diff --git a/ear/test/json.py b/ear/test/json.py
new file mode 100644
index 00000000..341f264c
--- /dev/null
+++ b/ear/test/json.py
@@ -0,0 +1,111 @@
+import attrs
+import ear.common
+import ear.core.metadata_input
+import ear.fileio.adm.elements
+from fractions import Fraction
+
+
+def value_to_json(value, include_defaults=True):
+    """turn EAR types into JSON
+
+    This is not complete but will be extended as needed.
+
+    The result can be turned back into objects using json_to_value
+
+    Parameters:
+        include_defaults (bool): add default values (True), or skip them (False)
+    Returns:
+        a json-serialisable object, composed of dict, list, bool, int, float,
+        str or None
+
+        objects are represented as a dictionary containing the keyword
+        arguments for the constructor, and a _type key naming the type
+    """
+
+    def recurse(v):
+        return value_to_json(v, include_defaults=include_defaults)
+
+    if value is None:
+        return None
+    elif isinstance(value, (bool, int, float, str)):
+        return value
+    elif isinstance(value, list):
+        return [recurse(v) for v in value]
+    elif isinstance(value, dict):
+        return {k: recurse(v) for k, v in value.items()}
+    elif attrs.has(type(value)):
+        d = dict(_type=type(value).__name__)
+
+        for field in attrs.fields(type(value)):
+            v = getattr(value, field.name)
+
+            if include_defaults or not _field_is_default(field, v):
+                d[field.name] = recurse(v)
+        return d
+    # types that need special handling; make this generic if we need to add more
+    elif isinstance(value, Fraction):
+        return dict(
+            _type="Fraction", numerator=value.numerator, denominator=value.denominator
+        )
+    else:
+        assert False, "unknown type"
+
+
+def json_to_value(value):
+    """turn the results of value_to_json back into objects"""
+    if isinstance(value, dict):
+        converted = {k: json_to_value(v) for k, v in value.items()}
+        if "_type" in value:
+            t = _known_types[value["_type"]]
+            del converted["_type"]
+            return t(**converted)
+        else:
+            return converted
+    if isinstance(value, list):
+        return [json_to_value(v) for v in value]
+    else:
+        return value
+
+
+def _field_is_default(field: attrs.Attribute, value):
+    """does an attrs field have the default value?
+
+    this depends on values being properly equality-comparable
+    """
+    default = field.default
+    if default is attrs.NOTHING:
+        return False
+
+    if isinstance(default, attrs.Factory):
+        assert not default.takes_self, "not implemented"
+
+        default = default.factory()
+
+    return type(value) is type(default) and value == default
+
+
+def _get_known_types():
+    known_types = [
+        ear.common.CartesianPosition,
+        ear.common.CartesianScreen,
+        ear.common.PolarPosition,
+        ear.common.PolarScreen,
+        ear.core.metadata_input.ExtraData,
+        ear.core.metadata_input.ObjectTypeMetadata,
+        ear.fileio.adm.elements.AudioBlockFormatObjects,
+        ear.fileio.adm.elements.CartesianZone,
+        ear.fileio.adm.elements.ChannelLock,
+        ear.fileio.adm.elements.Frequency,
+        ear.fileio.adm.elements.JumpPosition,
+        ear.fileio.adm.elements.ObjectCartesianPosition,
+        ear.fileio.adm.elements.ObjectDivergence,
+        ear.fileio.adm.elements.ObjectPolarPosition,
+        ear.fileio.adm.elements.PolarZone,
+        ear.fileio.adm.elements.ScreenEdgeLock,
+        Fraction,
+    ]
+
+    return {t.__name__: t for t in known_types}
+
+
+_known_types = _get_known_types()
diff --git a/ear/test/test_json.py b/ear/test/test_json.py
new file mode 100644
index 00000000..eff764c0
--- /dev/null
+++ b/ear/test/test_json.py
@@ -0,0 +1,55 @@
+import pytest
+from .json import json_to_value, value_to_json
+from ear.core.metadata_input import AudioBlockFormatObjects, ObjectTypeMetadata
+from fractions import Fraction
+
+
+def _check_json_subset(d1, d2):
+    """check that dictionaries in d1 are a subset of d2, and that all values
+    are representable in json"""
+
+    def is_type(t):
+        return isinstance(d1, t) and isinstance(d2, t)
+
+    if is_type(dict):
+        for key in d1:
+            assert key in d2
+            _check_json_subset(d1[key], d2[key])
+    elif is_type(list):
+        assert len(d1) == len(d2)
+        for v1, v2 in zip(d1, d2):
+            _check_json_subset(v1, v2)
+    elif is_type(bool) or is_type(int) or is_type(float) or is_type(str):
+        assert d1 == d2
+    elif d1 is None and d2 is None:
+        pass
+    else:
+        assert False, "mismatching or unknown types"
+
+
+@pytest.mark.parametrize("include_defaults", [False, True])
+def test_json_OTM(include_defaults):
+    otm = ObjectTypeMetadata(
+        block_format=AudioBlockFormatObjects(
+            rtime=Fraction(1, 2), position=dict(azimuth=0, elevation=0)
+        )
+    )
+    otm_j = value_to_json(otm, include_defaults=include_defaults)
+    expected = {
+        "_type": "ObjectTypeMetadata",
+        "block_format": {
+            "_type": "AudioBlockFormatObjects",
+            "position": {
+                "_type": "ObjectPolarPosition",
+                "azimuth": 0.0,
+                "elevation": 0.0,
+            },
+            "rtime": {"_type": "Fraction", "denominator": 2, "numerator": 1},
+        },
+    }
+
+    _check_json_subset(expected, otm_j)
+
+    otm_j_otm = json_to_value(otm_j)
+
+    assert otm_j_otm == otm