Audio Issues

The place for questions about shooting with Blackmagic Cameras.
  • Author
  • Message
Offline

josiahburdick

  • Posts: 2
  • Joined: Tue Nov 13, 2012 9:30 pm

Audio Issues

PostTue Jan 01, 2013 8:08 pm

I'm curious about the audio issues that were raised by Robert at Juiced Link in his video review of the BMC.

Black Magic, is there any way of knowing if/when these issues have been resolved, and whether they are a part of the next round of firmware updates?

Here's the video in question,
Offline

Keith Stark

  • Posts: 19
  • Joined: Wed Dec 12, 2012 5:05 pm

Re: Audio Issues

PostWed Jan 02, 2013 5:20 pm

wow,
no replies on this one..

My very humble 2C...

I thought the DC bias issue (if it's indeed true) is really interesting. I've seen it in other systems that had design issues. I think that's reason enough to go dual system. Depending on how the electronics are designed, it may not be able to be fixed in firmwhere (speculating....).

For what it's worth, I have an Edroll R-44, and for the record it sounds 5000 times better then any of the examples he has with the jucedlink in that presentation. Granted I'm using a pretty good mic with it (Shure KSM-32, boom ops hate me). I just don't see how a 5D recording 8bit at 44.1K or 48K (what ever it is) can be better then 24bit 96K, with analog limiting.

I plan on just using the R-44 dual system, like I have been (which I need to do for steady cam anyway), and running a line from the R-44 to the cam when it's convenient, but always recording on the R-44.

cheers,
K
Camera "Claude" Received 4/4
Offline

Robert Rozak

  • Posts: 7
  • Joined: Wed Jan 02, 2013 5:45 pm

Re: Audio Issues

PostWed Jan 02, 2013 5:56 pm

Keith,

I'm 99.99% sure the DC offset issue is in the firmware. I tested an earlier version of the firmware which didn't have the DC offset issue (but had other issues). I shared my test results (both firmware versions) with BMC back in August '12.

Obviously, I didn't design it, so I can't tell you what exactly may be going on ...


Regarding the signal-to-noise tests that included the R44, those came from a challenge from the ProVideoCoalition to use a low-sensitivity mic (dynamic mic), to stress the front ends of the recorders/preamp, to really compare how they perform under the worse case conditions. So yes, if you are using a higher sensitivity mic, your results with the R44 will not be as poor.

But, what the tests show that the front end matters ... a poor front end under stressed conditions recording 24b/96Khz will not perform as well as a well designed front end at 16b/48Khz in terms of signal-to-noise performance ... and with a little help from a good low-noise preamp, cameras are fully capable of recording with excellent signal-to-noise performance directly ...


Best regards,

Robert
Offline

jasonxinzhou

  • Posts: 79
  • Joined: Mon Sep 17, 2012 1:50 am

Re: Audio Issues

PostThu Jan 03, 2013 4:06 pm

Robert Rozak wrote:Keith,

I'm 99.99% sure the DC offset issue is in the firmware. I tested an earlier version of the firmware which didn't have the DC offset issue (but had other issues). I shared my test results (both firmware versions) with BMC back in August '12.

Obviously, I didn't design it, so I can't tell you what exactly may be going on ...


Regarding the signal-to-noise tests that included the R44, those came from a challenge from the ProVideoCoalition to use a low-sensitivity mic (dynamic mic), to stress the front ends of the recorders/preamp, to really compare how they perform under the worse case conditions. So yes, if you are using a higher sensitivity mic, your results with the R44 will not be as poor.

But, what the tests show that the front end matters ... a poor front end under stressed conditions recording 24b/96Khz will not perform as well as a well designed front end at 16b/48Khz in terms of signal-to-noise performance ... and with a little help from a good low-noise preamp, cameras are fully capable of recording with excellent signal-to-noise performance directly ...


Best regards,

Robert

Nice to meet you here Robert. I used to have a JuicedLink CX231 and I love it.
Offline

Keith Stark

  • Posts: 19
  • Joined: Wed Dec 12, 2012 5:05 pm

Re: Audio Issues

PostThu Jan 03, 2013 6:06 pm

Thanks Robert,
I guess I should correct myself first: Cameras like the 5D are recording a 16bit file, but due to the quality of the electronics I doubt it's '16 bit clean'. Then add in the data compression, RFI issues, etc. But this is a BMD forum, we don't care about DSLRs right ;)

I guess I'm making the mental leap then if someone buys a $700-$1200 audio recorder, they're probably not going to plug a $80-100 mic into it,...but stranger things have happened.

I was wondering if you've done a test like this:

'common' Rhode > JL > BMD
vs.
'common' Rhode > JL > 'Known' high quality recorder/interface

or possibly:

test tone generator > BMD
vs.
test tone generator > 'Known' high quality recorder/interface

Just to evaluate the actual line recording quality of the BMD, which is probably not worth it until the bias issue is resolved.

Cheers,
Keith
Camera "Claude" Received 4/4
Offline

Robert Rozak

  • Posts: 7
  • Joined: Wed Jan 02, 2013 5:45 pm

Re: Audio Issues

PostThu Jan 03, 2013 8:33 pm

Thanks! Great to meet all of you too!!!

let me respond with some comments ... on some of your comments ...

****
"I guess I'm making the mental leap then if someone buys a $700-$1200 audio recorder, they're probably not going to plug a $80-100 mic into it"

It's not how expensive the microphone is, but the microphone sensitivity and how well it can be placed. Not everybody is using a 50mV/Pa sensitivity ME66. The majority of people are using shotgun microphones in the class of something like the NTG2 at 15mV/Pa. Plus, different microphone types have less sensitivity, like the most popular hypercardioids (~10mV/Pa), or lavalier microphones (~5mV/Pa). So, even if your not using a dynamic handheld microphone, you're coming closer to it's sensitivity of ~3mV/Pa.

Plus, if you're not able to get that NTG2 boomed 1.5ft away for a wider shot, and it's more like 4ft away, the inverse square law is just push your microphone output signal down to the range of where the dynamic mic is.

Does that make sense? This is why the front end is so important. Some recorders have a great front end. Others ... not so much.


****
"Cameras like the 5D are recording a 16bit file, but due to the quality of the electronics I doubt it's '16 bit clean'"

I know that we're not focusing on DSLRs here, but looking at them is instructive for understanding what is going on, and you are correct in that it is not 'clean'. The limitation camera's like the 5D is the analog front end. But, a quality low noise preamplifier fixes that. It's what's known as the "cascaded noise figure of amplifiers" principle. Even if you have a poor signal-to-noise performing amplifier, if you stick a low noise preamplifier in front of that, the signal to noise performance of the entire system is set by the first amplifier. So, you can make the signal to noise performance of the 5D 16-bit 'clean' with a good low noise preamplifier.

Similarly, with those recorders that performed poorly in the signal to noise tests (even when recording at 24b/96kHz), you can fix their signal to noise performance by sticking a low noise preamplifier in front of them. Check out this old blog post on how to improve the signal-to-noise performance of a Rode NTG2 recorded by a Zoom H4n:
http://juicedlink.com/blog/2010/10/impr ... se-preamp/


So, you can record with excellent signal to noise performance directly in the camera with a quality low noise preamplifier, or in a recorder with a quality front end (or with a good low-noise preamp if the recorder front end is a weak link). But, there still are some differences between using a recorder vs recording direct to camera.

First is bitrate. IMHO, a higher bitrate (beyond 16b/44.1k) is not a big deal. More often than not, the long pole in the tent in terms of signal to noise performance is going to be the front end, not the A/D or bitrate. The AES had a great paper that showed that humans (the vast majority) cannot perceive improvement in audio quality at higher bit rates. I know I can't hear 144dB in dynamic range ...

Next is production flow. A recorder can have the advantage of being located remotely from the camera. Recording direct to camera has the advantage of not having to sync in post, and have multiple devices to remember (or forget) to hit 'record' on.

Third is frequency response. Here's where I would agree that there is an advantage for external recorders. Most cameras will not have as good frequency response as external recorders. They usually don't have enough room to fit the big coupling capacitors in camera's for the low end of the freq range. So, the right tool for recording the Chicago symphony would be an external recorder, not a camera. But for recording dialogue, recording to a camera (with a good low-noise preamp) is more than sufficient.


****
regarding the tests you requested above ...

Regarding a tone generator for signal-to-noise tests, the question always is what signal level to inject. That is why the ProVideoCoalition dynamic microphone challenges such a great way to evaluate, because it's a real world signal level at the low end of sensitivity that allows you to compare the recordings in the most stressed environment. Then, you act like a human HP8903B SignalGenerator/AudioMeter, and compare the noise floor of the different recordings turned quiet periods. So, I would go back and listen again to the recordings in the BMC audio applications video, and there are even more audio clips to compare in a previous blog post video. Once you have the sense on how things compare in the stressed situation, then the NTG2 test is less interesting. With a quality low-noise preamp, the cameras and some of the higher-end recorders performed very well in terms of signal-to-noise.


****
"probably not worth it until the bias issue is resolved"

Let me explain a little bit more about the DC offset. It is problematic in that you could be in a situation where you think you've got a juicy signal on the Ultrascope meter, but in actuality you do not because a significant portion of the meter is sensing the DC offset. The amount of DC offset varies with the gain setting in the camera, and it is higher at high gain settings, and lower at low gain settings. As mentioned in the video, you do not want to use a lot of digital gain (which will result in poor signal to noise performance). You want to set the camera gain to the sweet spot as indicated in the video (where you're getting close to no digital gain or digital attenuation). At that sweet spot gain setting in the camera, you do have some DC offset, but it's not that huge. You will take some hit in signal-to-noise. But, it is not a huge amount, as demonstrated by the signal-to-noise audio tests in the video.

So, if you're going to be using the camera and the current state of firmware, you want to set the camera gain to that sweet spot as indicated in the applications video, and leave it there. Use the low noise preamplifier to adjust the signal level so the Ultrascope meters (in dB) are happy. So, you can record to the camera and the current firmware with decent signal to noise, but it's a big pain. You will need to use Ultrascope for metering, or pre-calibrate an external meter to Ultrascope, or use an external monitor which has it's own meter. Then, you will want to remove the DC offset in post, before you do any manipulation. It's all doable, but a pain.


****

I hope readers find this conversation helpful ...


Best regards,

Robert
Offline

josiahburdick

  • Posts: 2
  • Joined: Tue Nov 13, 2012 9:30 pm

Re: Audio Issues

PostThu Jan 03, 2013 10:31 pm

I certainly have. =)

I know that for myself, even though I have Benchmark Micpre/adc, the vast majority of my interview work would be better served with the speed of going direct into camera and not lugging all that expensive equipment around.

Thanks Robert for chiming in.
Offline
User avatar

Josh Newman

  • Posts: 34
  • Joined: Fri Nov 23, 2012 12:08 am
  • Location: Sydney

Re: Audio Issues

PostThu Jan 03, 2013 10:59 pm

Robert is now my favourite forum member. Great info.

I'm an audio post guy by trade, and I have to agree with you about the 96Khz vs 44.1kHz stuff. 96kHz is great, but the difference from that to 44.1/48Khz is so unimportant, especially when we're talking about recording dialogue for something thats likely to end up being seen on the web.

Ordered my Riggy Micro last week. After listening to those tests, I figured I'll use it with my 5D (or as a better pre for the pretty horrible H4n) until the BMCC arrives.
Offline

Greg Huson

  • Posts: 188
  • Joined: Mon Oct 01, 2012 10:34 pm
  • Location: Culver City, CA

Re: Audio Issues

PostThu Mar 21, 2013 11:27 pm

Has there been any development on this problem?

I never really liked the dual-audio hassle BS. My Tascam DR-100 works great, but and this camera is already too much like a film camera, in that you have to hang a bunch of crap on it to use it.

I've recorded a bunch of interviews that now have to have the DC offset removed from the audio before editing. Total PITA. Would love for this annoying feature to 'go away.'

This camera has a perfectly good digital recorder in it and there's no excuse for this annoying little problem- this should of been discovered in QC and beta testing and solved before delivery - granted, it's not like, say, a bad sensor - but it's an unacceptable pre-release flaw that's drifted into manufactured and delivered product. Please fix ASAP!
GH
----------------------------------------------------
Greg Huson
Secret Headquarters, Inc
Post Production / Production
Culver City, CA
323 677 2092
www.SecretHQ.com
www.DigitalServiceStation.com
greg (at) SecretHQ.com

Return to Cinematography

Who is online

Users browsing this forum: Bing [Bot] and 20 guests