[linux-dvb] [RFC] [PATCH] Make S/N values to appear at cx24123 frontend

Trent Piepho xyzzy at speakeasy.org
Mon Apr 17 04:15:40 CEST 2006

On Sat, 15 Apr 2006, Mauro Carvalho Chehab wrote:
> Em Sex, 2006-04-14 =E0s 19:40 -0700, Trent Piepho escreveu:
> > oN fRI, 14 Apr 2006, Mauro Carvalho Chehab wrote:
> > > I've decided to use u64, instead of all those shifts. This way, we ca=
> > > improve the precision for lower BER rates. Here, it worked like a cha=
> >
> > It was only a single shift.  Anyway, the 64 bit math isn't being done
> > correctly.  Here is a graph were you can see the problem clearly:
> > http://www.speakeasy.org/~xyzzy/ber-error.png
> Hmmm... I forgot the (s64) at the calculus :) I'll fix it. I think it is
> better to make calculus with 64 bits to preserve precision, especially
> for low BER rates.

If you adjusted the table to eliminate the the last line, which can't be
reached with a 22-bit value, and added an extra line for 50%, you could get
the scale factor up to 13 or 14 bits without overflow in 32-bit math.  Is
64-bit math worth it for an extra 2 or 3 bits?

Really, the loss of smoothness comes from trying to scale BER from 0..5380
into just 1% of the range.

I was researching how to find SNR from BER, and think I have a better
understanding of how this works now.  If anyone understands this better, ma=
they can correct me if I'm wrong.

Normally BER is though of as a probability, that is a number between 0 and =
of a bit being in error, and named Pb.  It's normal to want this probably t=
be very small, on the order of 10^-6.  Usually when you see a graph of BER =
Pb, it will be log scaled.

The relationship between Pb and something called Eb/N0 is defined by the
modulation used.  Eb is the energy per bit, and N0 is the noise spectral
energy density.  Here is some good information:

According to wikipedia, the formula for QPSK, used by DVB-S, is:

Pb =3D Q(sqrt(2*Eb/N0))

Where Q(x) is the tail area under the unit normal distribution from x to
positive infinity.  You can think of is the probability that a random numbe=
with a Gaussian (aka normal) distribution is greater than x.  At least that
the definition on wikipedia, I think that might be wrong and it's more comp=
than that.

If I graph this, I get a plot that matches the one for PSK on the sss-mag

I think that Eb =3D S/fb, where S is the signal power and fb is the bitrate=
bits/sec, and that N0 =3D N/BW, where N is the noise power and BW is the no=
bandwidth in MHz.  This gives Eb/N0 =3D (S/N) * (BW/fb), giving us Eb/N0, a=
thus Pb, in terms of SNR.  For QPSK, the bitrate is twice the bandwidth, so
(BW/fb) =3D 1/2.  Convert SNR in deciBels to S/N with S/N =3D 10^(SNR/10). =
all that in, and graph the result:

Pb =3D Q(sqrt(10^(SNR/10)))

So I think THIS is the curve you should be trying to use to convert BER to
SNR.  The question I have it, what are the units of the 22-bit value read f=
the chip?  How do you convert this to the fraction of the bits in error?
Divide by the gross bitrate (bandwidth in MHz times two)?

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