Calibrating Mixer Levels with Camcorder Levels

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GrahamC
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Joined: Feb 8 2008

Hello All,

Apologies but this is probably going to sound like a really dumb newbie question (which of course it is).

I've just ordered a MixPre mic preamp which I'm intending to use with an ex-1. Trouble is I can't get my head round the relationship between the audio levels on the mixer and the audio levels on the camera.

If I just listen to the output of the mixer unit then surely that will be telling me nothing about the modulation level in the camera. What I hear through my headphones will be a function of the mixers' input gain and how loud I've set the headphone volume (on the mixer).

So presumably I need first to set up the camera's input gain level at some suitable figure to ensure that what I'm seeing on the mixer's peak meters is a true indication of what's going on at the camera.

I've bought the books, read the magazines and studied the posts but there doesn't seem to be anything which describes the procedure for doing this. I've read one article on "unity gain procedures" which seems to imply that you adjust mixer and camera gains so that they are both reading 0Db at peak levels. But then in another book it suggests that you should use the mixers' tone generator to establish levels at -20db.

Can anyone suggest a foolproof (idiot newbie proof) method for setting up mixer and camcorder levels?

Many thanks in anticpation

Graham

Rob James
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Joined: Jun 26 2001

Graham. it's a little late at night for me but here goes:

The important thing to decide is the amount of headroom you want. So far as I am aware the camera records in 16 bit which means the dynamic range is somewhat constricted. 0dBfs is the absolute maximum level you can record on the camera. So, what you need to decide is how much headroom you can afford to 'give away' to prevent any possibility of excursions over 0dBfs. Broadcast in the UK uses -18dB as a reference level. This means that signals into the camera inputs should never exceed -10dBfs. You may want to use something a bit hotter if the limiters in the Sound Devices box are as good as I'm led to believe. What you are aiming for is the maximum recorded level without distortion. If you can set the mixer tone generator up to give peak level then set the camera to either -10dBfs or anything up to -4dBfs and do some experiments to see if you get any distortion on peaks. If the mixer cannot be set to give peak level then you will have to do the math to achieve the same result. E.g. if you have to use -20dB then the camera meter should read 0dBfs minus the desired headroom and the -20dB tone so for 8dB headroom -28dBfs or, for 10dB headroom, -30dBfs and so on. Once this relationship is established and verified then the meters on the Sound Devices mixer should give a good guide to what you can get away with. As you have surmised the headphone level is necessarily arbitrary and you will have to play around to know if what you are hearing is a reasonable representation of what is being recorded. Hope this is at least vaguely useful. If not I'll try again tomorrow when slightly more sober :)

EDIT: Analogue reference tones are commonly generated at 8dB below peak. In the US this means -20dB for broadcast giving 12dB headroom before digital clipping.

Rob The picture is only there to keep the sound in sync

Z Cheema
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Joined: Nov 17 2003

It is also confusing with these dB's and things

if i ma recording from an external mixer i get them to feed a tone at a set Level does not matter what, I then set my camera to the same level, then get them to up an down 6DB just to see i am tracking it correctly. I know then that when it is zero on the mixer it is zero in the camera. The -20 is a reference for the the audio levels used in studios normally with the colour bars. Others here can tell you more

GrahamC
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Joined: Feb 8 2008

Diamond advice from both of you! Thank you very much. Mixer and Mic arrive tomorrow - camera plus suitable adult to operate it - next week. I'll let you know what quality we can achieve.

To put things in perspective, I'm not an absolute newbie to sound. We regularly record our voicetracks to broadcast quality in a professional studio and it's this level of fidelity that I'm trying to get close to. It'll be very interetsing to see if the MixPre can deliver!

Graham

DAVE M
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Joined: May 17 1999

you may find that the pro studio's adding a compressor or something from the fairy dust trolley to add some omph to the sound.

Rob James
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Joined: Jun 26 2001

With the right mics and room I see no reason why not. Bear in mind that untreated rooms generally record more 'live' I.e. reverberant than they seem to someone actually present in the room and background noises, air conditioning and traffic always sound louder as well. If you have a choice, go for a domestic room with plenty of soft furnishings, thick carpet shelves full of books etc.

Rob The picture is only there to keep the sound in sync

Rob James
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Joined: Jun 26 2001

I agree about the compressor but you will find that the majority of BBC people use little or no compression but they do use limiters and sometimes hit them gently. If you want to try a compressor in post start with non-agressive settings e.g. 2:1 and a threshold of maybe -6 or -8dB referenced to whatever your peak level is.

In the days when the BBC had it's own sound recordists compressors were almost never used on location and limiters only occasionally.

For voice recordings, (other than a certain style of commercial) the best compressor is a skilled operator's finger on a fader.

Rob The picture is only there to keep the sound in sync

GrahamC
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Joined: Feb 8 2008

Unfortunately we are stuck with an office. However, it's large and has a carpeted concrete floor. The chroma backdrop is heavy material so that should help. Also the DPA lavalier is a cardioid. I'm hoping that a directional lav such as this might help to reduce reverb.

GrahamC
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Joined: Feb 8 2008

Dave M Said: you may find that the pro studio's adding a compressor or something from the fairy dust trolley to add some omph to the sound.

The DPA comes with its own fairy dust! Its pre-equalised with a 4Db presence boost.

Any thoughts on the advantages or not of applyling to 80 or 160 KKz filters? I've heard that these can boost voice but my old prejudices tell me to add nothing to the sound at the recording stage.

DAVE M
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Joined: May 17 1999

I'd suggest recoring in flat and tweaking in post - that way you can undo it if you get too enthusiastic

Rob James
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Joined: Jun 26 2001

The DPA is an excellent mic. That said, omnis sometimes reject more background noise and reverb than cardioids which, by their nature can 'focus' it. Personally I wouldn't apply the filters when recording unless you are having a problem with high level LF e.g. rumble and even then I'd stick with 80Hz. FWIW film dialogue is routinely filtered at 80Hz and often at 8kHz as well. With some voices (mostly female) you might need to watch the presence boost to avoid the dreaded sibilance. However, I think the boost is there mainly to compensate for the position lapel mics find themselves in.

As you say the Chroma backdrop should help deaden the acoustic especially if it's one of those multi-layer jobs. Otherwise, if you are going to make a habit of this and you find the acoustic too live for your taste you might want to consider some portable screens at a later date e.g. http://www.ghostacoustics.co.uk/gobo.html or similar.

Rob The picture is only there to keep the sound in sync

GrahamC
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Joined: Feb 8 2008

Thanks again Rob. The screens look great - unlike my bank balance, which has taken a huge battering this week what with buying headphones, a mic, preamp and an autocue.

I think I'll have to make do with good old fashioned curtains for a while!

Rob James
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Joined: Jun 26 2001

Old duvets work remarkably well. It isn't too hard to contrive a means of keeping them spread and attaching them to a cheap and cheerful lighting stand.

I'm waiting for the change in the capital allowance rules in April before battering my bank balance. The suspense is killing me!

Rob The picture is only there to keep the sound in sync

GrahamC
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Joined: Feb 8 2008

Rob, the Mixpre user guide has this to say about the tone generator: the 1 KHz position activate a 0 dBu level 1KHz sine wave tone. Does this means its generating peak levels.

Graham

Rob James
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Joined: Jun 26 2001

Graham, I've just been reading the user guide. I'm ashamed to say I'm no wiser! However given the way the meters are calibrated I would be tempted to use +8 or +12 as the maximum OUTPUT level. The camera will need adjusting to this taking into account your chosen headroom. So for example, if you decide to go with 8dB of headroom then a peak output of +8dB should read -8dBfs on the camera. By extension, if the tone output is at 0dB then this is 8dB down from the mixer peak output and the camera meter should read -16dBfs when the tone is applied. If you choose a different mixer peak output level or different camera headroom you'll need to adjust these figures accordingly.
It is far from obvious to me exactly how the limiter acts. It really isn't clear if the second stage comes after the mixer level control (as I would expect) or not. To set the limiter thresholds you really need an external source of tone (e.g. analogue output from edit workstation) to be able to see exactly what it does. Since the minimum threshold is quoted as from +6dBu to +18dBu but not whether this refers to levels at the inputs or after the control pots I think you are going to have to experiment. However, if they work in the way I would expect, I'd set them to +6dBu if you set the maximum output level as above to +8dB. This gives you an extra 2dB headroom which is no bad thing since analogue limiters cannot work instantaneously (without a delay) and peaks that get past them can clip the digital recorder although these very quick peaks are unlikely to cause trouble.

Sorry I cannot be more specific. If I had one in front of me, I could work it out :)

Rob The picture is only there to keep the sound in sync

GrahamC
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Joined: Feb 8 2008

Rob,many thanks again. I dread to think how badly I would have coped without your input. Thanks heavens I'm going to be doing all this in the comfort of my own room with as much time as I need. I can't imagine how sound technicians on location manage this black art!

Rob James
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Joined: Jun 26 2001

No problem Graham. I'm sure you will have more questions when you actually have it all in front of you. The secret of locaion recording is to prepare for everything you can think of and then think fast when the one thing you haven't thought of happens.

Rob The picture is only there to keep the sound in sync

GrahamC
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Joined: Feb 8 2008
MixPre First Impressions

Well the Sound Devices MixPre finally arrived and I've had a couple of hours to play with it. First impressions are of impeccable build quality.

Used in conjunction with the DPA 4080 lavalier I'm getting a really good faithful reproduction of voice. The 80 Hz filter (recommended by DPA) seems to make a beneficial difference - though its very difficult to describe what that difference is! I personally can't register any appreciable diffrence when the 160 Hz filter is applied.

The tone generator outputs a 0Db tone as indicated on its LEDs and this seems to accord with the input I'm seeing via Sony SoundForge.

Unfortunately no camera yet so I'm still waiting to hear what recorded results are like. I tried several recordings to PC but unfortunately they've suffered badly from the poor quality A/D conversion inherrent in the on-board sound card.

Graham

Rob James
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Joined: Jun 26 2001

That's very encouraging and, as John Willetts and others will confirm, the mixer and mic are a darn sight better investment than the camera. (I.e. if you look after them you can sell them in ten years time with little or no loss) If DPA recommend 80 Hz cut then I'd certainly use it. You will probably only hear the 150Hz difference on big speakers or headphones with extended bass response.
I am impatient (as I'm sure you are) to find out how it works with the camera.
Have you managed to work out where the limiters sit in the signal chain yet?

Rob The picture is only there to keep the sound in sync

GrahamC
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Joined: Feb 8 2008

Rob, ah no I'll try that next. Can we post audio files up to this forum as attachment's. If so I'll put up a sample from the ex-1 next week.

Graham

P.S. I forgot to mention that, in the quest for ever better sound, I abandonded the Sennheisser radio system in prefrence for a wired option.

Rob James
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Joined: Jun 26 2001

Sadly, I don't think it's possible to post files as attachments. The only way is to put them on a hosting site and post the url.

Wired is always better than radio but a lot less convenient in many situations. It's good to remember the definition of a 'perfect' amplifier. I.e. a piece of wire - with gain. In other words, the less unnecessary processes audio goes through the better.

Rob The picture is only there to keep the sound in sync

GrahamC
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Joined: Feb 8 2008

Rob,

As you suspected I might, I am picking up some reverb from the room. If I'm to make my own homemade sound absorbing screens how high do they have to be? I noticed the Ghost screens are only 6'. Do you think this would be sufficient?

Graham

Rob James
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Joined: Jun 26 2001

Very difficult to be specific. For example I once worked in an ofice about 4m by 3m which had the most appalling acoustics. (Slap echo and horrible reverb) Almost completely solved by putting two Nobo (Low density fibre) notice boards opposite each other on the long walls. Also, you need to consider that the finished product will be watched in rooms with an acoustic. Provided that you even things out in post so that there are no disappeaing reverb tails on cuts then reasonably live rooms are unlikely to be noticed. In fact one problem I had as a TV dubbing mixer working on feature films was getting used to the idea that I needed a lot more reverb to be recognisable as an effect in the much bigger space of a cinema.

To answer the original question. If you go down the (cheap) duvet road with lighting stands then you can experiment to get the best results.

Rob The picture is only there to keep the sound in sync

GrahamC
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Joined: Feb 8 2008

Hmm that's an interesting concept Rob, it hadn't even ocurred to me that I should allow some reverb to stay in!

Rob James
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Joined: Jun 26 2001

Think anechoic chamber. If you could record in one you'd want to add reverb to make it sound natural. The difficult part on location is deciding what is excessive. All natural environments (including outdoors) have an acoustic which includes reflections, if only from trees and the ground. If you ask any post mixer what they would like, they will all tell you close miked dialogue with as little acoustic as possible. However, that is generally unattainable in the real world so we all compromise. Only experience (and/or sharing recordings with others) will tell you what is acceptable.
I am really pleased to find someone here who is clearly interested and concerned to produce the best possible professional audio results and to invest to produce that result.
One thing all sound people have in common is that we are never satisfied. (Yes, I know the same applies to cameras and editing!) All we can really hope to do is to get ever closer to an ideal recording. (Whatever that actually is)

Rob The picture is only there to keep the sound in sync

Z Cheema
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Joined: Nov 17 2003

A tutorial that may be of help

The Deadly Analog to DV Audio Level

http://www.digitalprosound.com/2003/02_feb/tutorials/analog_dv_levels.htm

Rob James
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Joined: Jun 26 2001

Thanks Z, much as I respect Douglas Spotted-Eagle that article poses more questions than it answers. E.g. nowhere does it indicate what excursion above 0dB analogue is permissible. If you are submitting material to many broadcasters this is pretty crucial. If the reference level is -20dBfs for 0dBu analogue then anything that goes over -12dBfs or +8dBu analogue will be rejected regardless of the fact that this is nowhere near digital clipping.

Rob The picture is only there to keep the sound in sync

GrahamC
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Joined: Feb 8 2008

Rob said "I am really pleased to find someone here who is clearly interested and concerned to produce the best possible professional audio results"

Yeah well to my mind poor audio is equally as bad a poor lighting. It completely "flattens" the richness of the media. I can never forget the otherwise excellent film "Local Hero" directed by Bill Forsyth. It was such a good piece of closely studied drama but very nearly ruined by a fuzzy and indistinct sound track.