On Thu, 19 Mar 2009, Mauro Carvalho Chehab wrote:
> that we should discuss about it for some time, comparing each alternatives we
> have, focused on SNR only. Later, use can use the same approach for the
> other quality measurements on DVB.
>
> In order to start such discussions, I'm summarizing both proposals under the
> same perspective.
>
> I'm also presenting some criteria that are important on this scope
>
> People are welcome to contribute and to argue in favor/against each one.
>
> So, on both proposals, we have 1 bit for signal and 15 bits to be used for
> storing the absolute numbering representation.
One could always use a normal 2's complement signed number to indicate
negative SNR. The only problem was the structure uses a u16 instead of an
s16.
> The SNR(dB) will be given by:
> SNR(dB) = driver_SNR_measure / 10;
Since the driver often needs to use a logarithm from dvb-math to find SNR,
you have code like this in the driver (from lgdt3305.c):
/* report SNR in dB * 10 */
*snr = (state->snr / ((1 << 24) / 10));
> The SNR(dB) will be given by:
> SNR(dB) = driver_SNR_measure / 256;
For the driver side, also from lgdt3305 which has both formats with an
ifdef:
/* convert from 8.24 fixed-point to 8.8 */
*snr = (state->snr) >> 16;
FWIW, converting to decimal to print using only integer math:
/* decimal fixed point */
printf("%d.%d dB\n", snr / 10, snr % 10);
/* binary fixed point */
printf("%d.%02d dB\n", snr >> 8, (snr & 0xff) * 100 >> 8);
On modern CPUs, integer division and modulus are by far the most
expensive operations, so the latter is probably faster since it replaces
two div/mod operations by a single integer multiply (cheaper than
division) and three much cheaper shifts or masks.
> Also, due to historical reasons, the frequency is represented as a 62.5 Hz
> step
> (or 62.5 kHz step, depending on a capability field), and this works fine: all
One could say the v4l1/2 use frequency in MHz in 28.4 binary fixed point.
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