The Stereophonic Zoom – Michael Williams

The Stereophonic Zoom is a document which describes a variable dual microphone system for stereo recording. The system described acts as a spacing unit which can alter the distance between microphones as well as the angle in which they face. The interaction between distance and angle creates a variable stereo width.

Williams argues against the implementation of a singular system for stereo recording because “rather than reduce the choice of systems, an effort must made to increase the number of systems available. Each sound recording engineer must have the largest possible selection of systems to choose from, in order to solve the specific problems presented by a particular recording situation and, to express his own personal interpretation, as freely as possible,”.

He describes the accepted standard listening condition pictured below:

stereozoom

Williams continues by describing the importance of the listening environment in determining the characteristics of the stereo width of a recording. He makes recommendations for treatment following the IEC guidelines for a standard listening room.

Williams goes on to describe how sounds are localized, which I describe throughout my comparison of stereo mic techniques, but include timing differences, varying the intensity between two speakers, or a combination of the two methods.

As per my earlier postings, I opted for an X/Y mic setup, meaning that I’m relying primarily on amplitude variation between the two channels for my stereo.

Williams goes on to describe the specs of the “stereophonic zoom” recording device in great detail, indicating distance v. angle, frequency responses, and the relation between direct in reverberation sound through graphs, and describes the test phases and limitations of the device during testing. This section, while interesting has little to do with my way forward, and can be summarized by saying, on paper this system looks good, though like all things audio, it’s subjective and I’d have to hear results before I’m sold.

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Organ Stops: Narrowing Down What to Record

A manual of an organ typically has 61 keys, or just over five octaves. That means recording 61 samples per stop, and that quickly adds up, with some organs having upwards of sixty stops (like St. James Cathedral’s Organ).

That said, I’m lucky enough to be recording the beautiful organ at Our Lady of Sorrows in West Toronto, which has a much smaller selection, with only two manuals and a pedalboard, totaling 27 combined stops. Still that works out to approximately 1400 samples, so I still have to narrow down what exactly to record.

To narrow down my selection, I had to understand exactly how the nomenclature of organ stops works. There are five types of stops: Principal, Flute, String, Reed, and Hybrid.

Stops

Principal stops are the “classic” sound of the organ, where it’s not trying to imitate the sound of another instrument. These are the stops which I will focus most of my effort on.

Flute stops are, not surprisingly, made to imitate the timbre of flutes, and similar instruments, like the piccolo.

String stops again are fairly straight forward, trying to imitate the sound of classical string instruments (although I can’t help but imagine how a modern “electric guitar” stop would sound). Our Lady of Sorrows’ Organ doesn’t have too many of these, so I’ve essential cut them out.

Reed Stops are a broader category, imitating brass, woodwinds, and even the human voice. These stops also use reed pipes as opposed to flue pipes, where the wind is pushed through past a brass reed, which vibrates to create the sounds. Whereas flue pipes rely solely on the vibration of air.

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and finally, Hybrid stops, are stops which try to imitate the combined sound of two types of stops, such as principal and flute, or string and reed, and so on.

From here, each stop is labeled with a number. This number signifies the length of the longest pipe in a given rank (with some exceptions), and the longer a pipe is the lower the pitch of that pipe. 8′ stops are what are known as the “native pitch”, or “unison”, with each halving of length resulting in a doubling of pitch (4′ is an octave higher than 8′ and 16′ is an octave lower than 8′). There are also “mutations” which play at non-unison intervals. For example, a 2 2/3′ stop will play at the 12th (or an octave of the 4th) of an 8′ pipe.

Manuals

The organ at Our Lady of Sorrows has two manuals, the Great Organ and the Positiv. This is a bit different from most two manual organs, which consist of a Great and a Swell [1][2].

The Great Organ contains the principle sounds of the organ as well as occasionally housing a solo-instrument stop such as the trumpet.

While the Positiv contains more solo oriented sounds.

The Pedals are typically meant to give the organ a bass-frequency foundation, although occasionally also can be played for solo-melodies.

Luckily for me, neither these manuals nor the pedals use swell boxes, which means that there will be no need to account for variation in note velocities.

The Final List

With this knowledge, and the handy-dandy stop-list of Our Lady of Sorrows’ Organ, I’ve come up with a plan to record 8 stops on the manuals, and 3 stops on the pedals, for a total of 582 samples.

The Great Organ: Principal 8, Octave 4, Nasat 2-2/3, Superoctave 2, and Mixture V 1-1/3

The Positiv: Salicional 8, Gemshorn 2, Sifflöte 1

Pedal Board: Oktavbass 8 , Choralbass 4, Nachthorn 2

Comparing Stereo Microphone Techniques

This post will describe and compare a number of stereo micing techniques. In the process of recording my samples I will have to mitigate a number of challenges, which will become the basis for my comparisons. They are:

  • Fidelity: Since a major technical goal of the project is transparency, I want to recreate the experience of being in the room as truthfully as possible.
  • Phase: Because the cathedral is such a live space, I have some concerns about how I’m going to deal with phase. This will be looked at in further detail in a dedicated post.
  • Consistency: Due to restrictions and availability of the Church I will need a set up that is easy to recreate across multiple days.

1. Spaced Pair or A-B

(1)

A spaced pair is a pair of omni-directional microphones, typically placed equidistant from the sound source. The stereo effect is created by the timing differences between the recordings off each microphone (2). Simply put a sound that is closer to the left mic will reach the left microphone faster than it will reach the right microphone. This has a few advantages: Omni-directional microphones typically have a better frequency response than directional microphones (see: Earthworks TC30K [omni] versus the Neumann TLM103), and spaced stereo usually has a larger perceived stereo width (3).

However this comes with some disadvantages. Firstly, spaced pair recordings aren’t mono-compatible. When mixed down there are often phase issues and/or comb filtering (3). This is a concern because I want the samples to be useful in as wide a range of uses as possible, also because I’m as of yet unsure how successful my experiments in stereo sampling will go, so having a good mono fall back is crucial. Secondly, even in stereo there can often be “dead space” in the centre, and avoiding this requires experimenting with the spacing of the microphones (3). This becomes problematic because it will take more time to set up (and time is fairly limited), and it will be harder to reproduce than a coincident configuration.

2. X-Y Pair

File:XY stereo.svg(4)

X-Y stereo is achieved with two coincident or near-coincident directional microphones, aimed between 90 and 130 degrees apart (5). Unlike the A-B system, where the stereo effect is created by recording timing differences between the channels, the X-Y uses small differences in amplitude to create a stereo effect. The advantage of this is that it doesn’t add the recorded timing differences on top of the timing differences from the speakers to the ears, making for a “cleaner” listening experience (6).

This placement also has the advantage of being very mono-compatible (6). This is because the microphones are in the exact same position (on the horizontal plane), meaning that they should theoretically be perfectly in phase (7). They are also much easier to set up consistently because both microphones are in the same position (although it requires keeping track of the angle between the capsules).

The major disadvantages of this technique being that the use of cardioid mics colours the sound and can limit the frequency response in the low end (see frequency charts linked above), and coincident recordings can sometimes be harsher than their spaced equivalent (3).

3. Blumlein Pair

Blumlein Pair(8)

The Blumlein Pair uses two bi-directional, or figure 8, microphones angled 90 degrees from each other. This creates a stereo image similar to that of the X-Y placement, although it also captures the room ambiance because of the rear pickup pattern (9). This seems like an ideal scenario for the organ, because the room itself plays a large part in the sound of the instrument, although this would also remove the possibility of removing extraneous sounds through microphone placement.

This technique also has the disadvantage of using mics that are often affected by proximity, where farther placements of the mic will result in less low frequency response (10). This is a pretty big problem in this situation because the ranks are located around the room and pretty far away from possible recording positions.

One thing that I’m unsure of and need to test/look into further is how the bi-directional recording will work in regards to phase, because my initial feelings are that the microphone will naturally capture an inverted version of a sound when in hits the opposite side of the microphone. The reality is obviously going to be more complex than that, but it’s something I’m going to look into.

4. Mid-Side

File:MS stereo.svg(1)

Mid Side micing using a directional microphone aimed directly at the source as well as a coincident bi-directional microphone 90 degrees off axis from the sound source. The stereo image, much like the X/Y technique, is created by differences in amplitude as opposed to timing or phase (11).  The stereo is created through the phase interactions between the bi-directional microphone and the mid microphone. A sound that’s to the right of the unit will be phase inverted, meaning that you can extract the right signal by subtracting the side signal from the mid signal (12). The big disadvantage here being the extra post-production steps required to create a usable signal. But this is mitigated by the ability to control the stereo image in post production and it’s excellent mono-compatibility.

In terms of the frequency response of the recordings, it’s subject to the same issues as both X/Y recordings and Blumlein recordings, in that both directional mics and figure 8 typically have poor or coloured low frequency response. This can be mitigated by using an omni-directional microphone in the mid position which increases the low frequency response and spaciousness (13).

Both the Blumlein and M-S are similarly easy to set up with consistency to the X/Y configuration, which makes them well suited to the project.

Additional Sources:

Further Reading