THAT line & mic output stage

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: THAT line & mic output stage

Post by carlmart »

Well, I'm glad you could see what little information there is on the audio side of those cameras.

Couldn't get a better spec yet, and I don't know if I will get it. Probably will have to figure out myself, or perhaps even measure them to be more certain on how to deal better with them.

OTOS I have plenty of practical experience, as I have been a location sound recordist, dealing with pretty difficult situations, for about 40 years. And audio has been one of the poorest capabilities video has been famous for.

I'm not sure if you have dealt with recording audio on video, be it professionally or at home. But if you did you may have seen how difficult is to have some control over it.

Maybe because I come from film, where audio was always recorded separately, which in video wasn't, my struggle with video has always been how to improve things so you can get better results, as comparable as possible to recording it separately.

As I'm also a director and producer, also editing some of my productions, I could follow the chain, which not most people did or do.

In the late '90s I designed and built an affordable two channel mic preamp, using SSM2017 chips, to be used with semi-professional cameras. Things could improve a lot using it. It was based on my own experience with a Hi-8 camera, which had unbalanced 1/8" mic inputs, and the trick was how to fool the internal AGC so it would not work, which was not easy but could succeed with. On DV cameras things could be even better if you used good mics (which people didn't), particularly if you used a sound person to handle it, which people didn't. I recorded several musical docs in DV, which later were praised precisely for the sound quality.

DSLR cameras share most of the limitations semi-pro video cameras had. Only this time people are accepting recording double-system audio, using a separate digital recorder.

But I think you can still improve things when recording audio in the camera itself.

I'm sorry for this long introduction, just wished to show that I think I know this matter better than most, particularly on the practical aspects.

The idea for this preamp, which was just based on my first experience but is more refined, came after my purchase of a Blackmagic Pocket camera. As I was also recording with another camera with XLR inputs, and I didn't have a sound person, what I did was use that first camera to feed audio into the Pocket. The XLR mics I used on the other camera were high quality ones (AKG), both cardioid and lapel types. Audio quality on the Pocket was excellent.

If you read the forums about this camera, you will read audio is considered very poor on this camera. Well, I could see it isn't, if you do things properly.

Yes, you should record audio separately too, but sometimes you may or will use the one on the camera itself.

Yes, I will be supplying a special cable to each camera model, handling amplifying and monitoring. There are several aspects we will be solving.

Sorry, the BMCC is the Blackmagic Cinema Camera, and it's the Pocket's big brother. It has balanced 1/4" jacks, but audio control is not much better than on the Pocket.
ricardo
Posts: 262
Joined: Sun Apr 10, 2011 7:24 am

Re: THAT line & mic output stage

Post by ricardo »

carlmart wrote:Well, I'm glad you could see what little information there is on the audio side of those cameras. ....
... loadsa good stuff ...
........ audio control is not much better than on the Pocket.
Perhaps we should have asked you this question at the very start.

In my MicBuilders LNprimer.doc, I define what it will be used with BEFORE specifying any technical details.

I'm going to make a guess that 10dBu (2.45Vrms) will be sufficient to drive any DSLR to FS. It will certainly drive FS on practically all multi-channel digital soundcards.

What is NOT a guess is that the special cable to go from Balanced to Unbalanced might be the most important design 'feature' for your device. Serendipitously, your choice of THAT 1606 is probably the most versatile and best solution to this. The TI & AD chips are not as good. And there are 'balanced' outputs which are truly awful for this important scenario.

Read Self & Rod Elliot on how to do this properly. Also Wayne on 'Balanced Outputs' on this forum.
.. trick was how to fool the internal AGC so it would not work, which was not easy but could succeed with ..
How did you do this?
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: THAT line & mic output stage

Post by carlmart »

Apparently, according to THAT, I will get a bit more than 10dBu from my preamp.

<The 1510 will swing to within 2V of the rail voltage and this 2V drop will be consistent as the power supply voltage drops. For example if the power supply is +/-5V (10V single supply with a Vcc/2 reference) then the maximum output swing will be 5V -2V for each side, or +/-3V (6V pk-pk).>

As mine will be regulated for 12v, swing should be +/- 4v (8v pk-pk), it would be about 14dBu. Enough to get to FS not just on DSLRs and digital soundcards, but on most pro video cameras too.

Yes, I knew I was choosing wisely when I went for the THAT output chip, instead of AD or TI. I'm not sure about the 1510, which I think it's quite similar to its AD or TI options with the improved input arrangement.

Designing primarily for DSLR cameras was the reason for using a single supply, as that simplifies what battery types they can use on it, similar to those used in video cameras. In this case 14.4v or 7.2v x 2 batteries, regulated to 12v.

THAT claims is not necessary to regulate down, but my EE partner insists on that, and I will go with his recommendation.
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: THAT line & mic output stage

Post by carlmart »

.. trick was how to fool the internal AGC so it would not work, which was not easy but could succeed with ..
How did you do this?
What I found out was that as long as levels were sufficiently high, short of clipping, the AGC wouldn't be activated, even when dialogue or music stopped.

I never did any measurements on the AGC threshold levels, it was an empirical approach that did work on all home-video cameras I found. What I did was fed a Shure mixer or an externally powered mic, with high enough level.

As a result it didn't "breath" down and up as AGCs did.
ricardo
Posts: 262
Joined: Sun Apr 10, 2011 7:24 am

Re: THAT line & mic output stage

Post by ricardo »

carlmart wrote:What I found out was that as long as levels were sufficiently high, short of clipping, the AGC wouldn't be activated, even when dialogue or music stopped.
Bear in mind that your Limiter will "breath" too.

That's why Limiter (also Compressor) design is nearly a Black Art. You may have to fiddle with Attack, Release, Tresholds & Rectifier side chains to get the sound you want.

Do report what you hear and relate that to your final design decisions.
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: THAT line & mic output stage

Post by carlmart »

I certainly will.

Though that is different to what happened to home video camera's AGCs.
Post Reply