--- In Paul Jacobson <> wrote:
> In this case my interest is in Blumlein Difference Technique (BDT) using =
baffled omni's rather than the better know Blumlein pair using crossed figu=
re-8 mics. Jecklin apparently developed his disk from the BDT idea.
Yes, I believe that is so. I also believe it has the potential to be an exc=
ellent stereo technique, offering the best of omnis - including no proximit=
y effect and consistent LF response with distance - with the added benefits=
of directionality.
It is amazing what Blumlein himself came up with, including MS, BDT and, of=
course, Blumlein. There is a saying about BDT that is quite funny. I canno=
t remember the year it was invented in, but let's say it was 1938. The sayi=
ng goes, "BDT... invented in 1938, forgotten in 1939"! Apparently it refers=
to the introduction of directional microphones a year or so after the intr=
oduction of BDT...
> The shuffler is used to convert phase difference to amplitude difference =
below 700hz. My understanding is that the conversion is derived from the p=
hysical spacing of the mic capsules, so it's not 100% clear how to translat=
e this into something like Greg's method of shuffling based on mid and side=
eq adjustments.
The method I use is predominantly intended for coincident miking, especiall=
y with either dual diaphragm designs or lower priced capsules where the pol=
ar response approaches omnidirectional at low frequencies, so all the low f=
requencies tend to pull towards the centre of the stereo image (very notice=
able when recording a string quartet that sets up with the cellist on the f=
ar right, for example). In fact, the method I described with Ozone is the s=
ame method shown in Figure 2 of the Gerzon article you have linked to, as r=
eprinted from Studio Sound magazine from some time in the '80s.
Gerzon discusses the effects of using it on a spaced pair that is relying o=
n time *and* amplitude differences to create a stereo image. In the article=
he refers to the problems it can cause for ORTF, but a similar set of issu=
es might arise if using it with BDT because, like ORTF, BDT uses time *and*=
amplitude differences (hence the baffle) to create a stereo image.
I am sure I have read about the type of shuffler you need for BDT. It may b=
e in Blumlein's original paper. If I remember correctly it uses interchanne=
l crosstalk as part of the process - feeding some of the left channel out o=
f phase to the right channel, and vice versa. Or perhaps that's what the MS=
process is doing anyway? I can't think the maths through right now...
> I've replicated Greg's setup in apQualir/apEQ as per Rob's suggestion and=
it looks like this will work pretty well, although I'm fairly certain apQu=
alizr isn't linear phase.
The effects of the phase errors due to non-linear phase EQ are described in=
the Gerzon article, and these can be negated if using linear phase EQ (whi=
ch wasn't readily available back when Gerzon wrote that paper - it existed,=
but was damn expensive!).
I find that with the iZotope/Ozone approach, being all linear phase and so =
on, the smallest amount of change is quite noticeable. A 1dB difference sha=
red between M and S (i.e. +0.5dB on the S, and a corresponding -0.5dB on th=
e M) is sufficient to move energy below 250Hz from 12 o'clock (centre) to 1=
0 o'clock (over to the left).
I don't remember Waves S1 being that effective, although I am in no hurry t=
o find out. There is no space on my system for Waves products these days:
Waves =3D over priced + stupid bundling policies + iLok key requirement =3D=
bah humbug!
- Greg Simmons
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