Hi Rob and the group,
<<RE: a "POP" sound in the audio during peaks with UCR211 receivers>>
The "pop" sound in the audio during peaks is caused by an overdriving
of the digital attenuator in the output of the UCR 211 and UCR201
receivers. The current design of the audio circuitry delivers internal
levels to the attenuator that are right on the edge of causing a
problem. When the transmitter is in limiting, the RF output is held to
an audio deviation of +/-70 kHz or less. In the receiver, the digital
pot overloads at +/-85 kHz of deviation. This is all fine and good
except for the fact that the transmitter limiter has a finite attack
time of a few milliseconds, as do most all limiters.
A finite attack time is a typical design compromise to keep very brief
sounds such as clicks or brief summed harmonics of the voice from
controlling the limiter's action. It is sonically better to simply
allow such brief signals to clip for a millisecond rather than control
the limiter level for 100 ms or more. The clipping level in the
transmitter for one of these peaks is about 100kHz of deviation. On
the units that produce a "pop" during peaks, the digital attenuator
overloads at about 85 kHz of deviation. When one of these brief
signals comes ripping through the receiver, the attenuator gives up
and the attenuation goes to 0 rather than the -40 dB that is called
for when the output level of the receiver is set for -30 dB. This
converts a harmless (mostly inaudible) transient to a very noticeable
"pop" sound. Another way of thinking about this is to consider that
the transient peak signal suddenly has 40 dB of gain applied to it
relative to the rest of the audio.
If the output attenuator is set for little or no attenuation, i.e.,
+10 dB line level, there is no change in attenuation with overdrive
because there was no attenuation to begin with. I would guess that
most systems are run with fairly high output levels, i.e. line levels,
so the problem does not occur.
The question was asked about why we didn't know about this earlier.
The only thing I can say is that some units do it and some don't
because settings and conditions have to be just right to cause the
problem. We have fixed two units here that Rob identified as having
the problem by reducing the internal audio levels by only one dB. If
you recall, John Gooch posted here about the repair of his units some
time ago with the following message: " ...In the end no particular
component could be found to be the cause of the problem. They replaced
several components in the receiver until the problem ceased."
Unfortunately, the techs didn't "fix" the problem with Rob's receiver
at that time, but instead changed some parts that affected the level
driving the attenuator just enough to keep the problem from occurring.
This got the unit working, but was not the real "cure" that is needed.
The conservative fix is to reduce the output stage gain by a little
less than 2 dB, which is what we will do on any units shipped after
today. Units that are in the field can be fixed here at the factory or
at our warranty stations at no charge, of course. The fix is simple,
consisting of changing an 18k resistor to a 13k resistor. In future
PCB layouts, a different opamp will be used; one that will run
rail-to-rail from the 5 Volt supply to limit its output to a level
that cannot overdrive the digital attenuator.
The 400 series receivers don't have this problem because the internal
opamps run from 5 Volts and can't overdrive the digital attenuators.
With this safety margin built into the circuitry, plus the fact that
in the digital audio processing we can also define the largest signal
that can be delivered to the output stage with a number, this problem
cannot occur.
Many thanks for the good input from the field and thanks for helping
to eliminate this subtle bug.
Best Regards,
Larry Fisher and the crew at
Lectrosonics
On 28 Jan 2004 17:12:16 -0800, soundmixer DeleteThis @zoominternet.net (Rob Lewis)
wrote:
>I have a mic/line level issue with my Lectro 211's/201's that is
>bothering me, More of a nusance than anything.
>
>When i set the Lectro's at -30dbu into the 442 at mic level i am
>hearing a clicking or sharp popping sound on loud yells, coughs or
>loud dialogue in general when the limiters engage on the transmitter.
>I have lowered the gain, tried different lavs, no change. I have
>tried this with the input limiters on the 442 on and off, but no
>change. Tried with output limiters on and off, no change. I dont
>have this problem when I use a hardwire lav or boom mic.
>
>Here's the catch. If i set the Lectro reciever output to a +5dbu or
>above setting into the 442 at line level input i dont have the problem
>at all. I notice at the -30dbu/mic setting the input limiters on the
>442 engage on a regular basis when things get loud. But at the +5dbu
>or above setting I rarely if every see the input limiter lights com on
>at all. (For reference, i set output limiters at +8 on the 442, done
>out at OVU on analog cams and -20dbu on digital cams.)
>
>I dont understand why this is happening at the mic level setting. I
>spoke to Sound Devices, Lectrosonics, some sales reps at unnamed
>dealerships, no one seems to have an answer or idea what is happening.
>
>I just want to know what is happening with my gear. I bought all new
>gear in May of '03 and within two weeks of purchase i had to have new
>pilot tone crystals in my Lectro 211's replaced. Is there something
>wrong with my mixer too. WTH.
>
>Sorry for the frustrated tone but is it too much to ask for gear that
>works properly when i need it too.
>
>Any suggestions or ideas would be greatly appreciated.
>
>Thanks
>Rob Lewis
>www.lewisproaudio.com
Larry Fisher
Lectrosonics
>> Stay informed about: Lectro 211/Sd442 Issue 
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