Equivalence systems: ORTF, NOS, DIN and EBS

Equivalence systems combine level and time differences on purpose. The two capsules are angled apart, so the pickup patterns create level differences, and spaced apart, so the path lengths create time differences. The two cues point the same way and add up.

The standard arrangements

ORTF takes its name from the French broadcaster that developed it. NOS comes from the Dutch broadcaster, DIN from the German standard.

Why this works

Each cue alone would need to be large to place a source at the edge of the image: roughly 18 dB of level difference, or 1.5 ms of time difference. With both cues present, each needs only a part:

• Because the shares add up, a source needs only part of each cue. For example, 6 dB together with 0.5 ms is already enough to place it fully in one loudspeaker.
• Narrow capsule spacings stay compatible with mono, because the time differences are small.
• The angle needed between the capsules is smaller than for a pure XY pair, so the pattern is used in a more favourable region.

What you gain over the extremes

Compared with XY, equivalence pairs sound more open and spacious, because the time differences blur the image slightly in a pleasant way. Compared with AB, they localise more precisely and sum better to mono.

What to remember

• The numbers are not magic. ORTF at 17 cm is an example that works well, not a law. The idea of the 17 cm is the distance between the ears; no physical reasoning requires it.
• The recording angle changes with spacing: moving to 30 cm narrows the angle noticeably.
• The same pattern with different angle and spacing is a different system, and the results differ in angle and in balance with the room.

Try it

Select ORTF, NOS or DIN in the stereo recording angle tool and then change the spacing to see how the recording angle responds. Use coincident pairs and spaced pairs for the two extremes.

Draft: written by an AI assistant, awaiting technical review.