Need to identify this hiss
 

Hello all - I'm looking to identify this source of this hiss, and hope that someone here can help!

The tools:

Canon 550D
Sennheiser MKH416P48
BeachTek DXA-6VU

The audio:

http://www.mathmissionfilms.com/audiohiss.mp3

The problem:

As you'll hear, there's a very distinct hiss that happens each time I record video using this setup. I am nearly positive that the problem lies with the BeachTek box, but as I do not own any other recording devices that can phantom power my mic, I have no way to test to make sure. The box has a ground switch on it, but on either setting, the hiss remains. If I adjust the gain, the hiss remains. Even when not recording, I can hear the hiss via headphones. I've had to use Soundbooth to isolate the hiss, and remove it, but as one would guess, that does terrible things to audio in different environments. I've read and used a Zoon H4N with a friend (his tech), but haven't established if it has the hiss as well; dreading that it's a problem with the mic, and not the box. Anyone with thoughts?

So it seems easy enough to do the differential diagnosis experiment where you record "silence" with just the camera, and then with the BeachTek box plugged into the camera (with any controls in minimum,and then maximum position, etc.) And then with the mic plugged in but buried in the middle of your bed inside a big pile of all the blankets and pillows you have in the place (to make something as close to acoustic "silence" as possible.)

Seems unlikely that anyone can diagnose the problem with a simple, compressed data sample with no variables to compare. Remember also that camcorders in general, and DSLRs in particular, were NOT designed to be decent audio recording devices.

When I get home this evening, I'll go through those steps and repost the audio. In the meantime, I found a single sheet of paper "manual" that came with the BeachTek box. Under its 10 points of interest, number 7 and 8 are the most interesting with respect to the noise:

7) Ground Lift Switch - Set the G1/G2 switch on the DXA-6vu to the position that gives you the least amount of noise. This switch allows the input and output grounds to be isolated to prevent ground noise on some Sony camcorders.

8) XLR Inputs - The two XLR inputs can accept balanced or unbalanced connections. To convert the input to unbalanced, simply ground pin 1 to 2 on the XLR cable. We recommend the use of sensitive condenser type microphones for the best results. Dynamic micropphones usually do not have sufficient output levels and may result in an excessive amount o fhiss from the high level of amplification necessary by the camera's audio preamplifiers. Microphones with a sensitivity rating of greater than -40 dBV are ideal.

Not sure if that helps at all...

The 416 is the type recommended and not noted for being noisy so barring a malfunction I'd expect the hiss isn't coming from there. The Beach is not a preamplifier so switching to something such as a JuicedLink or a field mixer where you can add some signal boost before sending it to the camera would help. I'd suspect that what you're hearing is the result of the noisy preamplifiers in the camera that virtually all video capable DSLRs are inflicted with.

I agree with Steve. Sounds like normal noise floor for a DSLR to me. You have to realize that all DSLRs and almost all video cameras have horrendously bad audio quality in general. Any audio professional listens to them and just shakes their head, that is why using something high-end like a Sound Devices recorder will always yield a superior result to any camera audio.

I have been testing the Juicedlink DT454 with my 5D MKII and FWIW, the noise floor is not that much better than your sample posted above. DSLRs may work fine in certain situations but when audio quality is important, it would behoove you to use a high-end, quiet recorder like an Sound Devices 702, especially when you have a nice quality mic like the 416. You get what you pay for and an $800.00 camera isn't going to have good quality audio, but neither is a $100k video camera, they all sound terrible. You should hear how bad the audio quality is on the RED too.

Dan Brockett

The ground lift switch on the BeachTek is unlikely to have ANY effect on the hiss you are asking about. OTOH, if you had a HUM issue, then ground-lift switch would be indispensable. Of course, you can do the very simple experiment of flipping the switch and see if it has any effect on your hiss (I would bet that it does not.)

Now, it is possible that you have a low-output microphone and/or you are using it too far away, and thus producing a signal level that is below what would be "normal". And the BeachTek docs are warning that they can't do anything about that since it is beyond the scope of what the BeachTek adapter was made for.

To be perfectly honest, I thought your sample sounded quite good considering it was recorded on a DSLR. Your issue may simply be unrealistic expectations.

This sounds like the normal AGC (automatic gain control) issues we have all had with the Canon DSLRs, and others. If you can turn off AGC, and adjust your mic levels going in from a preamp aperatus, you can get much cleaner sound. As I recall, we don't have the capability of turning it off on the T2i. On the Canon 5D you do. With my Juiced Link CX 231, before we got control of sound in Magic Lantern on the 5D, I had constructed a tone to feed to one channel with an mp3 player, that defeated the automatic gain. Then on the other channel, I could feed my mono mic testing for level adjustement. This actually worked nicely and I could find pretty good recording zone. Of course, you were limited to one track.

I'm using a Sennheiser MKH416 P48

I've recorded sound to the DSLR, as well as a Canon HF100 and directly to my computer via the mic input in the back; all have resulted in the exact same hiss. Leads me to believe that it's not the camera, and that it's a problem with the BeachTek box. Flipping the ground switch doesn't do anything.

I don't think I'm expecting something unrealistic; unless this is an attribute of the microphone, the simple solution is to dump the box, get something like a Zoom H4N and just deal with audio that way.

Yeah, limited to one track. Again, I think an answer may just be to get a stand alone recording device, and move forward from there. At least, until the T2i has gain control; not sure if there are firmware updates coming that would address that; doubtful.

Did some more reading and found a great video that I'd like to share with owners of the T2i

550D T2i AGC Automatic Gain Control Tests

It demonstrates the the AGC is the problem and that it increases the gain based on ambient noise. When the guy starts shouting, you can hear the noise drop down for a full second before it brings itself back up. From the literature that I've read, the T2i and 7D do not allow turning off the ACG. I read about a work around very similar as what you posted, where you flood one channel with sound so that the ACG drops the gain just enough to kill the noise, and record your audio with the other channel.

I'm not comfortable with a hack to disable it; and don't have the tools to pull off a dual channel thing to drop the gain. Guess I'm just looking at an external recorder.

To reduce the hiss, you need a preamp with gain. To keep the hiss from "breathing", you need a device with AGC defeat.

The Beachtek adapter you are using has neither gain nor the AGC defeat circuit. They later came out with a unit that has an AGC killer, and now have one with AGC killer and gain.

juicedLink came at it from the other side: their preamps (the CX series) have clean gain. The new DT454 has even more available gain and two modes of AGC defeat. These amplifiers are exceptionally quiet.

I've tested the Beachtek with AGC killer, but not the one with gain, so I'm not sure how clean it is.

The BeachTek box is NOT causing the hiss. The audio path in the BeachTek is passive and does not contribute to the noise level.

I wouldn't bet on the H4N necessarily solving the problem. It isn't noted for stellar mic input preamps either.

Well, it looks like upgrading to the newer DXA-5Da may be the best bet for now then. Any thoughts on the market value of a used DXA-6VU?

While that's technically true, a passive adapter doesn't help reduce system noise, while a clean, active preamp can.

Think of it this way. Let's say you have a signal that goes through a passive, straight wire, then feeds a cheap preamp that adds 10 "pieces" of noise and then multiplies both the signal and the noise by 100. Now you have "1,000" pieces of noise, along with a full strength signal.

Now imagine a system with a super-clean preamp. It multiplies the signal by 50 right up front and in the end it only adds one "piece" of noise. Next you send it into the cheap preamp. It adds its own 10 pieces of noise (plus 1 from the preamp makes 11) , but you only need it to multiply the signal by a factor of two. In the end, you get 22 pieces of noise along with the full strength signal. 22 is a lot less than 1,000.

The bottom line is that noise in the mic and any other components through the first round of preamplification are critical. Every little bit of that up-front noise gets amplified. If you can go through a squeaky-clean preamp and apply gain right up front, you can avoid much of the noise of a cheap preamp like you'll find in the 550D.

And, actually, the audio chip in the 550D (assuming that it's the same as is in the 5D), isn't all that bad. It's just programmed that way. ;)

The DXA-5Da doesn't have gain. You'll still have noise, as I show here:


I'd recommend the DT-454 (which I've tested to be clean), or the DXA-SLR (which I haven't had a chance to try), or an external recorder. Both the DT-454 and DXA-SLR have an AGC defeat circuit as well as gain.

It more importantly doesn't have phantom power options, so it's useless to me. The DXA-SLR seems the best choice for what I'm interested in doing. The $400 price tag will keep me away for a bit, unfortunately, as I don't have the cash for it (unless I can find a buyer for my DXA-6VU to fight some of that cost).

Just to be clear, you tested the DT-454 with the T2i? I'd really be interested in hearing a clip, if you have one available. No worries if not; just interested in hearing samples before I (try to) buy. Thanks everyone for your help. You guys are wonderful.

Gain or no gain, DSLRs still record lousy sound. The 5D MKII now has the manual audio levels and it still sounds relatively crappy.

Low dynamic range, not great S/N ratio.

Dan

Dan: I am curious, have you ever tried the beta Magic Lantern with the 2.04 firmware, and a mixer or one of the Juiced Links. Setting the Magic Lantern gain at 10 and and the digital gain on each channel at 0, you get a pretty decent sound recording capability in camera.

Shooting without Magic Lantern, I found you need to still feed with outside mixer or the Juiced Link, and you should not take the manual level any more than 3 ticks above 0.

That's why I was considering the Zoom H4N. Record audio directly to that, then sync up in post. Not ideal, but it's a way around the AGC; at least, until / if Canon decides to do a firmware update.

Does the firmware work with the T2i? I hear it used for the 5D, but I've not heard anyone using it with the T2i

No, sorry, I would like it to, because I have a T2i also.

I don't have a T2i, but I've tested the DT-454 with the 5D in auto mode. I'm pretty sure that all of Canon's DvSLR audio front ends are basically the same.

I've got to run right now, but I should be able to post some examples this weekend...

Figured that was the case. So, seems to me, you guys are suggesting that the only two options are:

1) a box that provides phantom power and AGC kill
2) Record on a stand-alone, sync in post.

Does that sound about right?

Yup. I found exactly this with a Canon HF200. I was using a Sennheiser G3 wireless. I ended up turning the mic gain up on the wireless receiver, and the mic gain down on the camera (quite a bit down, from it's "nominal" setting of -12 to about -30). This eliminated quite a bit of background hiss. Nice that the HF200 has manual mic levels so one can do this.

The "ideal" solution seems to be separating video and audio so you can record audio with equipment that's optimized for audio. Seems ridiculous that we have to do this, but there it is.

Hi Chris:

I tried the older ML when I was running the old firmware. To me, the result was not much different than the current 2.04 Canon firmware as far as audio quality. I think the main point for is that none of the DSLRs are going to even be in the same neighborhood of a competent audio recorder. The DT-454 I am currently testing is fine for dialog but I shot a month ago with it at a music festival where the recording is being pushed a lot more than it would be for dialog because of the full frequency and dynamic range of live acoustic music. The bottom line is that DSLR audio circuits can record fairly usable dialog. But it ends up sounding "thin" and the dynamic range is so-so. Tried using the internal mics, the RØDE NTG-3, the Audio-Technica AT875R and the Sennheiser ME-64. The sound quality naturally varied with the mic, but overall, it doesn't sound good. Thin and feeble sounding, even though the levels are fine.

I also own the H4N and it sounds like it has a thin veneer of acceptability for dialog but same thing, once you try to record very soft sound like whispered dialog or acoustic music through the internal H4N mic-preamps and the end result has a lot of zipper noise (quantizing error) on soft fade outs especially. The H4N can actually record really high quality stuff when fed line level input from a good quality front end. But that sort of defeats the purpose of using one with a DSLR. I am using the H4N these days mainly for phone interviews. I have used it with the Canon 5D MKII but to me, I find the DT-454 with the 5D MKII to be much simpler, faster and easier than double system sound when shooting alone.

My advice is, if you are just recording dialog, the Juicelink DT-454 paired with any of the DSLRs is okay. If you are shooting a feature, music, nature or other situations where the sound REALLY matters, use a "real" outboard recorder, typically something that costs over $1,000.00. None of the sub $1,000.00 recorders are what I would call "impressive" sounding, although some of them sound decent. I am pretty sold on Sound Devices mixers and recorders though, definitely the biggest bang for the buck in sound recording world.

Dan

Bruce Watson - That's exactly where I noticed the problem the first time around; I was using the HF100; seems the same problem existed then as it does now. As for the T2i, I think I agree. It's one or the other. Moral of the story: my old Beachtek box won't cut it. I either need to get a new one, or some other recording device and sync.

I completely understand, but just flat out don't have the cash for a $1000 setup. I do intend to use my setup for a feature, but understand that the budget won't allow for equipment purchases of that price.

Dan,

I agree. The 5D2 with a preamp is fine for dialog, but isn't great for critical music recording. For instance, the 5D2 has a low frequency roll off, which is great for removing wind noise when talking, but not good at all when recording a contra bassoon.

Also, the signal will show some distortion when really pushed hard. There seems to be some harshness in the A/D converter. When keeping the dialog below -12dB, it's usually fine, but be careful about riding the limits. Again for normal dialog - or loud dialog where you allow headroom - this isn't a problem. As I put it, you can get pro-level dialog (clear with low hiss), but not audiophile sound (that magical combination of edginess and creaminess.) With a preamp into the 5D with ML, you get can get cleanliness and edge, but not much cream.

By comparison, the H4n gives slightly more noise, softer edges, and the cream is still lacking. overall, I find the H4n to be a bit dull.

Like Dan says, if you want top results, plan on spending a lot more cash on mixers and recorders. But for good, pro-level (not audiophile level) dialog, a preamp into a DvSLR is a good solution - especially when shooting solo. (You can't keep your eye on an audio recorder when looking at a view finder. You won't know that the device ran out of batteries or storage space or just plain isn't recording until it's too late.)

The audio sample reveals more than hiss, and I believe this is the real problem. The distance from the mic to the sound source is too great - listen to the room sound - a good indication that the distance was too much, and the output level from the mic too little. Passive devices that don't introduce gain don't produce the noise, but the input circuit attempts to take this feeble signal and amplify it. Cheap audio circuits are pretty poor at this, and to get decent level, they have to apply too much gain and this is the noise we're hearing. If you provide the camera with decent input level, the cheap input design can do less work, and the noise level will be better. The solution would be a higher quality mixer or pre-amp for the mic, or simply get the mic in closer or use a more directional mic. Even the best pre-amp designs are noisy at their maximum gain settings.

Not at all ridiculous ... always use the right tool for the job. A camera is designed to record images. Any audio capabilities it has are at best an afterthought, sort of akin to the colour of the carry-case. An audio recorder is purpose-built to record audio, the right tool when the quality of the recording is important (as it always should be ... good audio is actually more imprtant than pretty pictures when it comes to telling a story).

Why a top recorder?
 

Given Dan's first quoted sentence, if I want quality, can't I just start with a good mic with its own preamps and ADC; connect the digital signal through an AES 42 to AES 3 connection kit to a cheap recorder such as the Hn4: would it work? what can go wrong?

Shem

Neumann makes some great digital mics that include preamps and A/Ds, but they're made for the studio, not video production.

Georg Neumann GmbH - Products/Current Microphones/TLM 103 D/Description

Most high-end mics are still analog. If you were to limit your mic selection to digital, there would be few models to choose from.

Yes Jon, digital mics never really have caught on. There are some good ones but large diaphragm studio condensers are not very practical for location sound.

Dan

Digital mics for field recording
 

I wasn't suggesting big Neumann mics, but rather the smaller Sennheiser rf condenser mics (including the MKH 8000 series) that John Willett has repeatedly suggested are the most suitable high end mics for humid conditions. These are just the digital equivalents of the 416s etc that moviemakers use.

Shem