Discussion:
Strange 'buzz' in Ambisonic recording
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Gerard Lardner
2018-05-07 16:30:10 UTC
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I'm very much an amateur here, so please forgive what might be a stupid
question!

I recorded a concert on Saturday (John Rutter's /Gloria /and Karl
Jenkins' /The Peacemakers/) using an Ambisonic mic and some others. I'm
encoding the A-format to B-format using VVEncode in Reaper, and panning
in the extra mics using Wigware Ambipan. The result is then decoded to
surround sound or to stereo using VVDecode; all in the same Reaper
set-up. I've used this approach before, usually successfully.

I find that, when the organ and brass are playing at full volume (I mean
in the orchestra, not just in playback), there is a 'buzzing' sound in
the playback. It sounds harsh, pitched at about 50 Hz. But if I listen
to the A-format files alone in Reaper or in VLC Media Player, there is
no buzz. Similarly, there is no buzz if I encode the A-format using the
stand-alone version of VVMic. The buzz is present both in the B-format
from the Ambisonic mic and, less strongly, in the panned B-format from
the soloists mic. It appears to be an artefact of my editing
configuration; but it didn't happen in a recording I made, using the
same configuration, last month. The only difference since then has been
that my old hard disc, which had developed some faults, was cloned onto
a new SSD.

Does this description suggest a specific problem/remedy to anyone? I
know I can produce a decent CD using the stand-alone VVMic route and
panning in the soloists into the stereo mix; but I'd like to get to the
bottom of why my Ambisonic configuration is doing this now.

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Fons Adriaensen
2018-05-12 07:37:16 UTC
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On Sat, May 12, 2018 at 12:35:56AM +0100, Gerard Lardner wrote:

> I have tried encoding using VVEncode and using X-Volver (in Reaper). The
> effect is there in either case, though it is less obvious when using
> X-Volver. However, I do not hear it at all if I listen to the files through
> the stand-alone version of VVMic. This makes me think I am doing something
> wrong in Reaper.

A had a quick look (more later, have to go now). In the stereo file
there are discontinuities spaced 512 samples or multiples of 512
samples apart. This looks like a problem in the convolution.

Please upload the B-format as well.

--
FA
Gerard Lardner
2018-05-12 01:43:06 UTC
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OK, I've put a short section (30 seconds) in
https://www.dropbox.com/sh/sa6mvgfxuo2cxao/AACrbYW96VbxwIB2ZFDJS17ua?dl=0

The A-format is in two 2-channel files; the file names should be
self-explanatory. The stereo file exhibits the buzzing sound right from
the start of the music.

Gerard Lardner


On 11/05/2018 15:13, Fons Adriaensen wrote:
> On Fri, May 11, 2018 at 01:03:57AM +0100, Gerard Lardner wrote:
>
>> Actually really only when the organ is playing; the brass is usually with
>> the organ, but not always. The buzz is present when the organ is playing
>> loudly.
> Could you make available a small part (20 seconds or so) of the original
> A-format file and the encoded B-format one for the part where you hear
> the 'buzz' ??
>

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David Pickett
2018-05-12 06:30:06 UTC
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Gerard,

could you put up the B-format also?

Thanks

David


At 03:43 12-05-18, Gerard Lardner wrote:
>OK, I've put a short section (30 seconds) in
>https://www.dropbox.com/sh/sa6mvgfxuo2cxao/AACrbYW96VbxwIB2ZFDJS17ua?dl=0
>
>The A-format is in two 2-channel files; the file names should be
>self-explanatory. The stereo file exhibits the buzzing sound right
>from the start of the music.
>
>Gerard Lardner
>
>
>On 11/05/2018 15:13, Fons Adriaensen wrote:
>>On Fri, May 11, 2018 at 01:03:57AM +0100, Gerard Lardner wrote:
>>
>>>Actually really only when the organ is playing; the brass is usually with
>>>the organ, but not always. The buzz is present when the organ is playing
>>>loudly.
>>Could you make available a small part (20 seconds or so) of the original
>>A-format file and the encoded B-format one for the part where you hear
>>the 'buzz' ??
>
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Fons Adriaensen
2018-05-12 09:35:24 UTC
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On Sat, May 12, 2018 at 02:43:06AM +0100, Gerard Lardner wrote:

> The stereo file exhibits the buzzing sound right from the start of the music.

See <http://kokkinizita.linuxaudio.org/linuxaudio/spikes.png>.

This shows 10000 samples of the left channel, differentiated twice.

Ciao,

--
FA
Gerard Lardner
2018-05-11 22:02:47 UTC
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The recording was made using a Brahma, no. 008.

I used the same set-up of mics and en/decoders in Reaper to record
another choral concert for Passion Sunday, the weekend before Easter. I
had no such problems with that recording (only coughing in the
audience!). But in that case there was only a small chamber orchestra
and a chamber organ.

I'll try to make time tomorrow to post some short extracts from the
recording causing trouble, both the A-format and B-format as requested
by Fons Adriaensen. I'll also try X-Volver, as suggested by David
McGriffy and see if there is any difference there. But given the
apparent success I have had with previous recordings, I am not jumping
to blame VVEncoder, Ambipan or VVDecoder; I'd sooner blame some
oversight or stupidity on my part. I don't do this stuff often enough to
claim to be any kind of expert, or anything more than lucky.

TBH, I had a problem with the previous recording where it was recorded
at 44.1 kHz, but Reaper seemed to think it should be played back as if
it was recorded at 48 kHz, resulting in the pitch dropping by almost a
tone. I sorted it at the time, but I don't really understand what was
happening and can't remember how I fixed it. I used that same Reaper
set-up for this latest concert, just dropping the new audio tracks into
it. Could that be part of the problem?

Gerard Lardner


On 11/05/2018 16:39, umashankar manthravadi wrote:
> Was it a brahma mic ? I got the impression it was not. If it is a Brahma, and I get its serial number, I can take a look.
>
>
>
> umashankar
>
>
>
> Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10
>
>
>
> ________________________________
>> From: Sursound <sursound-***@music.vt.edu> on behalf of David McGriffy <***@mcgriffy.com>
> Sent: Friday, May 11, 2018 8:44:33 PM
> To: Surround Sound discussion group
> Subject: Re: [Sursound] [allowed] Re: Strange 'buzz' in Ambisonic recording
>
> Given that this is a Brahma, it uses FFT based processing. My suspicion is
> that the artifact is actually something in the FFTs like a windowing
> problem or a bug handling the first FFT bins. At 48kHz, 1K block
> boundaries would be about 48Hz.
>
> Does using Xvolver give the same artifact? IIRC I read that code and tried
> to do the FFT filters in a compatible way in VVEncode so it could be they
> both have the same trouble or it could just be a bug in my code.
>
> David
> VVAudio
>
> On Fri, May 11, 2018 at 9:13 AM Fons Adriaensen <***@linuxaudio.org> wrote:
>
>> On Fri, May 11, 2018 at 01:03:57AM +0100, Gerard Lardner wrote:
>>
>>> Actually really only when the organ is playing; the brass is usually with
>>> the organ, but not always. The buzz is present when the organ is playing
>>> loudly.
>> Could you make available a small part (20 seconds or so) of the original
>> A-format file and the encoded B-format one for the part where you hear
>> the 'buzz' ??
>>
>> --
>> FA
>>
>> _______________________________________________
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Dave Malham
2018-05-09 07:02:03 UTC
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Against that, there's the statement that the buzz was also present "less
strongly, in the panned B-format from the soloists mic" where there
shouldn't be any bass gain at all. Sounds to me like something in the
playback set up. Can you go and listen to it on someone else's rig?

Dave

On 7 May 2018 at 21:41, Bo-Erik Sandholm <***@gmail.com> wrote:

> I suspect that depending of chosen virtual mic configuration there is so
> much low frequency gain added that you boost the level of the nearly always
> existing 50Hz hum so much that you actually hear it.
> It has sometimes happened to me that I need to lower the input level of the
> signal to the ambisonic decoder otherwise i got signal clipping in the
> processing stage.
>
> BR Bo-Erik
>
> 2018-05-07 18:30 GMT+02:00 Gerard Lardner <***@iol.ie>:
>
> > I'm very much an amateur here, so please forgive what might be a stupid
> > question!
> >
> > I recorded a concert on Saturday (John Rutter's /Gloria /and Karl
> Jenkins'
> > /The Peacemakers/) using an Ambisonic mic and some others. I'm encoding
> the
> > A-format to B-format using VVEncode in Reaper, and panning in the extra
> > mics using Wigware Ambipan. The result is then decoded to surround sound
> or
> > to stereo using VVDecode; all in the same Reaper set-up. I've used this
> > approach before, usually successfully.
> >
> > I find that, when the organ and brass are playing at full volume (I mean
> > in the orchestra, not just in playback), there is a 'buzzing' sound in
> the
> > playback. It sounds harsh, pitched at about 50 Hz. But if I listen to the
> > A-format files alone in Reaper or in VLC Media Player, there is no buzz.
> > Similarly, there is no buzz if I encode the A-format using the
> stand-alone
> > version of VVMic. The buzz is present both in the B-format from the
> > Ambisonic mic and, less strongly, in the panned B-format from the
> soloists
> > mic. It appears to be an artefact of my editing configuration; but it
> > didn't happen in a recording I made, using the same configuration, last
> > month. The only difference since then has been that my old hard disc,
> which
> > had developed some faults, was cloned onto a new SSD.
> >
> > Does this description suggest a specific problem/remedy to anyone? I know
> > I can produce a decent CD using the stand-alone VVMic route and panning
> in
> > the soloists into the stereo mix; but I'd like to get to the bottom of
> why
> > my Ambisonic configuration is doing this now.
> >
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> > edit account or options, view archives and so on.
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--

As of 1st October 2012, I have retired from the University.

These are my own views and may or may not be shared by the University

Dave Malham
Honorary Fellow, Department of Music
The University of York
York YO10 5DD
UK

'Ambisonics - Component Imaging for Audio'
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