Recording Help

J

JaredMACPA

Member
Hi,

I work for a non-profit recording webcasts. I haven't been here long and this issue predates me, but I don't know enough (read: anything at all) about audio to solve it. My partner is similarly unskilled in audio.

We have an issue, not on all recordings, but on too many, where though the audio of a recording seems generally low, we still get clipping and distortion when a speaker enunciates certain things.

Briefly, some details on our set-up:

We use Sennheiser Lavs ew100's, and often that goes into our HMC-40 via XLR. We often, when we're broadcasting live especially, use a Xenyx 1202 mixer. We feed the lavs and occasionally a Sennheiser handheld mic, into this and back to the camera.

I've attached a brief clip of some audio in a particularly rough patch, so that you can hear the problem for yourselves.

www.macpamedia.org/media/temp/WebcastSample.zip

Could anyone enlighten us on what we're doing wrong?


Many thanks!

Jared
 
1) It sounds like digital clipping. The audio is simply too loud at the point where it is converted from analog to digital.
2) When you say "the audio of a recording seems generally low", that doesn't really mean anything. An adjective like "generally low" isn't really defined. How exactly are you arriving at this "seems"? Monitoring where? Monitoring how? What does "generally low" mean exactly?
3) How does this symptom change when you are connecting the mics directly to the camcorder vs. through the mixer?
4) Do you hear this distortion when you monitor at the mixer?
5) What is the audio level indicator showing on the camcorder? Where is the indicator when you hear this clipping?
6) Do you get the same distortion with ALL the different mics (lavs and handheld)? Do the mics have fresh batteries?
7) Has this always happened with this camcorder, or is it a more recent development?
 
2. Perceived loudness. Especially when we're broadcasting live, we have at times gotten complaints from viewers that the audio isn't loud enough. I think, though, that generally that much is on the viewer's end and not on ours. Though I look at the rather scant amplitude of our waveforms and I listen and I, in many of our recordings, think it's not loud enough either. Considering how quiet it seems these sudden peaks confuse me. Though, as I said, I know next to nothing about audio...
3. As I said, I haven't been here long, but all the recordings I've noticed this on went through the mixer rather than directly into the camera. My partner isn't so certain, though couldn't provide a counter example. If we broadcast live, we ALWAYS go through the mixer. If we're just recording, we most often go directly into the camera.
4. I'm not typically the one monitoring, though yes, we do notice this when monitoring.
5. It does show peaking on the camera when this happens. But again, it seems very inconsistent; much of the recording sounds very quiet, and then we get these sudden, dramatic clips.
6. Yes, it happens with both the lavs and our handheld. The handheld I always feel is problematic because when we pass it around the audience, people like to put it as near to their mouths as possible. Not sure if the problem with the handheld mic is the same then as the problem with the lavs. We clip our lavs to a tie or a lapel, so it's not like they're suddenly being thrust in someone's face.
7. Can't comment as, again, I haven't been here long. Live webcasting is newer for us, though, and again that seems to be mainly where we're seeing the issues.
 
maybe your not attaching the mic to the cloth properly, that would cause random bumps that would clip the sound.
 
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can you provide numbers and gain settings from all the hardware.

the mic goes to the mixer -> mic in level with gain between A and B on average, that goes to the camera set to C with the gain level set between D and E on average and the final digital project file is sits in -18 to -30db before any mixing or adjustments?
 
Woah. Slow down there. Remember, I'm a total layman. I had one audio class in college like 4 or 5 years ago, and I didn't do so hot there...

And yes, this particular recording is at that level before I do any touch-ups.

Talk to me like I'm 4 and just discovering the world. :) ...because I am.
 
the knobs on the mixer

the audio settings on the camera, do you have agc setting or audio knob dials or settings on the camera or in the menus.
 
Not to contradict Richard, because his suggestion could also be the culprit here too. But another area to look is in your gain staging and mic placement.

I will explain:

Gain staging has to do with where everything in your signal chain is set for. If you, are increasing gain in one spot, decreasing it in another, and trying to make it up in yet another area.....you could be injecting clipping, somewhere the signal is too hot basically. Your final output can be "low", but clipping can still be there. Many times....this also causes.....summing amp overload. That is a bit harder to describe without getting too technical.

The mic placement (and talent voice control) is another culprit to consider. A mic can only handle so much volume due to the physical aspect of it's design. If the diaphragm/coil/electret receives an impulse above what it can handle, it's going to clip. Final output.....again......can be low, and clipping will still be heard.

You need to "walk" your entire signal chain.........checking what the signal is doing.

My suggestion:
Start with the mic placement......is it close enough to get clean audio, but far enough away to not overload when the talent gets too loud?
Check what the sensitivity of the transmitter is set for for mic.
Look at the gain setting on the receiver
If you are going straight into the mixer from the receiver......look at your "trim" settings on the board. Your fader should be at "unity" or 0 dB when doing this.
When going into your interface, do you have any controls there for "volume"/gain? If so....check there too.

Find a way to monitor at every point possible to listen for your clipping issue as you are checking.
 
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1) Get the mic as close as practical. But never more than 12 inches away.
2) Plug the mic into the XLR input #1 on the mixer.
3) Turn on the phantom power ("+48V") if the mic requires it. Or put a fresh battery in the mic if it wants that.
4) Adjust the mixer "EQ" and "PAN" controls to center position (12 o'clock) on all channels.
5) Adjust the mixer "FX" control to zero. (You don't need this.) on all channels.
6) Adjust the mixer #1 input "LEVEL" control (at the bottom) to somewhere between "0" and "+6" (i.e. between 12 o'clock and around 2 o'clock). This is the "normal" operating range.
7) Turn the mixer "LEVEL" all the way down (counterclockwise "-∞") on all the other inputs (2-12)
8) Slide the "MAIN MIX" control up to the "0" mark. This is the "normal" operating position.
9) Have the talent speak in a "normal" level".
10) Adjust the mixer #1 input "TRIM" control so that the "0" LED (above the "MAIN MIX" slider) comes on regularly and sometimes the "6" LED when speaking louder.
11) The "CLIP" LED (next to the PAN knob) should NEVER come on unless the talent yells unexpectedly.
12) Likewise, the "CLIP" LEDs above the "MAIN MIX" slider should never show up with very occasional and unexpected conditions.

If you monitor the output from the mixer (using the "PHONES" output and the "PHONES/CONTROL ROOM" knob) you should be hearing a good average level with no clipping.

You should be able to apply the same principles to plugging a mic directly into the camcorder, etc. But I don't know what all the features and controls are for your camcorder.


This all assumes that your talent isn't a low-talker (http://theseinfelddictionary.com/2007/12/30/low-talker/)
If you are dealing with a low-talker, you will need to use a headset mic with the tip within inches of their mouth and likely a compressor outboard processing.
Headset mic: http://www.rodemic.com/mics/hs1-p (just one example, there are several good products in this category)
Compressor: http://www.dbxpro.com/166XL/ (just one example, there are several good products in this category)

Candidly, it sounds like somethng in your audio chain is broken or malfunctioning. Or dramatically mis-adjusted.
The combination of low average levels, but severe digital clipping on rather LOW "peaks" is very unusual IME.
I agree with Doc Bernard that the gain-staging is a prime suspect, and the instructions above address that.
However it doesn't completely answer the question, at least as the symptoms were presented.
 
As to the digital clipping.......let me help define what that is. Aurally....you know what it is. But to understand how it happens is something a sound engineer needs to know.

When analog sound is transformed into a digital signal by an interface, it creates a snapshot of the electrical (analog) sound signal. How many times a second that happens is what sets your "sample rate" 44.1k=44,100 times a second, 48k=48,000 time a second.

But then there is the snapshot itself. Each snapshot becomes a digital "word" made up of 1's and 0's. Within that "word" are "bits". The amount of detail the snapshot captures is dependent upon how many bits are used to describe that "word". This is called the "bit depth". The higher the number, the more detailed the snapshot.

Okay.....with that explained......let's talk about digital overload. In creating these snapshots of sound and turning them into a digital representation, like I said, the A/D (analog to Digital) converter is making strings of binary numbers (1 or 0). if the word ever becomes all 1's.....you have digital clipping, because it can never be more than just 1's, despite there being more information being fed to the converter from the analog signal.

It is your job as an engineer, or someone doing that job, to make sure the converter never gets fed a signal that will become all 1's.
 
Yikes! I just read the original statement of the symptoms and realized that you are talking about a wireless mic!

That adds considerable complexity to getting all the gain-staging adjusted properly.
Specifically, it means that you have TWO MORE places where you need to get the gain set properly.
First, the transmitter gain must be set in the "Goldilocks Zone", not too "hot" and not too "cold".
If the transmitter gain is set too high ("hot"), it will result in clipping, possibly like what you are hearing.
If the transmitter gain is set too low ("cold"), the signal level will be low and the signal-to-noise ratio (SNR) will be low ("noisy").
But transmitter gain set too low will NOT cause clipping symptoms.

Then you need to set the gain of the receiver, also.
Again, "Goldilocks" applies, not too "hot" and not too "cold".
Have you read all the instructions for the wireless mic and applied them properly?
 
I thank you for your assistance, gentlemen. I'll start dissecting this piece by piece here shortly. Reading through all these responses a few more times. Ingesting! Thanks!
 
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My partner and I have talked. We are setting aside tomorrow to troubleshoot this with the information provided. I'll report back with any questions or clarification.

Thanks again!
 
Level too high on the peaks - unattended, no sound op solution - a limiter. The average level is low, so needs increasing - again, a simple solution would be a compressor, set on a moderate ratio, then left alone. The snag being these devices are usually line level operated, so really need interfacing via a mixer. Something like a second hand cheap Behringer on ebay won't cost very much. I'd also suggest maybe a small mixer - feed the mics into this, minitor on headphones and then either use the compressor/limiter on the channel inserts, or across the output, as it goes into the camera. However, if you just watch the levels to the camera, you could do the compression in the editing stage.
 
yes, i suspected the 2 opposing gain settings and low trim.
if the mixer can output line level and you set the camera xlr adapter to line level and turn off phantom power. it could solve the problem or minimize the variables.

the sound is noisy but also clear and i don't think they sell field mixers with preamps that noisy, so it's two gain settings obviously, the noise floor is at -43, i don't think that's normal with one preamp.
 
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