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Distortion but not clipping on mixer

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Distortion but not clipping on mixer

Postby matt_england » Mon Sep 09, 2019 3:52 pm

Hi!

Tearing my hair out at this one. Working on a film set with a device that captures both video and audio. To do the audio section an output mix is taken from the sound department at line level. Getting bad distortion on people talking loudly however the mixer isnt clipping. Same feed is going into the scarlett 2i2 and it sounds fine. This has happened on 2 different jobs with 2 different mixers. After the last time I took it back to the workshop and used a 3.5mm to XLR out of my laptop and played sound through and it sounded absolutely fine but distorts from any sound department that supply. Mixer is a Behringer ZMX8210 V2. Cheers!

Matt
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Re: Distortion but not clipping on mixer

Postby Hugh Robjohns » Mon Sep 09, 2019 4:01 pm

Is it an analogue or digital feed?

If digital, is it properly locked to the video frame rate?

Can you get the 'sound department' to send a reference tone level to check the signal level alignment?

H
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Re: Distortion but not clipping on mixer

Postby matt_england » Mon Sep 09, 2019 4:20 pm

Hi Hugh,
Thanks for the reply. Feed is analogue and is independant of the video source so will not need to be synced even though it is. Yes reference tone is send and aligned and still clips.

Cheers
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Re: Distortion but not clipping on mixer

Postby Hugh Robjohns » Mon Sep 09, 2019 4:25 pm

Ooh... intriguing! ;-)

What's the recording device?

What's the alignment level?

Is it a balanced or unbalanced input?

If the sound department's mix out signal is clean, and the recording device's record alignment is setup with a sensible headroom margin, then my thinking would be heading towards a problem with the sound department's mixer's line-driving capability, or a very low input impedance which is causing the line driver to collapse on peaks.

As an experiment, can you buffer the sound'department's signal with a transformer isolator, or an active line-level buffer of some sort?

H
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Re: Distortion but not clipping on mixer

Postby Wonks » Mon Sep 09, 2019 4:28 pm

The Behringer mixer has near mic-level input impedances at 3k ohm (balanced) and 1.5 k ohm unbalanced - and that's with the pad in. A normal line level input impedance is typically in between 10k and 50k ohm. I'm wondering whether the Sound Department's output driver is struggling to provide enough current into such a (relatively) low impedance, and you are getting distortion as a result on the loud sections.

Mixer data sheet : https://media63.music-group.com/media/sys_master/h70/hd6/8849573511198.pdf

The Scarlet's line inputs have a much higher input impedance (60k ohms), so are easy to drive into, hence no distortion.

I'd be tempted to put an active stereo DI box in between and see if that improves things. The Behringer can provide 48v phantom to power the DI.
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Re: Distortion but not clipping on mixer

Postby matt_england » Mon Sep 09, 2019 4:30 pm

Definitely an interesting one.

Balanced input. Standard 0dB tone then aligning to -20db on the mixers desk. Same as he does for most. The most intriguing things are this

A)The scarlett 2i2 which is piggybacking the feed accepts it fine
B)The distortion doesn't happen with a feed from a laptop or such

Cheers!
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Re: Distortion but not clipping on mixer

Postby matt_england » Mon Sep 09, 2019 4:32 pm

Wonks wrote:The Behringer mixer has near mic-level input impedances at 3k ohm (balanced) and 1.5 k ohm unbalanced - and that's with the pad in. A normal line level input impedance is typically in between 10k and 50k ohm. I'm wondering whether the Sound Department's output driver is struggling to provide enough current into such a (relatively) low impedance, and you are getting distortion as a result on the loud sections.

Mixer data sheet : https://media63.music-group.com/media/sys_master/h70/hd6/8849573511198.pdf

The Scarlet's line inputs have a much higher input impedance (60k ohms), so are easy to drive into, hence no distortion.

I'd be tempted to put an active stereo DI box in between and see if that improves things. The Behringer can provide 48v phantom to power the DI.

That would make a lot of sense but what does this mean for the output of the laptop? Does this have a much higher impedance?
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Re: Distortion but not clipping on mixer

Postby matt_england » Mon Sep 09, 2019 4:33 pm

Hugh Robjohns wrote:Ooh... intriguing! ;-)

What's the recording device?

What's the alignment level?

Is it a balanced or unbalanced input?

If the sound department's mix out signal is clean, and the recording device's record alignment is setup with a sensible headroom margin, then my thinking would be heading towards a problem with the sound department's mixer's line-driving capability, or a very low input impedance which is causing the line driver to collapse on peaks.

As an experiment, can you buffer the sound'department's signal with a transformer isolator, or an active line-level buffer of some sort?

H

My fallback plan is to run the signal into a Tascam DR60D Mk2 then take the 3.5mm out of either the Line out or the headphone out and send that into the mixer instead. Would this work?
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Re: Distortion but not clipping on mixer

Postby Brian M Rose » Mon Sep 09, 2019 4:39 pm

I know this is blinding obvious, but far better people than I fell for it in the early days of digital cameras.

'Reference Level' is 0dBm, right? So set the level at 0Db on the recorder, Right?
Well, actually no. It should be set at -20 dB (FS). In other words you have a full 20dB of headroom.
And this was a top TV comedy show, with one of the top sound recordists in the industry. Everything was working fine until there was a loud scream - and that did it.
OK, we were all learning back then (it was Digital Betacam recording via the mixer to the camera at line level). As I said, we were all learning back then, but it's all too easy to fall for it.
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Re: Distortion but not clipping on mixer

Postby Hugh Robjohns » Mon Sep 09, 2019 4:44 pm

Ah... I think we're on the same wavelength, Wonks.

I hadn't realised that the sound department's signals was feeding a behringer zone mixer rather than this av recorder directly. Is there a reason why it can't feed the recorder directly? What is this zone mixer bringing to the party?

The specs I found for the Behringer (v2 model) suggest the balanced input impedance is 5K rather than 3k (see link below), but that's still on the low side for some line devices to handle.

More worryingly, the maximum input level is apparently only +12dBu with the pad selected! Transient peaks could easily be perilously close to or even above that from a professional sound desk at normal-loud operational levels.

The zone'mixer's unbalanced input has a 20K input impedance, but a max input level of just +10dBu. And if a balanced line driver is feeding an unbalanced input that could also be the source of distortion on peaks...

https://media63.music-group.com/media/PLM/data/docs/P0CCY/ZMX8210_QSG_WW.pdf

The DI box idea is certainly worth trying and might prove the point one way or the other... but that zone mixer really isn't compatible with normal professional working practices.

If the Scarlet is strapped in parallel across the Behringer and the Scarlet is always clean, then it can't be an output driving problem from the sound department. It must be an input overload problem in the Behringer itself.

In which case, I'd suspect the sound department are allowing some high peaks through on loud dialogue and the Behringer simply doesn't have the headroom to cope -- it's at least 6dB below the EBU standard, and 12dB below most pro studio gear.

Using a DI box may well allow you to get signal into the Behringer cleanly by operating at a lower internal level, but I suspect you'll then run out of output level to feed the AV recorder at the proper reference level...

Is there -- or could there be -- any alternative to the Zone mixer because that looks to me like it's your problem?

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Re: Distortion but not clipping on mixer

Postby matt_england » Mon Sep 09, 2019 4:50 pm

Hugh Robjohns wrote:Ah... I think we're on the same wavelength, Wonks.

I hadn't realised that the sound department's signals was feeding a behringer zone mixer rather than this av recorder directly.

The specs I found for the Behringer suggest the balanced input impedance is 5K rather than 3, but that's still on the low side for some line devices to handle More worryingly, the maximum input level is only +12dBu with the pad selected! Transient peaks could be perilously close to that from a professional sound desk at normal operational levels.

The unbalanced input has a 20K input impedance, but a max input level of just +10dBu. And if a balanced line driver is feeding an unbalanced input that could also be the source of distortion on peaks...

https://media63.music-group.com/media/PLM/data/docs/P0CCY/ZMX8210_QSG_WW.pdf

The DI box is certainly worth trying and might prove the point one way or the other... but that zone mixer really isn't compatible with normal professional working practices.

H

Here's yet another curveball. We've been using several V1's of the ZMX8210 with great success, its just the V2 varients that are causing us these issues
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Re: Distortion but not clipping on mixer

Postby Wonks » Mon Sep 09, 2019 4:55 pm

There is certainly conflicting info between the full manual and the quick-start guide. The full manual document pdf was created in 2008, the quick start pdf in 2018 (as Behringer sadly now only seem to provide quick start docs for all their new equipment rather then full manuals).

There may have been a hardware mod in the meantime to partly mitigate the low input impedance, so it may depend on how old the zone mixer is as to what its actual input impedance is. However, there is no way of telling, without contacting Behringer directly, when any such modification was made.
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Re: Distortion but not clipping on mixer

Postby Hugh Robjohns » Mon Sep 09, 2019 5:01 pm

matt_england wrote:Here's yet another curveball. We've been using several V1's of the ZMX8210 with great success, its just the V2 varients that are causing us these issues

Curiouser and curiouser!

The V1 specs seem more or less the same -- same +12dBu max on the balanced inputs. Same +21dBu on the balanced outputs. The input impedance is lower on the V1 than the V2, but not by much.

The V1's unbalanced input can handle more level at +15dBu instead of the V2's +10dBu -- but you are using the balanced inputs yes?
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Re: Distortion but not clipping on mixer

Postby matt_england » Mon Sep 09, 2019 5:13 pm

Hugh Robjohns wrote:
matt_england wrote:Here's yet another curveball. We've been using several V1's of the ZMX8210 with great success, its just the V2 varients that are causing us these issues

Curiouser and curiouser!

The V1 specs seem more or less the same -- same +12dBu max on the balanced inputs. Same +21dBu on the balanced outputs. The input impedance is lower on the V1 than the V2, but not by much.

The V1's unbalanced input can handle more level at +15dBu instead of the V2's +10dBu -- but you are using the balanced inputs yes?

Correct, havent ever touched the unbalanced inputs, always used the balanced ones.
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Re: Distortion but not clipping on mixer

Postby Wonks » Mon Sep 09, 2019 6:11 pm

The V2s may possibly have a circuit error introduced when the input impedance was increased - or maybe just a metering one if that wasn't altered when the impedances changed. Totally guessing here though. Really want a V1 and a V2 on the bench to test them both out.
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Re: Distortion but not clipping on mixer

Postby MOF » Mon Sep 09, 2019 7:25 pm

I have come across devices that although supposedly pro spec at line level actually had insufficient headroom. No problem when listening to zero level tone but clipping when the plus 4-8dB signal hit it.
The only way around it was to send tone and increase its level until distortion occurred and then we knew how much headroom I had to leave.
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Re: Distortion but not clipping on mixer

Postby CS70 » Mon Sep 09, 2019 7:39 pm

Interesting. Any difference in cabling or anything surrounding the machine? If you keep exactly the same everything but only replace the box from V1 to V2 you get the issue?
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