Topic: Improved PAS technique - better imaging with good clarity

[al w.]

Just popping in to say I love Improved PAS! I've been using it for a while now and just made one of my fave tapes yet with it

hypers, 70°, 29cm

https://archive.org/details/eggy2024-05-04.alw?webamp=default

Thanks Gutbucket

[robeti]

Interesting topic! Checking in:)

[Gutbucket]

Thanks for the feedback and good to hear the Improved PAS technique has been working well for those employing it! 

Attached is the latest Improved PAS write up. 

To make it as easy as possible to use, the angle/spacing is now presented as a list of setup diagrams arranged by PAS angle, rather than a table of angles and spacings. The other change is that in addition to standard 2-microphone Improved PAS, the technique has been extended to 3-position PAS, which uses 3 or 4 microphones. Using 3 microphones, one is placed in the center between the other two.  Better is 4 microphones, with two of them sharing the central 3rd position in a coincident X/Y or Mid/Side arrangement. Tapers who are only interested in 2-mic PAS can just ignore the 3-position stuff.  I considered starting a new thread to discuss 3-position PAS separately, but since it's really just a further extension of the PAS technique from a single stereo pair to a mic array of 3 or 4 channels, I decided it would be best to keep everything together in this thread.

The 2-mic PAS portion of the new diagram-based PDF is based on the same data as the older table version, which was originally derived from the Sengiepelaudio.com on-line calculator tool.  That means you'll get the same answer if referring to the older Improved PAS table, the new 2-mic PAS diagrams, or the online Sengiepel tool.  The new 3-position PAS diagrams are based on data from the online Schoeps Image Assistant tool, which is capable of handling both 2 and 3 mic-position configurations. The 2-channel PAS solutions provided by Image Assistant are very close to those provided by the Sengiepel tool, but are based on a somewhat more extended data set. In comparison, Image Assistant provides additional information and control options (some of which are a bit hard to find, burred in the menu), yet tends to be less straightforward for most folks to use in comparison to the Sengiepel tool.  Additionally, I used a slightly different way of determining the appropriate offset between the PAS and SRA angles, some details of which are mentioned in the PDF write up.  Happy to discuss that and any other questions about Improved PAS in more detail if anyone is interested.

To skip the introduction and description of what is going and go directly to the diagrams, proceed to page 3 of the PDF for 2-mic Improved PAS, and page 5 for 3-position Improved PAS.

[Gutbucket]

Taking a step back to view the big picture- 

The extension of Improved PAS to 3-microphone positions illustrates the natural progression from 2-channel Improved PAS toward multichannel OMT microphone arrays using higher channel counts.  3-position Improved PAS setups using 3 or 4 microphones are the most recommended OMT3 and OMT4 setups.   

Four mics arguably represents the point of diminishing returns, and is as complex as many tapers choose go with a multiple-microphone array.  In simplified terms, OMT arrangements greater than OMT4 further extend this progression with the addition of a wide-spaced omni pair (when a spaced directional mic pair is already in use), a spaced directional pair (when wide-spaced omnis are already in use), and/or rear-facing microphones.  Each of those further additions are useful, yet how valuable when weighed against the increased complexity is the practical determination that each taper interested in exploring such multichannel techniques needs to make.  Four mics tends to be the right answer for most tapers looking to move beyond a straight stereo pair.

[morst]

This all assumes stereo playback, what if you planned to play back on surround systems?


 

[Gutbucket]

Works quite nicely!

The 3 position PAS stereo arrangements dovetail with mic-setups intended for playback over an L/C/R speaker arrangement.  Around 20 years ago when I started moving toward the use of 3 and 4 mic configurations I shifted to listening via both 2-ch stereo and surround playback, and have worked to maintain good compatibility of both since.  I find what works best for both playback schemes seems to be most right for either on its own.  It's a parallel evolution and serves as something of a cool double check.

Its simple to route the center Mid channel directly a center speaker / or sum the center X/Y pair to mono before doing the same.  The Side channel (or X/Y difference signal) can simply go unused, or you can play with sending some of that to L/R with a polarity inversion to one side.  Because its PAS, the direct-arriving front content is mostly coherent across the three front channels, it's just a physical extension from two channels using a phantom center to three using a physical center.

The surround channels are sort of a separate thing, best when sufficiently decorrelated from the front content.  Best to use additional rear-facing directional mics to sufficiently reduce pickup of sound from the front in those channels to allow for using enough playback level from them without the front content getting pulled around into the surround speakers. Wide-spaced omnis can be used to feed the surrounds and provide good room ambience, but even omnis tend to pickup a bit too much direct-arriving front content, and usually need delay applied to serve as surround channel feeds while perceptually keeping the front stuff in front.  But that application of delay is something of a crutch, and achieving sufficient level difference works much better.  My surround listening experiments indicate that something like a minimum of 6dB of forward attenuation is needed over that of an omni, but more is better because any additional attenuation just allows more freedom over the resulting mix levels.  Based on that I speculate that using a pair of wide-spaced subcardioids facing directly toward the rear in place of a pair of wide omnis might work well as a minimum-channel-count, dual-purpose stereo/surround recording arrangement that can provides sufficient forward attenuation to feed the surround channels appropriately while still achieving good wide-spaced omni-like low-frequency pickup and room impression.

Instead, I up the channel count and include an additional rear-facing baffled omni or supercardioid pair facing backwards in addition to the wide omnis.  The dedicated rear-facing channel(s) get folded int the stereo mix at low level to provide a touch of additional depth, room dimension and desirable audience reaction, or instead get routed directly to the surround channel speakers where they can typically be used at a higher level. Best of both words.

[morst]

.  Based on that I speculate that using a pair of wide-spaced subcardioids facing directly toward the rear in place of a pair of wide omnis might work well as a minimum-channel-count, dual-purpose stereo/surround recording arrangement that can provides sufficient forward attenuation to feed the surround channels appropriately while still achieving good wide-spaced omni-like low-frequency pickup and room impression.

I could try that just about any time that I don't have my wide card pair on stage... Hm. Have to clamp them separately to run them really far apart, but I can do that.

[Gutbucket]

Yeah, a wide spacing will help convey a more open room impression and natural sounding reverb quality when those channels are routed to the surround speakers.  If you need/want the deeper LF extension the subcards are providing, you can low pass that and route it to the front speakers if they have more LF capability than the surrounds.

When I first started playing around with making and installing DIY APE balls on DPA miniature omnis, my original intent was to try and emulate something of a subcardioid pattern for exactly this purpose.  Doing that was useful in a few ways but doesn't really work as well as a subcardioid because the resulting polar pattern isn't uniform across the full frequency range like it is for a subcardioid.

Until recently, I whenever I used APE balls on the wide-spaced miniature omnis in my multichannel OMT arrays (where it really doesn't matter as much anymore because I now have additional dedicated directional mics pointing both forward and back), I still stop to think about which way I'm going to point those APE equipped omnis.  It probably only matters for small channel count arrangements where I prefer to point them forward for stereo playback and backwards for surround channel use - but its a subtle difference at most.  Finally last year when reworking the rig I took the APEs off for good, which streamlined things a bit more out at the end of the 6' spread and eliminated that small dilemma!

[Gutbucket]

^
A follow up note directed to folks most interested in straight 2-channel improved PAS-

This thread is intend to promote Improved PAS and my primary aim is explaining it as simply and clearly as possible.  For that reason I'm going to encourage taking further discussion of Improved PAS's applicability to surround playback and its extension to higher channel counts over to the current OMT thread or to one of the dedicated surround recording threads here at TS where the focus is the discussion of more complex stuff such as that.  More than happy to continue the discussion there, which I feel will help keep this thread more generally accessible to everyone.

I used to think of Improved PAS and multichannel OMT-arrays as being separate approaches, and am now pleased to have found a way of dovetailing the two.  For anyone interested, the intent of improved PAS serves as a strong foundation for further extension to multichannel OMT.  But for most folks recording using a pair of microphones from a taping position in the audience, there is no need to take things further than the "automatic optimization" 2-channel Improved PAS provides.