Solving Sound problem with AGC

[Chris Barcellos]

So in the last film I shot, I learned the AGC lesson. It makes recording decent sound to the camera, next to impossible.

Not one to leave things alone, I recalled the Beachtek coming solution, and one that Dan Chung talked about using a 1K tone fed into the camera. I tried that, and Dan was right, but the problem was when you were monitoring through a mixer, the tone masked the voice. So I realized I needed an inaudible signal.

Last night I decided to try my hand at it. I went into my copy of Audacity ( a free sound program on the net ) and found that it included a tone generator. Since Beachtek is apparently using a high frequency inaudible sound, I started making tones, until I found a point where it was inaudible. I settled on a 15,400 setting. And then I made it 15 minutes long. Next I rendered it to an mp3 file. I then loaded the file to my IRiver mp3 player, and hooked up the earphones out to the auxillary imput on my old Beachtek DXA-4., and started the player playing the tone. I ran my Sennheiser mic into the other channel, an set the imput at about two clicks below top.

I recorded voice... Result, no noise during the silent periods. AGC was shut down.

I subsequently ran the same thing through my Eng 44, plugging the mp3 player into the auxillary port where the inaudible track ran in both channels, and then recorded to one of the other imputs using the Sennheiser. A winner again.

I also found by adjusting the volume on the mp3 output, you will affect the level of gains setting in the camera. So the volume control in effect can act to raise and lower your levels.

Give it a try, you will be pleasantly surprised.

[Silton Buendia]

Great info thanks for posting this!

Originally Posted by Chris Barcellos

So in the last film I shot, I learned the AGC lesson. It makes recording decent sound to the camera, next to impossible.

Not one to leave things alone, I recalled the Beachtek coming solution, and one that Dan Chung talked about using a 1K tone fed into the camera. I tried that, and Dan was right, but the problem was when you were monitoring through a mixer, the tone masked the voice. So I realized I needed an inaudible signal.

Last night I decided to try my hand at it. I went into my copy of Audacity ( a free sound program on the net ) and found that it included a tone generator. Since Beachtek is apparently using a high frequency inaudible sound, I started making tones, until I found a point where it was inaudible. I settled on a 15,400 setting. And then I made it 15 minutes long. Next I rendered it to an mp3 file. I then loaded the file to my IRiver mp3 player, and hooked up the earphones out to the auxillary imput on my old Beachtek DXA-4., and started the player playing the tone. I ran my Sennheiser mic into the other channel, an set the imput at about two clicks below top.

I recorded voice... Result, no noise during the silent periods. AGC was shut down.

I subsequently ran the same thing through my Eng 44, plugging the mp3 player into the auxillary port where the inaudible track ran in both channels, and then recorded to one of the other imputs using the Sennheiser. A winner again.

I also found by adjusting the volume on the mp3 output, you will affect the level of gains setting in the camera. So the volume control in effect can act to raise and lower your levels.

Give it a try, you will be pleasantly surprised.

[Nandan Rao]

You already said "no noise"...but I want to grill you for more information regarding the quality. Beachtek keeps saying their new DXA-5D will "greatly reduce the hiss" from the camera, which makes me nervous because Im planning on buying one as soon as it comes out, but I really need something that eliminates the hiss, not "greatly reduces it"...so my question for you: "is there ANY audible hiss? Did the tone eliminate the hiss completely?"

Thanks for doing this by the way, REALLY appreciate the effort and ingenuity on your part.

Nandan

[Chris Barcellos]

Hi guys:

Well I just did a little testing, enough to satisfy me that with a little bit of work, excellent sound will be produced. Here is a wma file with those first two tests.

The first part is throught the XlR adapter only, without mixer. At first you will hear the camera straining to raise audio gain, to pick up the TV in the background, then I plug the mp3 player in, and the AGC shuts down.

In the second part, I am actually running through the Eng44 mixer, where it seems to work better, as I can control more things. I know a little work will get exact right settings.

Take a listen.

[Ray Bell]

Chris... good test and great results... thanks for the post..

[Christopher Witz]

so would it be possible to create a very small, wafer battery powered, in-line tone generator? for one channel of course.

Hold the phone.... maybe the new ipod shuffle would do the trick! Small, cheap, sexy.... kind of like Madonna but younger!

[Jon Fairhurst]

Originally Posted by Nandan Rao

...but I really need something that eliminates the hiss, not "greatly reduces it"...so my question for you: "is there ANY audible hiss? Did the tone eliminate the hiss completely?"

The key to eliminating hiss is to use a great preamp/mixer. That's where the high gain is. After that, feeding a relatively high-gain signal to the camera (as well as the pilot tone to overcome AGC) should minimize hiss.

I haven't tested it yet, so I can't speak to how low the hiss can be.

The preamp/mixer will also give you a headphone output, which is important for controlling levels. (It's not as good as monitoring from the recording device, but better than nothing.)

Even if hiss were completely eliminated (it won't be, but should be low), you will often have other unwanted sounds, such as heating, ventilation, and air conditioning (HVAC). So regardless, you will want to have noise reduction in your toolkit.

On the PC, I've found that GoldWave's NR works well. There's a free demo with something like 50 uses for free, and last I checked, the cost was only $50. You just copy a portion of the clip with noise-only, and apply it to the rest of your clip.

If the hiss is low and you want it to be zeroed out, NR will usually do the trick.

[Ray Bell]

Originally Posted by Christopher Witz

so would it be possible to create a very small, wafer battery powered, in-line tone generator? for one channel of course.

Hold the phone.... maybe the new ipod shuffle would do the trick! Small, cheap, sexy.... kind of like Madonna but younger!

Yes, good idea.... of course the Iphone would work just as well...

[Christopher Witz]

Originally Posted by Ray Bell

Yes, good idea.... of course the Iphone would work just as well...

but much bigger.

For what it's worth.... I've been shooting a huge testimonial project and using 2 5d2's with rode videomics on them.... so far it's worked out very well....

[Chris Barcellos]

Last night I tried a y adapter at the mic input to see what would happen trying to add the signal input along side the a mic input. I had a Sony stereo mic in one of the stereo inputs, and the Iriver in the other. It did not work. Its possible my connector was not proper for the camera, but it sounded like one signal was interfering with the other, as you would expect. I have no doubt that if I actually split the channels, mic into one channel, and IRiver into other, that the result would be different. I will stop by radio shack today to see if I can locate that adapter.

I am coming to the conclusion that for editing reasons, it is better to keep the tone track separate. In post, in Vegas, I can eliminate one track, and double the other to both sides, and have a wave form that actually shows the peaks and valleys of the voice track. When the two are combined via mixer and/or XLR adapter, you have a solid block along most of the track, with occasional peak over from the voice track.

[Guy McLoughlin]

Originally Posted by Chris Barcellos

Here is a wma file with those first two tests.

Hi Chris,

Just to see what the 5D MK II audio would be like once processed, I took your audio file into Sony Sound Forge 9 and did the following:

1- Split the audio tracks and deleted the tone track.

2- Manually adjusted the volume of the voice track.

3- EQ'd the voice track to eliminate some of the high end ( -2dB at 9Khz )

4- EQ'd the voice track to boost some of the lower range ( +4.5dB at 200Hz )

5- EQ'd the voice track to eminate any low end rumble. ( - Infinite slope starting at 80 Hz )

6- Used the Sony noise reduction plug-in to bring down the background noise by -6dB.

7- Used the Sony WaveHammer plug-in to compress the dynamic range by a factor of 2:1 at -16dB.

8- Exported the voice track as a WMA file.

...This is the quick and dirty method of processing audio that I use for work. To me, the processed audio track from the 5D MKII sounds not bad at all.

- Guy

[Chris Barcellos]

Guy:

Thanks for the work on that. I will review the process you employed tonight on some other material I have.

[Chris Barcellos]

Guy, had an interesting thing occur tonight, and with my minimal sound experience, I can't figure it out.

So I have the mono setting engaged on my XLR adapter, so every is going to both tracks.

Not a big deal, since I can't hear the tone anyway. So I convert my files to Cineform, open Vegas, and drop them on the line. Play on the line, sound is still clear of any audible tone. I edit, and render to the Cineform 4.xxx codec, and play the resulting file, there is a continuous high tone being heard.

So then I render the sound to a wave file, and it is free of tone, great, I drop it on the track, turn off the original sound track, and render again to same Cineform file. More tone in the final render.

Of course, I will render to separate track to eleviate any issue of the tone in the voice track, but it makes an interesting problem.

Got any ideas ?

[Chris Barcellos]

Found a resolution to issue in last post. I had the settings at 48000 hz in audio in the HD template I was using. When I dropped it to 44,000 herz, no more tone.. My sound guy thought it could be a harmonics issue... So maybe push the tone track up a little more to 17,000 area..

[Guy McLoughlin]

Hi Chris,

Personally, I would stick with sending the tone track to one channel, and then removing the tone track when editing. Unless you can be asolutely sure of removing the tone frequency completely from the finished audio ( say with a narrow band filter ), I would be concerned about it effecting things somewhere down the road.

Another option that you might consider is to add a high quality digital audio recorder as your main audio source, and use the 5DMKII audio simply as a sync track. I use a Sony PCM-D50 recorder for much of the video I shoot. It's an astounding recorder, even when just using the built-in mics, and it will run for 10+ hours on one set of 4 AA batteries.

Sony PCM-D50 ( about $450 from B&H Photo )
Sony | PCM-D50 - Professional Portable Stereo Digital | PCM-D50