I still haven't been able to reproduce the problem, but if the phenomenon i=
s known then I'll assume I've fixed it with the new battery.=0D
=0D
The frequency of the bias was too low to be audible in this case, but was s=
trong enough to cause audible clipping if my subject had been louder.=0D
=0D
Peter Shute=0D
=0D
Sent from my iPad=0D
=0D
On 27 Oct 2013, at 12:28 pm, "umashankar" <<umash=
>> wrote:=0D
=0D
=0D
=0D
I think this is a variation on what is called motorboating, the voltage sup=
plied varying with signal and causing positive feedback. in an audio amplif=
ier drawing tens of miliamps this really sounds like a motorboat. I guess i=
n a mic head amp drawing less than a milliamp, the frequency would be once=
in a few seconds range.=0D
=0D
umashankar=0D
From: Peter Shute <<>>=0D
To: "<=
om>" <<=
com>>=0D
Sent: Sunday, October 27, 2013 5:09 AM=0D
Subject: RE: [Nature Recordists] ME66/K6 with signal bias=0D
=0D
Thanks for looking at it, Vicki. I assume because the bias is really a supe=
rimposed wave with no overall bias, it should show up in a spectrogram as a=
signal at 0.25Hz. If I set Audacity's spectrogram window size to 32768 (th=
e maximum) and the max spectrogram frequency to 100Hz (the minimum), I see =
a bit of a lump in the 0-10Hz area at each spike in the waveform, two big o=
nes and a little one.=0D
=0D
I don't think it's really that useful to look at the spectrogram though, th=
e problem is plain to see in the waveform. I played with fixing it by dupli=
cating and inverting the track, then using eq to filter out anything from t=
he duplicated track above about 20Hz, then mixing it with the original. The=
bias disappeared from the waveform, and it sounds ok to me.=0D
=0D
I'm still not absolutely convinced it's caused by the flat battery because =
I've had the mic on for a couple of hours now with that same battery, and i=
t's still not doing it. I think it was about 10 degrees C when it was happe=
ning, and it's nearly 20 here in the house, so that might be boosting the b=
attery output a bit. I might try putting it in the fridge for a while.=0D
=0D
Peter Shute=0D
=0D
________________________________=0D
From: <=
com> <=
com>] On Behalf Of vickipowys <vickipowys@=
skymesh.com.au>]=0D
Sent: Sunday, 27 October 2013 9:52 AM=0D
To: <=
m>=0D
Subject: Re: [Nature Recordists] ME66/K6 with signal bias=0D
=0D
=0D
=0D
Peter,=0D
=0D
How curious! I downloaded the sound clip and looked at it in iZotope=0D
RX. No anomalies show up in the spectrogram but they do in the=0D
waveform, and the levels jump upwards according to the levels meter=0D
(but not audibly), in a cyclic pattern unrelated to the birdsong.=0D
=0D
I have no explanation for this phenomenon. But it does seem to be a=0D
lesson for people to keep their batteries fresh. A chap I know=0D
changes his K6 gun mic battery daily 'whether he needs to or not', on=0D
recording trips.=0D
=0D
Vicki=0D
=0D
On 27/10/2013, at 9:18 AM, Peter Shute wrote:=0D
=0D
> I recently left my ME66 switched on overnight, and when I=0D
> discovered this in the morning and switched it off and on again,=0D
> the battery confirmation light was very dim.=0D
>=0D
> The first recording I made after that seems ok, but a couple after=0D
> that show a bias in the signal, showing in the waveform as a funny=0D
> shaped wave, repeating every 4 seconds or so. Later in the day this=0D
> decreased to about 1 second, although the recordings sound ok to me.=0D
>=0D
> I've uploaded a sample to http://snd.sc/1a7KEuG=0D
>=0D
> Is this bias the normal result of a dying battery? When I tested it=0D
> again just now before changing the battery, it seemed ok again,=0D
> perhaps because the battery had recovered a bit. I've left it on to=0D
> see if I can replicate the problem.=0D
>=0D
> I'm not at all concerned about saving the recording, only about=0D
> whether the mic is faulty or just needed a fresh battery.=0D
>=0D
> Peter Shute=0D
>=0D
=0D
=0D
=0D
=0D
=0D
=0D
=0D
=0D
=0D
------------------------------------=0D
=0D
"While a picture is worth a thousand words, a=0D
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.=
=0D
=0D
Yahoo! Groups Links=0D
=0D
=0D
=0D
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=0D
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