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[Keith Broughton]

I have had this issue with both a beltpack and rack receiver as well as a different beltpack and ENG receiver. Both Sennheiser G3
When monitoring the audio directly from the receiver, it sounds like there is a gate on the audio.
If the room is fairly quiet, there is no background sound heard until there is enough sound to "open" the gate and then there is a slight his along with the audio.
I checked for RF level on the receiver with the TX off and it looked clean. Changed channels but no difference.
The same receiver on the same frequency works as expected with a hand held TX.
This doesn't happen all the time and has me baffled.
It seems to be a beltpack TX issue.
Thoughts?

[Kevin Maxwell]

I have had this issue with both a beltpack and rack receiver as well as a different beltpack and ENG receiver. Both Sennheiser G3
When monitoring the audio directly from the receiver, it sounds like there is a gate on the audio.
If the room is fairly quiet, there is no background sound heard until there is enough sound to "open" the gate and then there is a slight his along with the audio.
I checked for RF level on the receiver with the TX off and it looked clean. Changed channels but no difference.
The same receiver on the same frequency works as expected with a hand held TX.
This doesn't happen all the time and has me baffled.
It seems to be a beltpack TX issue.
Thoughts?

Is it just out of the headphone jack that you hear that? Or does it also sound the same when plugged into a mixer and monitored out of the mixer. Are you by any chance using both the XLR and the ¼” connector on the receiver? I a problem when I tried to split the outputs (for House and Monitor feeds) of some G3 receivers doing it that way but it didn’t sound like gating.

How is the antenna on the body pack? They can get messed up depending on usage. I have had to replace them on a lot of rental packs for one sound company. How close do you have the transmitter to the receiver when testing? Some wireless systems I have encountered don’t like it when you have the transmitter too close, as in right on top of the receiver. Have you played around with changing the squelch settings on the receiver? If the transmitters are the 300 or 500 series are the transmitters you are testing all set to the same power output settings.

Have you checked to see what the RF environment is on the frequencies you are testing and have you tried to re tune them to different frequencies. Just the other day I was helping a friend in his church with a wireless that sounded funny and even though we weren’t picking up anything when the transmitter was off something seemed to be coloring the sound of the unit. I knew that there were digital TV stations broadcasting on all available channels of that system with from a very powerful broadcasting station to a medium powered station. We then changed to a different Shure wireless system still the relatively cheap model but the new digital version and all of the tonal problems went away. 

[Russell Ault]

[...] When monitoring the audio directly from the receiver, it sounds like there is a gate on the audio. [...]

My guess is that what you're hearing is the expander half of the G3's compander circuit.

-Russ

[Luke Geis]

I think you are hearing the companding circuit of the unit. This is generally more evident when using higher squelch settings and having less than optimal antenna setup.

The squelch is essentially a gate for radio noise. When the incoming signal is at or around the noise floor, you will hear the gate woosh in and out. This occurs when the radio signal strength is too weak, or the source audio signal from the transmitter is too low.

If the presenter speaks and the audio meter barely moves on the receiver, the reason for hearing the gate is because the gain at the transmitter is too low and the squelch is too high. If it has plenty of audio level but seems to cut in and out, it could be the squelch is too high and the radio signal strength too low for that given squelch setting.

You can get the effect if you try hard enough on any squelch setting. Generally, it has to do with the audio signal strength; if it is too weak, the companding circuit starts to show itself.

[Keith Broughton]

I will have to check the squelch and mic gain settings and see if I can replicate the problem.
I was not on the ENG shoot so I have to look at the kit when I can get into the shop but so far, I can't replicate the rack system problem in shop.
I have been operating Sennheiser gear since it was G1 and am very familiar with it but have only heard this "gating" 2 or 3 times and always with a lav mic, never a hand held.

[Russell Ault]

The squelch is essentially a gate for radio noise. When the incoming signal is at or around the noise floor, you will hear the gate woosh in and out. This occurs when the radio signal strength is too weak, or the source audio signal from the transmitter is too low.

Companding and Squelch are two separate functions. Squelch works entirely in the RF domain, Companding in the AF domain. Squelch is either based on carrier strength or pilot tone strength, and neither of those are affected by audio signal level, so a weak RF signal with strong audio will still squelch, while a strong RF signal with no audio will not. Any decent quality RF system should have a visual indication on the RX when Squelch is kicking in.

The goal of Companding is to increase the apparent dynamic range of the system. The range from max signal to noise threshold of an analogue FM signal is limited by its bandwidth (which, for wireless microphones, is in turn limited by regulation), and this dynamic range is small enough that significant noise is introduced into the received signal even with appropriate gainstaging. To help combat this, a compressor on the TX is used to reduce the dynamic range of the signal (allowing a hotter signal to be transmitted), and an expander on the RX is used to restore the dynamic range of the signal and reduce the level of the added noise. Given that the added noise is basically white noise (and therefore louder at higher frequencies), pre-emphasis and de-emphasis can be used with companding to further increase the noise reduction and dynamic range increase.

Of course there's no perfect way to "un-compress" an analogue audio signal (especially in real time), so all RF companders feature some artifacts. Gating very quiet signals is one of them. Another classic is erroneous response to ultra-high-frequency (i.e. above human hearing) sounds (often referred to as the "key test" because it literally involves jingling a set of keys by a wireless microphone and seeing how much the signal is affected/degraded).

I will have to check the squelch and mic gain settings and see if I can replicate the problem.

My bet is that the TX gain was set too low for the application.

I have been operating Sennheiser gear since it was G1 and am very familiar with it but have only heard this "gating" 2 or 3 times and always with a lav mic, never a hand held.

Gainstaging wireless handhelds tends to be a little easier than gainstaging lavs for a couple of reasons. Most people don't swap handlheld capsules very often (so the transmitter is always dealing with same sensitivity of transducer) and most handheld applications typically have a fairly narrow range of nominal signal levels, both of which reduce the need for gain adjustments and the size of those adjustments when they are seen as necessary. Conversely, lavs are often swapped and each model has a different sensitivity, and changing lav placement can make a huge difference in nominal level (moving from face mounting to chest mounting represents a drop of something like 20 dB or more).

-Russ

[Keith Broughton]

Gainstaging wireless handhelds tends to be a little easier than gainstaging lavs for a couple of reasons. Most people don't swap handlheld capsules very often (so the transmitter is always dealing with same sensitivity of transducer) and most handheld applications typically have a fairly narrow range of nominal signal levels, both of which reduce the need for gain adjustments and the size of those adjustments when they are seen as necessary. Conversely, lavs are often swapped and each model has a different sensitivity, and changing lav placement can make a huge difference in nominal level (moving from face mounting to chest mounting represents a drop of something like 20 dB or more).

-Russ

I take your point on gain but I have had situations where I am running 25 or 30 mics for a theater production and they are set to a wide variety of gains, some rather aggressive reduction for loud singers.
When listening to the channel while actors are being miced backstage, I have not heard this "gating" effect. In fact, I can clearly hear clothing noise, just like you would with an open wired mic.
It's so rare and hard to replicate in shop that I have not found a cause and effect...as yet.
I will get the ENG kit and test and report results.

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