The
Graph Sketch site has recently come to my attention.
A few days ago I was looking at the upper and lower limits of the logarithmic presentation of the transfer function (Hlog) of an xDSL circuit and the representation of the "no value is available" condition.
The last three paragraphs of Section 8.12.3.1, headed "Channel characteristics function per subcarrier (CCFps)", from
ITU-T Rec. G.992.3 (04/2009) read as follows --
"The channel characteristics function Hlog(f) shall be represented in logarithmic format by an
integer number m(i), where i is the subcarrier index i = 0 to NSC – 1. The m(i) shall be coded as a
10-bit unsigned integer. The value of Hlog(i × Δf) shall be defined as Hlog(i × Δf) = 6 – (m(i)/10).
This data format supports an Hlog(f) granularity of 0.1 dB and an Hlog(f) dynamic range of
approximately +6 dB to –96 dB.
An Hlog(i × Δf) value indicated as m(i) = 2^10 – 1 is a special value. It indicates that no measurement
could be done for this subcarrier because it is out of the PSD mask passband (as relevant to the
chosen application option – see annexes) or in the BLACKOUTset (see clauses 8.13.2.4, 8.13.4.1
and 8.13.4.2) or that the attenuation is out of range to be represented."
As a consequence, I had an equation that I wished to visualise --
y = 6 - (((2^x) - 1) / 10)On entering that equation, all was then
revealed.
