Virtual Loopback

Property	Value
Abstraction Id	{942BF02D-93C0-4EA8-B03E-D51156CA75E1}
Mnemonic	LOOP

Overview

A Virtual Loopback is a mechanism for two or more applications to communicate with each other over MIDI.

Suggested Uses

If you want to have loopback endpoints which are always available for routing between applications, but do not want them to be controlled by the applications themselves, this is the right kind of endpoint to set up.

Configuration

As with all configuration file changes, we recommend using the Windows MIDI Services Settings application, once we make that available. For now, you may edit the JSON directly. But please note that JSON is quite unforgiving: the format is specific, and all keys (including the GUIDs and property names) are case-sensitive. In addition, there’s no usable provision for comments in a JSON file, so we can’t include examples in the file itself.

That out of the way, here’s an example configuration section for the Virtual Loopback MIDI endpoints.

"endpointTransportPluginSettings":
{
    "{942BF02D-93C0-4EA8-B03E-D51156CA75E1}":
    {
        "_comment": "Loopback MIDI",

        "create":
        {
            "{0C1B3439-593F-4B7A-8950-4698D97B0897}":
            {
                "_comment" : "the above GUID is the unique association Id for this pair",
                "endpointA":
                {
                    "name": "Perm Loopback 1A",
                    "description": "This is a loopback I created in the configuration file",
                    "uniqueIdentifier": "3263827"
                },
                "endpointB":
                {
                    "name": "Perm Loopback 1B",
                    "description": "This is the b-side of the loopback I created in the configuration file",
                    "uniqueIdentifier": "3263827"
                }
            },
            "{B21B4973-3F85-48A0-8BA3-B35F44683D36}":
            {
                "_comment" : "the above GUID is the unique association Id for this pair",
                "endpointA":
                {
                    "name": "Perm Loopback 2A",
                    "description": "This is a loopback I created in the configuration file",
                    "uniqueIdentifier": "5150-1984"
                },
                "endpointB":
                {
                    "name": "Perm Loopback 2B",
                    "description": "This is the b-side of the loopback I created in the configuration file",
                    "uniqueIdentifier": "OU812"
                }
            }
        }
    }
}

Each loopback endpoint pair is identified by a GUID for the association id. The association GUID must be a valid unique GUID and shall not be an empty (all zeroes) GUID.

Key	Description
(Association Id)	Unique GUID for this pair of endpoints. The GUID itself is the property key under the “create” node.
endpointA	Data for the first endpoint
endpointB	Data for the second endpoint
(endpoint) name	Required. This becomes the transport-supplied name for the loopback endpoint.
(endpoint) description	Optional. This becomes the transport-supplied description for the loopback endpoint.
(endpoint) uniqueId	Required. This is a short (32 characters or fewer) case-insensitive unique Id for the endpoint. When combined with the loopback A/B prefixes in the service, it must be unique across all loopback endpoints in Windows. You can use the same unique id for each endpoint in the same pair, but not the same as other pairs.

Note: Loopback endpoints created in the configuration file cannot be removed by the API. Only loopback endpoints created via the API can be removed via the API. This is by design to help avoid applications changing user configuration.

Implementation

Internally, the Virtual Loopback is implemented as two endpoints which are cross-wired, so anything sent out to Loopback A arrives on the input of Loopback B, and vice versa. Each declared pair has an exclusive relationship, and there’s no practical limit to the number of loopback pairs you can define.