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Audio Levels: my camera vs FCP

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Piper
Audio Levels: my camera vs FCP
on Nov 13, 2006 at 2:20:43 am

I have a JVC GY-HD100, HDV 720p camera (records the mpeg audio), my favorite mic is a Sanken CS-1, and I run it through a Sound Devices MixPre mixer. I have Ty's "Audio Boot Camp" book (a worthwhile purchase by the way), and have searched this forum and the Final Cut Pro forum extensively about audio levels.

I don't know if this can be answered simply, but the gist of my question revolves around the fact that my camera's default setting for reference tone is -20db (although -12db is available), and FCP's default is -12db (although -20db is available). FCP's manual says only use -20db for high end equipment or high end work like feature films, and some FCP users, including Jerry Hoffman over on the FCP forum, seems to say -12db works just fine if not better. Other info I've read says to use -20db on digital cameras (like Digital Betacam or HDCAM); that also supposedly leaves more head room for peaks. Although -12db for recording is said to leave less headroom, it brings the average level farther above the "noise floor". I've tried both on the camera, and cannot discern any difference in noise. Of course, my 63 year old ears, after playing in a Rock and Roll band and then a bagpipe band may have some effect on that.

Should the FCP setting be the same as the camera? And if there isn't a clear choice of which setting to use, can anyone suggest a testing senario I might try do determine the best settings for each?

Just to further complicate the matter, the camera has an additional menu choice for mic input of -50db (default) or -60db. Not sure what that is.

Any help would be appreciated.

Thanks,

Gary Morris McBeath
SaltAire Cinema Productions


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Will Salley
Re: Audio Levels: my camera vs FCP
on Nov 13, 2006 at 7:17:27 am

[Piper] "my camera's default setting for reference tone is -20db"

I assume this is for the camera's internal tone generator. For external reference tone, the camera doesn't care. It helps if the type of metering (peak or RMS) match that of the mixer. What does matter is that the reference tone levels set ON THE MIXER match the relative levels on the camera. The same level should then display in FCP. If the mixer, in your case a fine analog device, reads at 0dB with 1k tone, the camera's digital meter should be set to either -12 or -20 (I use -20, but just keep it consistent and make good notes of it). Again, that level should be displayed by Final Cut's meters as well.

System Info - G5/Dual 2 - 10.4.8 - QT v7.1.3 - 8GB ram - Radeon 9800Pro - External SATA Raid - Decklink Extreme - Wacom 6x8



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Proper Modulation
Re: Audio Levels: my camera vs FCP
on Nov 13, 2006 at 4:17:12 pm

This post was really confusing.

Reference tone has nothing to do with headroom and peaks.

It's merely a way to make sure that the source material you recorded starts at a figurative zero on your destination.

FCP's tone has nothing to do with a camera's tone.

Just do this..

Record -20db tone on your camera.

Play footage through FCP. If the -20 tone you recorded on the camera doesn't show up as -20 on the FCP meters, it means you have to bring your faders up or down until they match. That's really it.

I'm really wondering what that dude you quoted meant when he said one level of tone was beter than another. That makes no sense.



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Piper
Re: Audio Levels: my camera vs FCP
on Nov 13, 2006 at 6:40:16 pm

OK, now I think I get it. The reference tone is just that, a reference tone, not necessarily where the 0 analog tone on the mixer will record; that is up to me to set, both in the camera and then in FCP.

My post is probably confusing, since I have been confused (most of my life actually!), and didn't really know how to ask the question.

I appreciate the time you took to sort through it and provide me with an answer. Thank you.

Gary



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Peter Groom
Re: Audio Levels: my camera vs FCP
on Nov 15, 2006 at 1:54:45 pm

It really doesnt matter what the reference tone is at, although the relationship between tone and peak audio must be defined.
In the UK the norm is -18 tone = PPM 4. Peak is at ppm6 (ie +8db above ref)
However cameras dont usually have a marking for -18 on their meters so sound recordists will line up to -20 =ppm4 and studios will deliver for TX at -18 ppm4 with audio at ppm6 max.

This way you never go near clipping which is very nasty indeed.

Peter
Dubbing mixer


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Ty Ford
Re: Audio Levels: my camera vs FCP
on Nov 15, 2006 at 7:37:50 pm

To clarify the importance of reference tone.

It's there so the person operating the mixer knows how loud the peaks can be.

As I explain in my little book.

The Sound Devices 442 mixer has a default tone level of 0 dB. When you send that from the mixer to the camera, with a digital camera, you set the record level at the camera to - 20 dB.

For analog cameras, you set the tone level lower, depending on the camera, usually around -15 dB.

Now your mixer meters are calibrated to the camera meters.

BIG WATCH OUT!!!!!.....MAKE SURE THE OUTPUT OF THE MIXER (LINE OR MIC) IS ALSO SET TEH SAME WAY ON THE CAMERA. MOST CAMERAS WILL LET YOU SEND LINE LEVEL FROM THE MIXER TO THEIR INPUTS, EVEN WHEN THEY ARE SET TO MIC AND NOT LINE!!!!!!!

You'll want your mixer meters to indicate peak reading. It's the peaks that kill. NOW, ANOTHER MESAGE FROM MY BOOK ---- WHEN DO YOU LISTEN? ALWAYS.

From the number of posts from people with audio problems, I know that NOt listening during shooting is a major contributor to screwed up audio.

"Well, it sounded ok before we started to record........:)", is not an appropriate excuse. :(

you can send line level to a mic level input and hear audio, but when the action really starts and the mic is put in the rigbht spot.....SPLAT....now you're distoring.

And by listening, I mean listening to the input of the camera, not just the output of the mixer.

Audio-camera interconnect cables have two balanced cables for audio and an unbalanced stereo return that plugs into the camera's headphone output on one end and plugs into the mixer return jack on the other end. There's a switch on the mixer that lets you toggle between hearing the mixer output and the return from the camera. You won't know what things sound like unlke unless you listen to the camera return.

Even then, if something's wrong with teh record head, you can still end up with crappy audio.

Regards,

Ty Ford

BTW, using tone in a digital camera is almost useless. You want the tone from the mixer so you can calibrate the two devices so you don't have to look at the camera audio levels while recording...although that is not a bad idea.

I find that sometimes there is a discrepancy between camera and mixer peak levels even after calibration. Keeping an eye AND AN EAR is very, very important.

Regards,

Ty


Ty Ford's "Audio Bootcamp Field Guide" was written for video people who want better audio. Find out more at http://www.tyford.com


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Peter Perry
Re: Audio Levels: my camera vs FCP
on Nov 15, 2006 at 8:49:18 pm

This is really an excellent thread. This topic causes a lot of confusion, and I hope everyone who doesn't have a firm grasp of these concepts reads this thread.
What is being addressed only indirectly in this discussion is the differences in metering.
Analog equipment typically use VU meters... which show an average value of the signal and are woefully inadequate in showing peak levels... because in the analog world 0 dBVU is close enough, because analog recording devices are more forgiving when it comes to levels. In fact, engineers recording on analog tape typically let the levels exceed 0dBVU, especially on electric guitars, because analog tape saturation can be pleasing to the ear.
Not so with digital equipment. Any signal in the red AT ALL rrepresents digital distortion, and digital distortion is ugly sounding, and not at all desirable. Thus, digital equipment typically use PPM meters, or peak meters, which are much better at reflecting what is really happening with the signal. On peak meters, 0dBFS, or full scale, is the MAX signal before distortion occurs.
So, let's say you connect a mixer that has VU meters to a camera that also has VU meters. I would set the 0dBVU tone on the mixer, and I would likewise set the tone at 0dB on the camera, as long as you have the level match of line level out to line level in, or mic level out to mic level in, as Ty stated.
The confusion comes when you connect, say, an analog mixer with VU meters, to a digital camera with peak meters. If I am not mistaken, and guys, correct me if I am wrong, the SD 442 has VU meters. So 0dBVU is sent from the mixer to the camera, where the signal is set at -20dBFS. The -20dBFS came about simply through trial and error as the best level to set the the record device at so that the person watching the levels hit 0dB on the mixer can be reasonably sure that the signal is not hitting the red on the record device while still getting plenty of level.
I think it is essential to talk about the differences in metering and what the numbers mean to get a better understanding of what is going on.
Again, great thread. This is why the COW exists.
Peter


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Ty Ford
Re: Audio Levels: my camera vs FCP
on Nov 15, 2006 at 10:54:41 pm

Thanks Peter,

All Sound Devices mixer meters are SWITCHABLE to read VU (average/RMS) and Peak at the same time. This is a WONDERFUL feature. You see the RMS as a more continuous line of lit LEDs, some unlit LEDs and then lit Peak LEDs. I ALWAYS set my Sound Devices 442 meters to read RMS AND PEAK simultaneously.

I forgot to do that once and set tone at -20 dB, while viewing RMS only. Setting tone that way was no problem. When we started to shoot, I was feaking out because the levels were WAY LOW. Then I realized didn't see any space between the RMS and PEAKs, so I wasn't in RMS AND PEAK metering mode, I was just in RMS mode. I switched to RMS AND PEAK and BANG! There were my PEAK levels....actually a bit higher than I usually run them because I had cranked everything up a bit because the levels looked so low. Fortunately, the limiter was containing all of the PEAKS and the audio was fine, just a slightly thicker than usual.

FTR, Calibration tone has no peaks like the human voice or other sounds. It's a continuous sine wave. As discussed in my book, when you set it to -20 (on a digital camcorder) you have from 0 dB all the way to +20 dB for safe audio. The nice thing about the Sound Devices mixers is that they have very good limiters. I set my limiter threshold at +16 dB. Any audio that exceeds +16 dB is caught by the limiter. The difference between +16 dB and + 20 dB is 4 dB, that's a little extra headroom; my safe margin. I need that because sounds like hand claps or a three-ring binder clacking opening have very quick, loud sounds called transients. They will go through the limiter so quickly that they can't be caught. That extra 4 dB (between +16 and + 20) is where these peaks end up, and they are still safe.

BTW, the more you look at the combined RMS and PEAK metering, the more you find that the relationship between RMS and PEAK of people's voices are all over the road. Sometimes the difference between RMS and PEAK is pretty small, other times it's quite large.

Regards,

Ty Ford


Ty Ford's "Audio Bootcamp Field Guide" was written for video people who want better audio. Find out more at http://www.tyford.com


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Piper
Re: Audio Levels: my camera vs FCP
on Nov 16, 2006 at 12:24:26 am

Ah, a whole lot more great information. Thanks again to all.

My mixer is not the 442, but the smaller MixPre. Its meters, a group of 7 led's, are peak only. The thresholds, however, can be set with a set of recessed pots on the left side, but the specs say the limit are from +6 to +18. I'll experment with that.

Just some further info I've found relative to my camera. Its meters are somewhat crude visually (a small row of square dots on in the viewfinder that turn yellow), and setting the camera's ref tone to -20db places a small indicater dot over that portion of the camera's meter. (Doing that also lowers the recording level on the camera from the -12db setting; part of the reason for my confusing reference tone with recording level).

Then, when I send the mixer's tone to it, I found the camera's faders can be set anywhere from 3.5 to 7 (on a scale of 0 to 10) and still leave the camera meter at -20db indicated.

So I recorded a test with all the settings, captured into FCP, and found the camera fader reading that brought the level into FCP at -20db on the FCP meters. I don't know if this is exactly -20db, but it is likely close, and at least now I have a good reference point for consistency from production through post. Peaks also fall in about the same spot in FCP as they did on the MixPre.

Again, my thanks to all of you, and especially to Ty for his great book.

Gary


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Ty Ford
Re: Audio Levels: my camera vs FCP
on Nov 16, 2006 at 1:36:27 am

Thanks Gary, and DAMN, yes the MixPre only has PEAK meters. My bad saying they have RMS/PEAK, but the important thing is to have those peaks. The tone from the MixPre, of course, is still an RMS sort of signal with no PEAKS. Technically, you could argue that the top of the tone sinewave is a PEAK. Yes, sigh, it is, but the RMS to PEAK RATIO is 1:1.

I'd rather be dragged through fire naked than get people lost by over discussing this, but RMS to PEAK ratio is simply the difference in height between the RMS level and the PEAK level. An example of a BIG RMS to PEAK ratio would be clapping your hands in front of the mic while you had a noisy HVAC running in the background. The handclaps would show the PEAKS while the more continuous HVAC would have more of a constant (RMS) level.

OK. Enough. I HATE the audio metering on a lot of the MiniDV and HDV cameras. My XL2 has dots on the eyepiece screen. I have no idea what they mean. However, of the side metering (on the body) there is actually a meter that does have a -20 blip. Hooray!

If you have a good mixer that has an output limiter, here's another way to set your camera input levels. Set tone from the mixer for what you think is -20. Talk into the mic very closely at a level louder than you expect to record while you observe the camera and mixer levels. See where the PEAKS hit on the mixer and camera meters. With the limiter engaged, adjust the camera levels upward to about 3 to 6 dB below 0 VU (for digital camcorders). This tests the limits of the mixer limiter for everything but very sharp, quick sounds.

As you shoot, adjust the inputs to peak the audio at or just below the limiter thresholds. Occasionally the level will go over and you will engage the limiter, that's OK.

Regards,

Ty Ford



Ty Ford's "Audio Bootcamp Field Guide" was written for video people who want better audio. Find out more at http://www.tyford.com


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