From my AM broadcast days and my pro audio days, yes it is not unusual
to find that a given mike has more positive output than negative output
in terms of the voltage waveform. Thus if phased correctly, and we are
only using one mike, the positive modulation peaks will be greater than
the negative modulation peaks. There is advantage to this with regard
to AM stations. At the same time, when multiple mikes are used then
they all must be "in phase" in order to prevent a comb effect of
frequency cancellation. It is standard convention that a positive
pressure on the mike diaphragm, then amplified to the speaker system
causes the cone of the speaker to move forward or compress the air into
the room. Positive pressure thus causes positive pressure. Some
audiophiles claim they can hear the difference. The jury is still out
on that point.
From a SSB point of view, it starts basically a modulated carrier, i.e.
AM, which then has the carrier suppressed by the balanced modulator, and
then one sideband removed by passing the DSB signal through a sharp and
steep side filter to produce SSB. Hence, then one can theoretically
attain more positive modulation than negative. Don't confuse amplitude
energy with frequency spectrum energy. At the same time, in a SSB
transmitter, excessive audio can over modulate the carrier, before
carrier suppression and before the sideband filter, and the same effect
as over modulating an AM transmitter will exist. Distortion and
splatter. Look at the opposite sideband of a signal on the air while
using a SDR or spectrum display. Clearly some signals will show these
artifacts while others do not exhibit the artifact.
Final rule, except in a SSB transmitter the positive output is limited
by the available capacity of the amplification stages which must remain
linear. In the SSB world, maximum PEP value capability is the
ceiling. On the other hand, in an AM transmitter, the modulation must
be limited to 100% negative so as not to cut the carrier, but can be
allowed to exceed 100% positive, in fact upwards of 125% to 135% is
quite normal, as long as the transmitter is capable of handling the
increased positive peaks without distortion. Makes for "loud" AM
stations. And of course there are "processors" which control the
negative going values while enhancing the level of the positive going
values in order to enhance the modulation. Of course if the hams AM
transmitter is not capable of 135% positive modulation, splatter and
distortion would be generated making for unfriendly neighbors near the
frequency.
Oh my, this is a lot more than I care to recall.
73
Bob, K4TAX
On 9/14/2018 3:24 PM, Nicklas Johnson wrote:
An audio engineering friend noticed in some voiceover recordings something
that I've also noticed in my own waveforms, that a lot of the time, human
vocal cords seem to produce a biased waveform (seemingly a positive bias
most of the time).
I vaguely remember reading either here on this list, or maybe in QST,
something about this as it relates to setting microphone gain (and bias) so
that the resulting modulated signal is making more efficient (I'm probably
not remembering the right word) use of the available amplitude (ie, not
hitting the peak early on the positive side, but falling well below it on
the negative side).
My Google-foo isn't working out for me today though; does anyone remember
an article or posting on this topic and/or have a link to it? I haven't
been able to track it down again.
Nick
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