AGC Disable - Magic Lantern vs. Juicedlink?

[Michael Clark]

Right now I've filming with a T2i using Magic Lantern. I have a Juicedlink cx231 (doesn't have AGC disable). Problem is, if you mount the CX231 to the bottom of the camera, it gets in the way of removing a battery on the T2i. So Juicedlink has recommended I buy their DT454 ($100 more, does have AGC disable) to replace my CX231, as the DT454 was built with DSLR cameras in mind. The main plus would be that I could exchange camera batteries without unscrewing the JL with an allen wrench. For the price increase though, I'm looking for what other upgrades it would have. I know it has levels which is a plus. But my question is, is there any difference in the AGC disable between what a mixer and ML would provide?

[Chris Barcellos]

Juiced Link AGC disabler will sacrafice one of the stereo track to which a signal is transmitted that effectively shuts down the AGC. Whe the 5D first came out, I did the same thing with an MP3 player and an inaudible tone.

However, Magic Lanter still gives you better control over sound, by providing adjustments to gain in both digital and analogue, which affords you better control over noise. I use the Juiced Link CX 231 and love its preamp capablity.

The battery change issue wasn't as much a problem on the 5D because the batteries lasted longer. The battery run on the T2i is much shorter. I have built little contrapations to avoid the issue over the years, trying to offset the XLR adapter. When I mount on rails, I can mount the adapter under the rails with effect. But mostly, I just put up with the inconvenience.

[Jon Fairhurst]

Hi Michael,

Chris is right. ML gives better control (analog and digital) than other methods. I've done tests on the 5D2, and ML gives better S/N than Canon's manual audio, and manual gives better S/N than AGC distable methods.

That said, I have the DT454 and when recording audio in a noisy environment, such as the NAB show floor, I use manual mode and monitor from the preamp, rather than from the camera. It's simpler than needing to put ML on the cards, start it up, pop the battery, etc. However, for narrative productions, I definitely choose ML (or separate sound recording) over AGC or Canon manual mode. The bottom line is that ML isn't always needed and it's simpler when you can avoid it. But it sure is clean!

Chris also makes a good point about mounting to rails. I mounted my DT454 on the chest rod of a Redrock "The Event" rig. It helped keep the weight balanced and didn't affect the camera height that way.

I see two good approaches. You could get a rails setup and stick with your existing preamp. The Redrock DSLR baseplate, mini tripod mount, and a couple of rails is a nice starter kit. It's not as rock solid as the cheeseplate-based tripod platform, but you can build it out into a quick-release handheld rig.

Redrock Micro - DSLR/HDSLR Base Plate
Redrock Micro - DSLR Mini Tripod Platform
Redrock Micro - DSLR Tripod Platform
Redrock Micro - microMount

On the other hand, going with the DT454 has advantages if you shoot documentaries or casual video. Being able to just turn on the camera and preamp and record without ML is a very nice option when shooting solo. The highest possible signal to noise isn't that important when there is a lot of ambient noise.

So... rails if you want battery access, might want to upgrade to a follow focus and/or handheld rig, and will always run ML. Or DT454 if you want battery access and the simplicity of avoiding ML for certain projects.

[Tom Morrow]

Also consider the sound devices mixers. The new mixpre-d can mount under the camera like the juicedlink... not sure about batteries. I have an old mixpre without that mounting method, and today I finally figured out quick and dirty way of mounting it. I had laying around one of those flash brackets designed for wedding photogs to get the flash above the eyes for verticals. I unscrewed the flash shoe from that, and gaffer-taped the mixpre onto that arm that normally holds the flash.

Gaffer tape actually is almost perfect for this purpose... lots of surface area on the mixpre, and no need to drill any holes for mounting.

When I'm shooting tripod-mounted, the mixpre lives in a sound bag below the camera or on my person, so I can adjust the audio on the fly.

[Tom Morrow]

This thread is good for me in terms of realizing the grass is always greener on the other side. I had been lusting after the real-time on-camera metering of magic lantern, and wanting to ditch the bulk of the mixpre. But after reading about the limitations of magic lantern (no third party battery or AC adapter, needs to be on card, needs to be enabled after boot, not completely reliable) I'm getting cured of my magic lantern envy. Strange how sometimes software solutions can be the most unwieldly.

For sheer convenience I think the Rode videomic pro would win. The 20db gain lets you avoid DSLR preamps, and so the only hassle needing to adjust levels on the dslr itself, not live because canons only let you adjust the levels from the menu. You can't shoot or view the subject while you see or adjust level.

[Jon Fairhurst]

The VMP is nice when an on-camera mic is adequate and when there is no wind. It's a good, fair weather, travel mic.

It's no narrative mic though. For narrative, you want to get the mic as close as possible to the talent - and that means you need XLR connections. You also want a straight mic that fits in a blimp. (According to some, the switches on the back of the VMP make it prone to wind noise.)

You still want to disable AGC. There are three options: Magic Lantern, Canon's manual audio control, or the tone/noise method of products like the JL DT454. Of the three, Magic Lantern is the lowest noise solution.

If I were covering a riot or natural disaster solo on foot, a VMP would be my #1 choice. It's light, simple, and would capture the ambient sounds. And I'd want to tape up the back of it and encase it in a furry windscreen. But for narrative, it would never touch my camera.

As always, the right solution depends on the project at hand.

[Tom Morrow]

I'm curious about the thinking that Magic Lantern is the lowest noise solution. I don't really understand why Magic Lantern would get you any stronger signal or lower noise floor than say my external Sound Devices Mixpre. (or a Juicedlink, assuming they have clean preamps)

In both cases you want to turn the camera preamp down all the way. But magic lantern doesn't have any other preamps, so you can't boost a shotgun microphone signal to get good signals like you can with an external high quality preamp. I understand that magic lantern has digital amplification, but you can also do digital amplifcation just as well if not better in post-production if needed, and it's better in post-production because you don't have to worry about clipping.

-Tom

[Tom Morrow]

One thing I want to mention for those who aren't aware is that newer generations of the Canon cameras don't need AGC disable because they have AGC on/off in the menu system of the camera itself. If you have a t2i that needs AGC disable, you might be able to sell it and buy a t3i which has AGC on the menu.

[Chris Barcellos]

Originally Posted by Tom Morrow

I'm curious about the thinking that Magic Lantern is the lowest noise solution. I don't really understand why Magic Lantern would get you any stronger signal or lower noise floor than say my external Sound Devices Mixpre. (or a Juicedlink, assuming they have clean preamps)

In both cases you want to turn the camera preamp down all the way. But magic lantern doesn't have any other preamps, so you can't boost a shotgun microphone signal to get good signals like you can with an external high quality preamp. I understand that magic lantern has digital amplification, but you can also do digital amplifcation just as well if not better in post-production if needed, and it's better in post-production because you don't have to worry about clipping.

-Tom

Tom:

Jon can explain better, but point of ML adjustments is you can adjust your digital and analog gains as fits you needs best, while the Canon adjustments do not allow that. The thinking was that it is best to lower digital gain as much as possible, and then bring down analog to meet the input coming into camera.

I was able to use my Sign ENG 44 mixer quite nicely, feeding my 5D. I could pretty much reduce gain setting to 0 on Digital, and under 10 on the analog, if I recall properly. With my CX231 I tried to keep Analog at 10, but had often to go up to 17, sometime adding a little digital gain too.

These days I shoot double system mostly, so I haven't used it this way for about a year. So I could be off on current thinking.

[Jon Fairhurst]

Chris is right. I've done the measurements myself. ML is quieter than the Canon firmware, which is quieter than using a signal to disable AGC - at least with the 5D2 and JL DT454.

In all cases, I used a juicedLink preamp to provide a strong clean signal to the camera.

With Magic Lantern, you can control the analog and digital gains separately. With the Canon firmware, the analog gain is fixed at +31dB. The analog preamp in the camera is noisy, so having the analog gain that high amplifies the noise. The gain control in the Canon firmware controls the camera's digital gain only.

With the tone/noise method of killing ACG, you are also stuck with +31dB of analog gain in the camera. As I recall, the AGC killer doesn't knock the camera's digital gain down as far as the Canon firmware can, so you get about 6dB more noise this way.

In fact, I found that with a JL preamp and ML firmware, the S/N was lower than that of the H4n or DR-100. That said, recording into the camera always has a high pass filter engaged, so it's fine for dialog, but not so good for full range music. Also, the JL/ML/camera audio sounds slightly more harsh to my ears than the external recorders at high signal levels.

From a signal to noise standpoint, recording with a JL preamp and Magic Lantern delivers as little noise as you can get from a 16-bit recording. None of the other solutions that I measured (H4n, DR-100, JL+Canon firmware, JL+AGC killer) deliver as much as 16-bits worth of signal to noise quality.

So, if you want clean, whispered dialog, the JL/ML solution wins. For recording full range music, the external recorders win. If you want higher quality, you need to budget for the Fostex FR-2LE or above...

[Tom Morrow]

> With Magic Lantern, you can control the analog and digital gains separately.
> With the Canon firmware, the analog gain is fixed at +31dB.
> The analog preamp in the camera is noisy, so having the analog gain that
> high amplifies the noise. The gain control in the Canon firmware controls
> the camera's digital gain only.

Are you sure that the gain control in my 60d's menu controls digital rather than analog gain? I was under the impression from numerous other sources that lowering the Canon's audio menu gain would lower the ANALOG gain so as to turn down the noisy preamps. And I have recorded using this approach audio that sounds very noise-free and artifact-free (via SD mixpre into 60d), so I doubt that the audio gain was fixed at the highest noisiest value of +31db.

Sorry for asking so directly, but I'm really curious Jon if you have a reference or other proof that the Canon firmware doesn't control the Canon analog gain.

[Jon Fairhurst]

I don't know about the 60D, but on the 5D2, Tramm was able to read the registers to see what the Canon firmware settings were. He sent the register information to me, and I confirmed it.

Also, the analog gain can only be controlled in large steps. To adjust the gain properly, one would need to play the two controls off of each other. For instance, to go up, one would raise the digital gain to a point, then jump down as you boost the analog gain. Raise the digital gain again, then drop as you take the analog gain to the next step etc. I did gain measurements tick by tick and it was clear that this was not happening.

The final clue is that the overall noise is lower with ML than with the Canon firmware set to one tick above minimum.

So, between reading the registers, tracking the gain profile, and reading the noise, I'm pretty confident that the Canon firmware adjust the digital gain only.

The upsides are that it would have been simple to write, and it provides a monotonic, predictable gain curve from camera to camera. The downside is that the noise level isn't as low as it could have been, but it sure beats AGC. :)

[Tom Morrow]

I sent a PM to Jon asking for the results of that test... I have an oscilloscope for measuring noise floors, but it's always easier to see what others have done.

If it really is true that the Canon firmware doesn't adjust the noisy preamps, then a lot of Juicedlink's marketing seems misleading, where they talk about bypassing the noisy preamps in the camera with their clean preamps.

If the preamp noise level in Canon cameras really is fixed then that does free me up to do some things I wouldn't otherwise do, like using my lighter h1 for preamp, level adjustment, and metering rather than bothering with the cleaner-preamp but bigger bulkier SD mixpre.

[Jon Fairhurst]

I'll have to search out my old notebooks to find the test data. I ran those tests over a year ago.

One thing about the digital gain in the AK4242, it seems to be able to go well over unity. That means that you can turn the digital gain down and still go full scale. It's tied in with the AGC circuit. When the chip's digital AGC lowers the gain, you still get full headroom, otherwise you would get lower gain, but still clip.

Anyway, when you turn down the gain in this manner it does lower the noise as well as the signal. And by cranking up the gain in an external preamp like the JL, you still get a full range result.

This is unlike adjusting down the digital gain in your NLE. In that case, you reduce the headroom and anything that already clipped still clips at a lower level.

Still, the most compelling evidence is that when Tramm read back the data from the chip, he found that it's programmed for +31dB of gain - even when the camera is set to manual gain, one tick above the minimum. The analog control (m-gain) has large coarse steps only. With ML, I set it at +10dB when I can and +17dB when the signal is quiet. There are no steps in between.

[Tom Morrow]

Thanks Jon. I don't necessarily need all your test data, but a few data points such as what the gain-bands are, along with the procedure used to get that data, would be very enlightening.

This is unlike adjusting down the digital gain in your NLE. In that case, you reduce the headroom and anything that already clipped still clips at a lower level.

Adjusting digital gain down in an NLE doesn't reduce headroom it increases it. If you take a signal that peaks at -12dBFS and adjust the digital gain down so it peaks at -20dBFS, you now have more headroom above that peak (20>12). So I'm not clear on what the point you were making was about over-unity gains. Please explain further if it was important.

Still, the most compelling evidence is that when Tramm read back the data from the chip, he found that it's programmed for +31dB of gain - even when the camera is set to manual gain, one tick above the minimum.

Well hopefully the analog gain could be done off-chip before it gets to that chip. Perhaps this is what you are talking about when you write:

The analog control (m-gain) has large coarse steps only. With ML, I set it at +10dB when I can and +17dB when the signal is quiet. There are no steps in between.

Aha! That makes lots of sense... so the Canon firmware DOES adjust analog gain, just not with as much resolution as would be desired. I've heard people suggest using a few clicks from zero as the Canon gain setting, and perhaps the few-clicks are related to this banding that you talk about.

So I suppose what I'm interested in is what the bands are, how wide or narrow. If a simple production gain strategy like you propose (always +10db for loud, +17db for quiet) can preserve headroom/noisefloor well, that would be certainly worth pursuing over unwieldly outboard solutions and riding the gain. In fact such a strategy could be implemented with the native non-ML firmware just as easily I presume (with digital gain adjusted in post, in the NLE)