGC design, etc.) and external (antenna gain,
booster amplifiers) that may affect this value. In theory a
weak signal with a lot of noise will have the same strength
as a weak signal with little noise. In the first case the
signal may not be useable whereas it may be acceptable in
the second case.
SNR is probably more useful as it is more indicative of the
quality of the signal received rather than just the
quantity. SNR is a ratio in dB. Increasing positive values
indicate that the signal is stronger than the noise.
Negative values indicate that the noise is stronger than
the signal. SNR is less affected by internal and external
influences because additional amplification/attenuation
usually affect the signal and the noise equally.
I am not familiar with DVB chips. They may provide signal
strength directly. The chips supported by the lgdt330x
driver provide SNR directly. The specification gives the
exact formula to compute SNR in dB for any value read from
the chip.
SNR is not in the "Video for Linux Two API Specification
Draft 0.13". I have no problem reporting SNR in dB or as a
percentage of full scale. Either way the value returned is
useful for aiming an antenna or indicating cable connection
problems. I propose adding some more details to the "Video
for Linux Two API Specification":
[proposal snipped]
Mac-
This topic does not apply to V4L2 --- We are talking about the DVB
subsystem. Yes, we all know that a single tuner is shared between the
analog and digital functionality in hybrid devices, but this is
irrelevant to this discussion. The DVB API is the issue at hand. V4L2
has nothing to do with it.
Cheers,
Michael Krufky
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