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

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s