Hi all,
K9YC wrote:
> The decibel IS a ratio.
I think all here know the meaning of a decibel.
The reference level is often taken as understood -- which means it's
understood in the context of some particular usage. Sloppy, perhaps,
but we all use shorthand language for convenience.
A particularly relevant example: a reference level used in WSJT-X is the
signal level corresponding to one A/D count -- thatis, one
least-significant bit in the 16-bit digitized audio.
The WSJT-X user is advised to set the audio level at "30 dB" on the
level meter (with the adjacent slider at midscale), when no strong
signals are present. This action has the effect of setting the rms
level of the digitized audio to about 10^(0.05*30) = 31.6 counts.
By audio standards, this is a low audio level. Audacity certainly
thinks so. That's because for high-quality audio you don't want
quantization errors creeping in; at the 3% level they would easily be
heard by a discerning listener.
However, those same small quantizing errors have a negligible effect on
signal-to-noise ratio of the JT65, JT9, etc., signals we're interested
in, because they are swamped by the background receiver noise level.
With the rms noise level set at around 30 counts, the quantization
errors (+/- one-half of a least-significant bit) don't matter at all.
Sure, we could set the level higher. WSJT-X works OK with the noise
level set anywhere between about 20 dB (rms=10) and 60 dB (rms=1000).
But the 16-bit A/D converter necessarily clips at +/- 32767, about 90
dB. If you set the background noise level much higher to, say, 60 dB,
you have only 30 dB of "headroom" for strong signals.
-- 73, Joe, K1JT
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