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Using DR-701D with ambisonic mic

Discussion in 'Digital Recorders' started by fieldSounds, Nov 30, 2019.

  1. fieldSounds

    fieldSounds Active Member

    Joined:
    Nov 30, 2019
    Location:
    Germany
    Hi all,

    This is my first post here, so be gentle with me, please ;-).

    So, I'm about to start using my DR-701D and an ambisonic mic - one of Sennheiser Ambeo VR or Rode NT-SF1, I haven't yet decided - to record ambient sounds in the field, i.e. in the wild. Things like birdsong, running water, wind, weather, etc. I've already been using a Rode NT4 with good results, but I want now to step up a little.

    I've done some research online on this gear combo, and I've heard from one source - - that the recorder needs to be very carefully calibrated before using an ambisonic mic. The author of that clip uses a Phonic digital signal generator to do this, but that's beyond my budget. He also mentions an iPhone sig-gen app which could also be used to calibrate (but he doesn't say how), but I don't possess an iPhone. I've seen somewhere a simple-looking all-in-one device which can plug into an XLR socket and do...something like check for shorts, etc. and which might also provide a signal to calibrate with.

    Here's the question: do I actually need to calibrate my recorder for recording those ambient sounds with an ambisonic mic, or is such calibration only important to keep the sound-field "steady" for audiovisual VR purposes when using such a mic to record, say, a string quartet from really close-up?

    Thanks for any help,
    Rob

    PS - I've tried to edit this to change the youtube video from being a big eye-sore media insert, to just a tidy clickable link, but I can't figure out how to do that, sorry.
     
  2. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    Quebec, Canada
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    Hi and welcome to RO !!

    I listened to the video and hes calibration explaination makes no sens.
    He says all channels need to have the same gain input, then he links them. So he is assuming that the preamps are 100% matched. This is rarely the case.
    Next he talks about the input level calibration and headroom for the performer while using an other source, How does he know how many db output from the tone generetor will match the level captured by the mic when the performer play??

    Maybe someone will correct me but here's my thoughts:

    The guy isn't an audio engineer, if he was, he would be talking about gain staging and not calibration. A true calibration would be to gain match each inputs of the recorder by fine tuning the gain circuit. To do so would mean sending a test tone, in phase and out of phase and adjust the gain inputs until they null at the output of the unit.

    I wonder if when you link the channels they keep their gain differences.. If they don't it can't be calibrated externally...

    Linking the 4 channels is ok, if there is a slight difference between the levels of the channels, it can be compensated in post. (by loading the files into a computer and a DAW)
    I would just plug the mic into the channels, make the musicians play a loud part of a song and set the gain correctly.
    If you can't ask the musicians, be there early for rehearsal or soundcheck or when they are all tuning..
    In the field, is the same game.. put the gain to an healthy level usually between -18db to -10db of average level.

    His idea of using a tone generator isn't all bad, but it would work if he would play the tone through a speaker placed where the performer would be and guessing at what level they will play.
    Guessing isn't very appealing to me tho..

    Hope this helps
     
  3. fieldSounds

    fieldSounds Active Member

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    Nov 30, 2019
    Location:
    Germany
    @pcrecord, thanks for the reply. Not being a pro sound engineer, I can't speak to things like
    but I do hear where you're coming from.
    My thoughts entirely.

    Sounds like I'm good to go!

    Thanks again,
    Rob
     
  4. Boswell

    Boswell Moderator Distinguished Member

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    I'm sorry to say I didn't rate that video very highly. The presenter had picked up a few techniques necessary in surround sound recording, but he clearly had not understood very well why they were used. For example, as in any live classical recording, you set your channel gains so that they never overload in the loudest passages, then attenuate further by a (say) 10dB. But then you do not change them during the movement or the piece, so the linking of gains is an unnecessary feature. Any adjustment to the gains would be done in post-processing.

    What you do have to know at mixdown is the relative electronic gains of the channels, so using the signal generator at performance time is a valid way of calibrating this. A few seconds of 1KHz tone from a generator plugged into each channel in turn at the start is sufficient, as those tone amplitudes can be measured at mixdown and any channel differences noted. Some surround engineers prefer to record the rear channels at a slightly higher gain, as there is less direct sound in those, and the difference is then taken out when applying the corrections from the calibration tones.

    There is little point of in-situ acoustic calibration of the surround microphone, e.g. by supplying a tone from a loudspeaker, as the variations in the acoustic properties of the hall are liable to make a nonsense of any technical measurements. A high-quality surround microphone is usually carefully measured in an anechoic chamber at time of manufacture and sold with the calibration graphs.
     
    pcrecord, dvdhawk and Kurt Foster like this.
  5. fieldSounds

    fieldSounds Active Member

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    Nov 30, 2019
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    Germany
    @Boswell
    Now THAT makes perfect sense to me! Now that you say it, I'm reckoning that's maybe what the video guy (who just happens to be "a professor in the Music Production and Engineering department for Berklee College of Music") must have meant.
    About what I had expected.

    It looks like I can easily by-pass the calibration stage for my field-recording needs, although I'm intrigued by the 1kHz tone idea above and may follow that up at some time.

    Thanks,
    Rob
     
  6. Boswell

    Boswell Moderator Distinguished Member

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    It should be obvious, but I didn't make clear that the tones should be recorded after the gains have been nominally set.
     
  7. fieldSounds

    fieldSounds Active Member

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    Yep, OK, got you, makes sense: set all gains nominally, gang'em, and record a couple of seconds of 1kHz tones on each channel before recording sounds. Use the tones in the DAW to set levels on each channel for mix-down.

    Looking forward to creating some cool binaural output, once I've decided on which mic I'll buy.

    Thanks again!
     
  8. fieldSounds

    fieldSounds Active Member

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    Nov 30, 2019
    Location:
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    Just re-read your post and picked up that comment, which underlines what I've already learned re not changing gain once the recording has started, so ganging/linking gains for ambi would indeed be superfluous. Thanks again.
     
  9. cyrano

    cyrano Active Member

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    Feb 4, 2018
    Location:
    Brussels
    Linking would be less bad to the ambi sound stage if you changed gain by accident, or because you had to, for some reason.

    I had a job recently, where one soprano was a lot louder than all the rest. I needed to reduce gain when she came on, in the second part. Had set it a bit high, due to lacking time and no sound check...

    She was the one professional amongst amateurs and she needed everyone to hear it.

    If the gain change is the same on all four channels, it's easy to correct. If it's one channel, good luck correcting it.
     
  10. fieldSounds

    fieldSounds Active Member

    Joined:
    Nov 30, 2019
    Location:
    Germany
    There's also of course the situation where one might record, say, some bird-song or waves on a beach, at one setting of the linked channels, and then another recording at, say, 10dB greater or less across the board. Just to see what differences might arise in noise levels, for instance.
     
  11. cyrano

    cyrano Active Member

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    Feb 4, 2018
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    Brussels
    Yep.

    It's just because I recently forgot to link. I was recording 7 channels and forgot 4 of those were "special"...

    So, being able to link gain is kind of a biggie. In my case, I forgot and the machine didn't stop me. Digital is good at helping. With the ADA8000 I was using, linking isn't an option as it has analog gain. Real Knobs!
     
  12. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    I would call this recording a safety take.
    Some units allows you to record 2 different files 1 normal and one at -10db or -20db.. so you push record once and have both recordings as results.. ;)
     
    kmetal likes this.
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