On 17/06/2018 08:45, Claude Frantz wrote:
the Qt naming of audio streams is not helpful but you should not need of
adjust the Rx audio stream level in the digital domain, in fact doing so
can only degrade the signal. Rx audio level adjustments must be made
before the signal is digitized and any digital gain controls should be
left at 100%.
If I do not reduce the level using pavucontrol, the thermometer is
always red. This is probably not a good situation.
Hi Claude,
to help understand why digital level controls do not help with level
setting, let's consider an audio signal that is to be digitized by an
analogue to digital convertor (ADC) and for simplicity we will use a
4-bit PCM audio protocol. This means that sampled signal voltages are
translated to integral numbers between 0 and 15 inclusive. Now let's
consider a voltage that exceeds what the maximum level of 15 represents,
it is still represented by the value 15 like all voltages that are above
the maximum level that can be sampled. Similarly all voltages below the
level that will be represented by the digital value of 1 will be zero.
So to summarize, the translation is linear (for some definition of
linear) only for the signal voltages between two specific levels. So now
to the important part, a digital level control can only increase or
decrease the waveform digital values by some factor, for example a
waveform digitized as above with a maximum sample value of 15 and a
minimum sample value of 2 can be digitally attenuated by up to a factor
of 2 while maintaining some linearity. It cannot be attenuated any
further without losing information, nor can it increased without losing
information as the samples with a value 15 must remain so i.e. their
representation is clipped. In fact even attenuating such as waveform by
a factor of 2 loses some information as there must be a rounding off
error, for example all sample values between 14 and 15 must be
represented by a value of 7, and so on for lower values.
Hopefully this will help to understand what I mean by my statement above.
With respect to your interface, I would expect a headphone output to be
at a higher level than that of a line output level. I would also expect
a headphone output to be affected by some audio gain control on the
transceiver whereas the line output may well be a fixed level. The
headphone output is probably designed to drive a different load
impedance than the line output so the level sampled at the ADC may be
higher or lower. Either way the line output is preferable as it should
be a fixed level. That in turn means you will probably have to add
attenuation unless you are very lucky or the rig has a line output level
setting option (many modern rigs have this adjustment in their settings
menus) given there is no universally agreed line level specification.
The rule of thumb that we recommend is that the audio input level to the
ADC should be adjusted such that the WSJT-X level indicator shows at
least 30dB when only background noise from a quiet band is present.
Background noise levels above 30dB are acceptable but the higher they
are, the less headroom remains to handle strong signals that might come
along.
73
Bill
G4WJS.
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