Stereo? Much more interesting than I thought.

[rob aylestone]

I've always been keen on recording acoustic stuff, live - in a range of spaces from horrible to excellent and I've noticed lots of the things I've always done now being either downplayed or misunderstood on various forums and social media groups. Recently I was getting really grumpy about people banging on about close miking in "Blumlein' without any regard to the history of stereo recording. Or where people bang on about using Decca Trees - without realising that adding and changing the format and number of mics, where they are and how they are oriented sort of makes it an array that has little to do with the original configuration.

The other thing is where people put up a stereo pair of mics, but then add piles of 'spot mics' - something I have always struggled with. You know where you have a bunch of people, or musicians spread over a large area and cannot hear a vital bit in the stereo mics, because they're just not loud enough to cut through.

I set up some mics and a stereoscope in my video studio and thought I'd record some stuff and see if I could work out what exactly is happening.
WARNNG - it's very boring, and contains results that might make your brain explode once you work out what exactly is going on, but I think it makes my failures with making spot mics blend it easier for me to now understand, and how the stereoscope does reveal some of the things that are going on. The thing that hit me was the realisation that if you were to record 4 sources in mono, then pan them left to right - that is crazily different to having two mics in an X/Y configuration and spreading the 4 sources apart. The stereoscope reveals why the sound is so very different - before even thinking about reverberation and effects.

The video is hard work, but I'd really be interested in what you think - did it make sense? Was I just sprouting rubbish or explaining it badly. The results of the stereoscope are not at all what I expected, but it really does make the room's impact on sound very visible.

 

[Manslick]

Never heard of Blumlein.

 

[Papanate]

The video is hard work, but I'd really be interested in what you think - did it make sense?

Made sense to me - although I didn’t understand what you meant by people using a Blumlein pair or Decca Trees but putting more mics up - - if you put more mics up is that just additional mics adding to the Blumlein Pair or Decca Trees - sounded like you were saying that it isn’t a Decca or Blumlein anymore - or maybe you are saying that is convoluting the mic techniques?

 

[DrewPeterson7]

I'll try to come back to this when I have the time, I'm always up for a good deep dive.

 

[ecc83]

You can all download a very good series of articles from www.soundonsound.com about the history, theory and practice of much of stereo recording. I shall try to find a link.

But how many of you have your speakers* setup to give a good solid stereo image. I have recently been fortunate enough to get a pair of Neumann KH-150s and the image improvement over my Tannoy 5As is staggering, especially BBC recordings.

Remember recordist's rule 101: "If you can't HEAR it. You can't FIX it!"

*Headphones have no place in a discussion about proper stereo.

Dave.

 

[rob aylestone]

The thing with Decca Trees and Blumlein is that the purpose was to produce a more accurate rendition of a sound source in a space. Blumlein in his patent detailed what suspiciously looks like X/Y - with two cardioids at 90 degrees, and the one most people think of as 'Blumlein' with the crossed fig-8s - using the ribbons that were in every large US and UK studio - the RCAs in the US and STCs in the UK. As a technique, for larger nice spaces, Blumlein does do a great job. However, using a pair of ribbons close in to the sound source - is not what he described in the patent. Well the mics are, I suppose, but all the stuff about relative levels between the front and rear lobes make no sense when you slap them in front of a guitar. What you get is pretty much what we all do with acoustic guitars and two mics - one does the darker sound from the body/hole and the other from the neck joint and frets. That is not Blumlein - especially so, because the ratio of front to rear is so unbalanced, the room sound hardly gets a look in. My space actually makes the rear of a Blumlein pair a phasey imprecise mess - not at all what he was getting praise for.

Alan Blumlein's patent is well worth reading, because even his wiki entry misses out some crazy information.

Think about his idea for a moment. We look at it as two fig-8 mics mounted at 90 degrees to each other - from above, an X, One mic faces away from centre 45 degrees, and the other mic faces away from centre at 45 degrees. In his design, the two mics were connected to the recording or reproduction change with a gizmo he called the 'shuffler'. Now the brain crunching bit.

Stop for a moment and take the X plan you have and rotate the X by 45 degrees. You now have one fig-8 mic pointing towards the subject, and the other fig-8 mic aiming left and right. You ALL know what that is! M/S - and his shuffler is what we now call a matrix, where we take the two mics and add in the side signal to the front, to get stereo width. Blumlein's version is M/S with a rear pickup. So Blumlein = M/S with a 45 degree spin and effectively three faders - one signal being split, once channel inverted and added back in - Blumlein's shuffler. The maths in his formula revolves around the square root of half - as in total signal calculated to split into 2, Cosines in effect.

Have you ever heard anyone make the link between the common X/Y with ribbons and M/S? I never made the link at all!

The Decca Tree is another even more complicated design that gets converted into not quite a Decca Tree by most people, because few of us have omni M50 mics. We probably have two cardioid capsules that recreate omni, back to back, but they do it differently to the M50 where the mic element is surrounded by a sphere. There is some crazy maths to describe how they even work - Pi features in the maths. Decca Trees and Trees with outriggers, are all 'enhancements' that might with some sound sources guve better width or depth perception. In this technique, which is pressure operation, not pressure gradient, The signal travelling around the circumference of a sphere rather than a conventional omni involves maths I can't do - I know the result is a trade off with HF performance. The Decca Tree worked, but Blumlein warned tampering with HF spoiled the stereo.

Far too complex brain work for me.

 

[ecc83]

🆘 Claim your FREE Stereo Miking eBook today!

As promised.

Dave.

 

[HumanPlanet]

I enjoy watching your videos. Always interesting topics, informative and well presented.

 

[rob aylestone]

Thanks for that - I actually like the ones where the results don't go to plan. The really annoying thing is the office upstairs. I've mentioned it far too many times. It used to be an insurance office, with carpet - but now it's a posh nail bar, and the laminate floor and office chairs just goes straight through.