2. What is ORTF Recording?

Introduction

In this blog I will be exploring the applications and acoustic theory of the ORTF microphone recording. As discussed in the first blog, this project is exploring how the theory of ORTF recording may be able to provide an alterative to binaural audio in interactive media.

History of ORTF Recording

ORTF is a stereo microphone technique that was developed by the Office de Radiodiffusion Télévision Française in the early 1960’s, from which it got its name (Francis Rumsey and Tim McCormick, 2014, pp. 517–519).

The Office de Radiodiffusion Télévision Française was a radio broadcasting and television department, appointed and heavily controlled by the French government in the mid-20th century, before being disbanded into the independent company Radio France’ following the ‘May 68’ protests that challenged the censorship and influence of the French establishment (Bachmann, 1988; Comité D’Historie, 2020).

Nonetheless, the Office de Radiodiffusion Télévision Française is credited for many innovations in audio technology throughout its operation, including research and development in stereophonics for radio and television (Remouit, 1974).

The ORTF Configuration

ORTF is a near-coincidence stereo recording technique that spaces two cardioid microphones 17cm apart with a microphone angle of 110° (Eargle, 2004; Francis Rumsey and Tim McCormick, 2014).

The term ‘coincidence’ means that the microphone capsules are as close together as physically possible, ensuring that sound arrives at each capsule at the same time.

A near-coincidence pair, therefore, is one that has some spacing between the capsules, but typically less than would cause considerable time differences in the sound arriving at each microphone. The 17cm spacing was carefully considered in its invention to roughly approximate the spacing between each ear of the human head (Francis Rumsey and Tim McCormick, 2014).

The microphone angle or ‘splay’ of the microphones, was also considered to approximate the directivity of human hearing (Francis Rumsey and Tim McCormick, 2014).

Diagram of ORTF Microphone Configuration

Here is an example of a ORTF recording of acoustic guitar. If you have headphones, please put them on to listen to this recording.

ORTF was the preferred recording technique of the Office de Radiodiffusion Télévision Française for most of their applications, including radio, live music performance, and television programmes (Remouit, 1974).

Since then, ORTF has also become a very popular microphone technique in studio recording, as it is credited with having a preferential 'spatial image' and sense of 'realism' over other stereo microphone techniques (Francis Rumsey and Tim McCormick, 2014). Additionally, the technique is great for recording wide sound sources, such as string quartets, orchestral sections, and piano, as it has a large recording angle (explained further in the 'Recording Angle' section, and in the 'Phantom Source Shift' blog) (Wittek and Theile, 2002).

Time difference and Level difference

Since the two microphones in ORTF are distributed on the horizontal plane (are level with each other, but spaced apart), we can use the terms Interaural-Time Difference (ITD) and Interaural-Level Difference (ILD) to explore the behaviour of sound arriving at each microphone capsule.

The two terms ITD and ILD, are more commonly associated with binaural hearing (the psychoacoustic term for human hearing), which will be covered in the following blog. However, when using the term ‘interaural’, we are generally referring to the difference between the left and right channels. It is useful to use these terms for ORTF recording as they will help explain what makes ORTF a ‘spatial’ and ‘natural’ sounding stereo microphone technique.

In essence, the ITD and ILD captured by the microphones help to rebuild the stereo image (apparent direction and spatial width of the source) when played back through headphones or loudspeakers.

Interaural Time Difference (ITD)

The ITD is the time difference of sound arriving at each microphone capsule. As discussed, this is determined by the spacing of the microphones, but it is also determined by the direction of the sound source.

For a source that is directly in front of the microphones, the ITD should be 0ms, but as the angle of the source increases the ITD will also increase to reach ITDmax (Wittek and Theile, 2002). The value of ITDmax for an ORTF pair is approximately 0.5ms (Williams, 1984), however, it is possible to calculate the ITD for any source direction using the equation:

ITD= m/c* sin⁡(θ)

Where 'θ' is the source direction, 'c' is the speed of sound in meters per second (ms-1), and 'm' is the mic spacing in meters to give the ITD result in seconds (Williams, 1984; Howard and Angus, 2009, pp. 107–115). The maximum ITD for binaural hearing (using a formula that accounts for head diffraction) is around 0.67ms. Therefore, ORTF is a close but not completely accurate approximation of binaural time difference (Howard and Angus, 2009).

Interaural Level Difference (ILD)

The ILD is the level difference of the sound arriving at each microphone capsule. This is determined by the microphone's directivity pattern (polar pattern) (Williams, 1984), which describes the area around the microphone that is sensitive to sound.

In ORTF the microphone polar pattern should be cardioid, which is sensitive to sounds that are in-front of the microphone (on-axis), but not sensitive to sounds behind the microphone (off-axis).

Due to the cardioid polar pattern and the microphone angle, as the sound source direction increases, the source becomes increasingly on-axis for one microphone, and increasingly off-axis for the other, which creates a level difference between the two microphones.

The ILD can also be calculated for any source direction using the equation:

ILD=20 * Log10((cos⁡(θ - β/2))/(cos⁡(θ + β/2)))

Where θ is the source direction, and β is the microphone angle (Williams, 1984). Therefore, ILDmax equals around 0.9 for a source at 90°, which when converted to the decibel scale gives a difference of approximately 20dB.

This value is very similar to binaural hearing, which also gives a value of 20dB ILD at 90° (Howard and Angus, 2009).

ORTF ITD and ILD against Source Direction. Image captured from Schoeps Image Assistant (SCHOEPS GMBH, 2022)

The Recording Angle

The recording angle describes the area in front of a microphone configuration that is captured by both microphones (Williams, 1987; Wittek and Theile, 2002). The recording angle for ORTF is approximately 100° (Eargle, 2004), as shown in the diagram below.

Sounds that are within the recording angle should sound clear and balanced, but sounds that do not fall in the recording angle will not be captured clearly by both microphones.

ORTF 100° Recording Angle

In William’s study of near-coincidence microphone configurations, he found that different combinations of microphone spacings and microphone angles could also reproduce a 100° recording angle (Williams, 1987). These different combinations were plotted on a graph to form a continuous line.

The idea here is that you could choose any point on the line, and it would tell you exactly what microphone spacing and microphone angle would produce a 100° recording angle.

Additionally, he tested other microphone configurations, which provided different recording angles between 60° and 180°. These results are visible in the graph below, taken from (Eargle, 2004). The benefit of this data is that sound engineers can select stereo microphone configurations based on the width of their chosen sound source.

Recording Angle Chart (Eargle, 2004).

Here is a great video that demonstrates the difference between spaced, coincidence, and near-coincidence microphone pairs. I definitely suggest watching through this video and seeing if you can use the graph above to calculate the recording angle for these different configurations.

Reflective Summary - Driscoll's What Model (Driscoll, 2007)

What?

This week I have explored the theory surrounding near-coincidence microphone techniques. In particular, I have explored the ORTF configuration, and discovered equations that enable you to estimate the ITD and ILD values of the configuration based on source direction.

I have also discovered the concept of the recording angle, and how this applies to the Williams localisation curves.

So What?

The project aims to focus on the ORTF Recording technique, since this has been documented extensively as a close match to natural binaural hearing. However, after learning more about the Williams Curves, perhaps the scope of the project could be expanded to include different virtual stereo microphone configurations?

This could be particularly beneficial for spatialised music rendering considering that music in video games is usually not tied to a physical position. What I mean by this is that music is typically 'non-diegetic', which means that it doesn't exist within the environment that is portrayed on screen. This grants the audio team more flexibility when spatialising non-diegetic sound.

Therefore, utilising different stereo-microphone techniques for different music tracks, could provide a virtual mixing environment that is similar to that of physical studio recording, thus providing the audio team with more creative tools for sound source positioning in video games.

What Next?

In my next blog I will be exploring the theory of Binaural Audio, and how it is currently used in Interactive Media. I will also be exploring the limitations of binaural synthesis, which I hope will provide more insight into the rationale of this project.

Conclusion

ORTF is a stereo microphone technique that uses a near-coincident pair of cardioid microphones, spaced 17 cm apart with a 110° microphone angle. The technique was developed by the Office de Radiodiffusion-Télévision Française in the early 1960’s for broadcasting and television but has since become a popular technique in studio recording due to its natural-sounding spatial image.

ORTF closely approximates natural human hearing (binaural hearing) by spacing the microphone capsule to the width of the average human head, and splaying the microphones apart from each other. This allows the technique to capture both ITD and ILD cues.

The recording angle is around 100°, meaning that the microphones capture the area ±50° in front of the microphones. Sound sources that are located outside of the recording angle will be off-axis to both microphones, and will not be picked up well in the recording.

References

Bachmann, S. (1988) ‘La suppression de l’ORTF en 1974: La réforme de la “Délivrance”’, Vingtième Siècle. Revue d’histoire, (17), pp. 63–72. doi:10.2307/3768797.

Comité D’Historie (2020) ‘Office de radiodiffusion-télévision française (1964-1974) (ORTF)’. Guide des sources. Available at: http://gtc.hypotheses.org/32723 (Accessed: 4 May 2022).

Driscoll, J. (ed.) (2007) Practicing Clinical Supervision: A Reflective Approach for Healthcare Professionals. Edinburgh: Elsevier.

Eargle, J.M. (2004) The microphone book. 2nd ed. Oxford: Focal Press.

Francis Rumsey and Tim McCormick (2014) Sound and Recording : Applications and Theory.

Howard, D.M. and Angus, J.A.S. (2009) Acoustics and psychoacoustics. 4th ed. Amsterdam Heidelberg: Elsevier.

Remouit, J. (1974) ‘Broadcasting in France’, in. Audio Engineering Society Convention 49, Audio Engineering Society. Available at: https://www.aes.org/e-lib/browse.cfm?elib=2491 (Accessed: 4 May 2022).

SCHOEPS GMBH (2022) Image Assistant | SCHOEPS Microphones, Schoeps Mikrofone. Available at: https://schoeps.de/en/knowledge/image-assistant.html (Accessed: 7 May 2022).

Williams, M. (1984) ‘The Stereophonic Zoom: A Practical Approach to Determining the

Characteristics of a Spaced Pair of Directional Microphones’, in. Audio Engineering Society Convention 75, Audio Engineering Society. Available at: https://www-aes-org.libproxy.york.ac.uk/e-lib/inst/browse.cfm?elib=11692 (Accessed: 4 May 2022).

Williams, M. (1987) ‘Unified Theory of Microphone Systems for Stereophonic Sound Recording’, in. Audio Engineering Society Convention 82, Audio Engineering Society. Available at: https://www.aes.org/e-lib/browse.cfm?elib=4963 (Accessed: 4 May 2022).

Wittek, H. and Theile, G. (2002) ‘The Recording Angle - Based on Localisation Curves’, in. Audio Engineering Society Convention 112, Audio Engineering Society. Available at: https://www.aes.org/e-lib/browse.cfm?elib=11307 (Accessed: 4 May 2022).