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Level 2

276 points

Distorted Audio

I have recently recorded a musical concert with a Canon XL1S video camera using 16 bit audio, two audio channels -- one channel for a line out from the main mixer and one channel for an ambient mic through a second mixer. Both channels were set at line level input and both were recorded at attenuated levels due to large fluctuations in sound levels and volume from the musicians/group singers. I heard no distortion through my headphones while taping and recording levels were set low enough not to distort. Upon editing the sound in Final Cut Studio, I find that that both channels are suffering from some distortion, but the ambient mic run through the second mixer is suffering the most distortion. For future reference, can you tell me why attenuated audio would distort in the editing process and not be heard during the recording process? Thanks.

Small Town Gal

Mac Pro, Mac OS X (10.4.11)

Posted on Apr 20, 2011 4:43 PM

26 replies

Apr 21, 2011 7:57 AM in response to Barbara Smits

Hi Barb -

As Kind of previously mentioned, Cameras and "Auto" audio not so good of an Idea 🙂 When the camera is set to "Auto" it applies a limiter to the signal to force it to be consistant... so, using a limiter, to get a consistant level the camera has to gain up the signal so much that the difference of hearing a 10db (Input) : 1db (output) smoosh of the signal That originated at say.. -20db... a threshold probably in the neighborhood of -15db, maybe even -20db.. The camera will gain up the levels by 20-30db to get a consistant -4db signal... Now obviously something as dynamic as a choir.. it's going to be adding and pulling as they get louder and softer, but never fast enough. Basically the limiter is crushing all form of dynamic range that would have been there, and causing tons of saturation because although it doesn't look like it's peaking... it's so over-saturated that it might as well look like a stick of butter (correct me if I'm wrong 🙂)

That Said - and as previous said - Broadcast standards are usually -15db to -12db when going out to broadcast. Sometimes even as loud as -10db. Recording at -20db is really not that quiet. and it really protects you from having to worry about getting caught off guard and peaking if you're trying to ride the faders.

What kind of audio equipment are you using? what kind of mics and preamps? How many mics and preamps? sound board? or small multi channel mixer? With even just a small investment in these things you can improve the quality of your recorded sound by tons! purchasing a Class-A Preamp ($100ish per channel) will dramatically clear up the sound and lower the noise floor, so when you record something at -20 and want to bring it up.. it won't have a lot of garbage in the signal.

Hope this helped some.

Apr 21, 2011 8:17 AM in response to NLEdit

I guess I didn't read closely enough when the posts got longer. 🙂

Perhaps the sound guy clipped the mix before it went to the sends.

Barbara Smits Author

Level 2

276 points

Apr 21, 2011 8:27 AM in response to thrillcat editorial

GUYS, GUYS,

I have done many of these recordings and, believe me, I can hear distortion through my headphones when it's there. This sound was as clean and mellow as it could be and I was sure that I was going to get a good recording. I wonder if it could be a bad channel on my mixer? All of my equipment is pro equipment, from mixer to recording mic, so there is no cheap stuff there that should cause a problem. I did notice before I did the recording that I was picking up some distortion when I tested the mic and I dropped the gain level on the mic until the distortion was gone and I was not registering any red peak levels during the actual recording process. So, I will try to get the sample posted here shortly...and thanks for your interest.

Barb.

Apr 21, 2011 8:39 AM in response to Barbara Smits

Don't try to stifle a discussion, this is how problems are solved. 🙂

The more things the sound went through, the more chances there are that distorcion was introduced before it got to your camera, which means you wouldn't have seen any red peaks during recording.

The only stage where distortion should be impossible to add would be the capture via FW. We're trying to talk it out and figure out where the distortion is being introduced.

Barbara Smits Author

Level 2

276 points

Apr 21, 2011 8:58 AM in response to Blacknight582

Dear All and Black Knight in particular:

I don't mean to squelch the discussion -- go at it!

I have edited plenty of music videos that were recorded at levels so low that it becomes an editing circus to get a good final product...colleges particularly like to do this when they are recording student recitals...sheesh!!! There's hardly anything there to work with on the quiet parts.

Black Knight is perfectly correct in saying that the visual sound wave when the distortion occurred looks like a stick of butter, which it does, so I assume that the rest of what he said is the correct answer...using the attenuated evens things out, or tries to, raising the quiet parts, but over saturates the loud parts in the process. So, the answer is to lower the recording levels and stay away from attenuated audio because IT DOESN'T WORK!!!

Thanks again, and continue the discussion if you wish. By the way, I have made plenty of very good recordings in my time, regardless of what anyone thinks!

Barb.

Apr 21, 2011 9:07 AM in response to Barbara Smits

I hadn't even seen that response, but yes, that's what happens.

You're better off setting your audio very low and boosting it in post than setting it high and letting the attenuator keep it from 'peaking'. All that really does is removes dynamic range.

I like to call the attenuator, and any typical normalize function, the CHAIN SAW, aka WAVE HAMMER.

Barbara Smits Author

Level 2

276 points

Apr 21, 2011 9:23 AM in response to Barbara Smits

Dear BlackKnight:

In answer to your question, "What kind of audio equipment are you using? what kind of mics and preamps? How many mics and preamps? sound board? or small multi channel mixer? With even just a small investment in these things you can improve the quality of your recorded sound by tons! purchasing a Class-A Preamp ($100ish per channel) will dramatically clear up the sound and lower the noise floor, so when you record something at -20 and want to bring it up.. it won't have a lot of garbage in the signal."......

I would say that I would be very interested in purchasing a Class-A Preamp for my Canon XL1S. I want to be able to plug directly into the camera with my ambient mic without going through a mixer, but if Channel 1 is set to line level, channel two is also line level and cannot be changed. I wanted to buy a converter that plugs directly into the microphone, changing it from mic level to line level, but my audio tech here advised me to go the other way. He said that the left channel, channel one, on the Canon XL1S is the proprietary channel, so he said to buy a converter that converts line level to mic level (rather than mic level to line level), and then adjust the volume control for channel two accordingly. There is one problem, however, that being that there is only ONE level adjuster on the Canon XL1S and a balance control knob, so there is no level control for channel two. How will I ever adjust the volume without a mixer -- perhaps a small one that I can put on my tripod? Any suggestions?

Apr 21, 2011 10:12 AM in response to Barbara Smits

Ah - So here's what you're saying 🙂 -

"I wanted to buy a converter that plugs directly into the microphone, changing it from mic level to line level, but my audio tech here advised me to go the other way."

That "Converter" you talk about is a mic preamp. I'm going to dumb it down (even) knowing that you have experience, but for everybody that's reading, and maybe we all learn a little more 🙂.

'Microphone level' is considered to be down at the -60db>-50db range... this means that if you just plug a mic into something that is expecting line level (be it consumer line@ -10db or pro line @ +4db) you're at least 40db lower and will sound like a whisper even though somebody could be screaming into the mic. By buying a mic preamp, and plugging the mic into the preamp it has knobs that raises the level of the mic into a line range (-10 or +4db) normally +4db...

My suggestion for using a Class A preamp is suggesting that you purchase a preamp that has been built to a standard that most lower level mixers are not. On a mixer, you have 4 all the way up to 48 channels of preamps, Aux, EQ, panning, and Faders.... These preamps will get you the conversion from Mic level to line level, but they are not built to the standad that these Class A preamps are built to. And they will have more harmonic distortion, they will have cross talk between channels. (Cross talk = When a Person speaking into a microphone is on Ch1 and a guitar is on Ch2; you solo Ch1 in post-production and turn up the levels, and! Holy Cow! There's a guitar IN the vocal track!! it's quiet, but it's there for sure... but I didn't want a guitar... ok rant over- you get the idea 🙂 )

Barb here is a small, 2 channel example of a class A Preamp.

http://pro-audio.musiciansfriend.com/product/PreSonus-BlueTube-DP-Stereo-Dual-Pa th-MicrophoneInstrument-Preamp-With-12AX7-Tube?sku=184132

This preamp actually has tubes (cool attempt at "warming the sound up") built in, lots of headroom, and VU meters to show you constant level (note: VU is not showing peaks... so you would not want to slam this up to -8db on the meters... stay around -20db>-18db)

If you're needing more channels for more microphones, there are many different options out there to choose from. They would replace your mixer

So your signal flow would go like this :

Microhphone (picking up a choir in this case) >> XLR Cable >> Mic Preamp >> 1/4" TRS (for line out of mic-pre and then adapted to whatever your canon takes, be it XLR or other. Having an adaptor is not a big deal, but I would build the cable personally..) cable >>Camera

I think that covers it! Any further questions, feel free to post (obviously)

Cheers!

Barbara Smits Author

Level 2

276 points

Apr 21, 2011 10:44 AM in response to Blacknight582

Dear Blacknight,

As my old boss used to say, "You are a shining star in a sea of darkness!" Thanks for the info -- I will look into it and if I have any more questions I will post them at the end of this forum so that we have all of the questions in one forum for others to look at.

I like someone who gives me answers so that I can make corrections and proceed from there rather than just telling me that I'm an idiot. As I am sure you know, every video taping and recording situation is different, and we all must rejoice when we come out with a good product in the end...I've certainly learned that in thirty years in the business!

I would love to buy all of the latest and best equipment, but believe me, unless you are a bigtime Hollywood producer or the son of a recording studio magnate, there is not much video business out there today...so, I adapt where I can without spending a fortune...but I am willing to spend money to get the solutions that I need and do a great job for my clients.

Thanks again -- and I may have more questions!

Barbara Smits

Apr 22, 2011 4:56 PM in response to Barbara Smits

Does your camera have a headphone jack? I believe what you hear at the camera is probably more reliably what is being put onto the tape. I personally check at the board and on the camera. Some mic preamps have headphone jacks on them. As far as being small enough to mount on your tripod, I wouldn't suggest it just because if it shifts or falls for whatever reason that video is hosed you know? I'd hate for something like that to happen. Basically you would be purchasing a preamp that has the basic functions of a mixer. If you're wanting Eq or anything like that, then a nicer sound board is in order. What sound board mixer are you currently using?

Mar 5, 2015 8:57 AM in response to Barbara Smits

Thank you for posting Barbara. I don't have anything to add (not that you need it) but I too have issues with live music recordings (usually rock blues bands with single line from desk and ambient mic to Panasonic HX201 directly) but the distortion is recently (though at the same venue the engineers are always different, never happy to allow me, a wee older wifey, to sound check with the bands) so extreme that I have crackles and pops. But unlike you I am still completely at sea with audio and am reading avidly. Your experiences and input do make me feel hopeful that I can get this addressed. Thank you again!

Distorted Audio