Songcast Direct

Overview

Songcast Direct (SCD) can be used to send decoded audio from any computing device to an OpenHome device. The Sender device is responsible for decoding audio to PCM, framing this in a simple protocol and making it available via a simple TCP server. The Receiver pulls data from this server, giving the Receiver complete control of the audio clock.

A Songcast Direct sender performs a broadly similar role to a Songcast sender. Applications that perform their own decoding to PCM will find SCD is easier to integrate and offers higher audio performance:

There is a constant TCP connection between sender and receiver, avoiding the need for application-level resend support
It uses the clock of the receiver, allowing for higher quality playback if the sender is on a desktop computer
It offers the receiver limited control over the stream – initially seeking within a track or skipping between tracks. While the primary control UI may reside on the sender device, this allows for integration with any Ir handset for the receiver.

Sample Code

Sample code exists for all key aspects of the Sender – server, framing and control.

Discovery

SSDP

First search for av-openhome-org:service:Product:2 then either

Check Attributes on Product service for "Transport"
Check Modes on Transport service for "scd"

mDNS

Search for _openhome._odp service to identify ODP endpoint

Control

Control of the Receiver is out of band, via the standard OpenHome network APIs. These are available over UPnP or ODP (a single connected socket).

Protocol

The protocol used for communication between Sender and Receiver contains the following message types:

Type	Description	Sent by
Ready	Signals availability and version support to other party	Receiver & Sender
MetadataDidl	Metadata relevant until the end of the next track. Will cause the receiver to reset its reported time indicator for the audio stream. Should be sent after all Audio for any preceding track. Uses DIDL-Lite.	Sender
MetadataOh	Metadata relevant until the end of the next track. Will cause the receiver to reset its reported time indicator for the audio stream. Should be sent after all Audio for any preceding track. Uses OpenHome Metadata format.	Sender
Format	Format for the following audio. Must be sent before any audio.	Sender
Audio	Decoded audio. Can only be sent after a `Format` message describing its sample rate etc.	Sender
MetatextDidl	Metadata relevant to a portion of a track. May be sent [0..n] times during a track. Uses DIDL-Lite.	Sender
MetatextOh	Metadata relevant to a portion of a track. May be sent [0..n] times during a track. Uses OpenHome Metadata format.	Sender
Halt	Indicates that a break in audio follows. E.g. at the end of a track with no further tracks to be played, or when the Sender has paused.	Sender
Disconnect	Indicates that the originator of the message is closing its SCD session. No further audio will be available. The message receiver should disconnect its socket.	Receiver & Sender
Seek	Indicates that the Receiver wants to jump to a different point in the current track. Is only sent for tracks that the Sender has indicated support seeking.	Receiver
Skip	Indicates that the Receiver wants to immediately jump to the next or previous track.	Receiver

From OpenHome

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