Revision as of 09:54, 10 May 2011

Songcast OHZ Protocol Specification (Version 1.0)

Introduction

OHZ, together with OHM and OHU, form the Songcast family of OpenHome protocols for transporting audio around a home network.

OHZ provides the ability for a uniquely identified Songcast Sender to redirect its Receivers to different OHM or OHU URIs.

How does a Songcast Sender use OHZ?

In order to use OHZ, a Songcast Sender must:

Publish an OHZ URI instead of an OHM or OHU URI
Broadcast the OHM or OHU URI associated with this OHZ URI whenever it is requested by a Receiver (solicited broadcast)
Broadcast the OHM or OHU URI associated with this OHZ URI whenever it changes (unsolicited broadcast)

Why would a Songcast Sender use OHZ?

There are two known reasons for the use of OHZ.

To allow devices that are both Senders and Receivers to manage complicated patterns of one-to-many communication, such as a Receiver listening to a Sender that is itself listening to another Sender. In this instance, the intermediate Sender can redirect its Receivers to the root Sender rather than rebroadcast Audio messages, which would accumulate network delays.

In order to allow a Receiver to continue listening to a Sender even though the configuration of the Sender has changed from Unicast to Multicast, or vice versa. Or when the Multicast channel is changed while the Sender is in Multicast mode.

Message Format

The OHZ protocol has the following message format.

Bytes	Name	Description
Header
4	Signature	0x6f, 0x68, 0x7a, 0x20 ('ohz ')
1	Version	1
1	Type	The type of message
		0 - Zone Query
		1 - Zone Uri
		2 - Preset Query
		3 - Preset Info
2	Length	The length in bytes of the whole message including this header
	Minimum	8
	Maximum	5000
Audio
1	AudioHeaderLength	50
1	Flags	Bit 0 - halt
		Bit 1 - lossless
		Bit 2 - timestamped
		Bit 3 - 0
		Bit 4 - 0
		Bit 5 - 0
		Bit 6 - 0
		Bit 7 - 0
2	SampleCount	The number of samples in this frame
4	Frame	The frame number
4	NetworkTimestamp	The time when the previous frame was sent over the network (if the timestamped flag is on)
4	MediaLatency	The length in milliseconds of the audio buffer
4	MediaTimestamp	The time when this frame was placed into the audio buffer {if the timestamp flag is on)
8	StartSample	The sample number within the current track of the first sample in this frame
8	TotalSamples	The total number of samples in the current track
4	SampleRate	The sample rate in samples per second
4	BitRate	The bit rate in bits per second
2	VolumeOffset	The volume offset in signed binary milli-dB (+/- 32dB)
1	BItDepth	The bit depth (8, 16, or 24)
1	Channels	The number of interleaved audio channels
1	AudioReserved	0
1	CodecNameLength	The number of bytes in the name of the codec
m	CodecName	The codec name (in UTF-8), where m = CodecNameLength
n	Pcm	Pcm samples, where n = Channels * BitDepth * SampleCount / 8
Track
4	TrackSequence	Sequence number for this track.
4	TrackUriLength	The length in bytes of the track uri
4	TrackMetadataLength	The length in bytes of the track metadata
p	TrackUri	The track uri, where p = TrackUriLength
q	TrackMetadata	The track metadata in DIDL-Lite format, where q = TrackMetadataLength
Metatext
4	MetatextSequence	Sequence number for this metatext.
4	MetatextLength	The length in bytes of the metatext
r	Metatext	The metatext in DIDL-Lite format, where r = MetatextLength
Slave
4	SlaveCount	The number of slaves
s	SlaveList	The list of slave endpoints, where s = 6 * SlaveCount

@@ Line 5: / Line 5: @@
 OHZ, together with [[Av:Developer:Songcast:Ohm|OHM and OHU]], form the Songcast family of OpenHome protocols for transporting audio around a home network.
-OHZ provides the ability for a uniquely identified Songcast Sender to redirect its Receivers to different OHM or OHU Uris.
+OHZ provides the ability for a uniquely identified Songcast Sender to redirect its Receivers to different OHM or OHU URIs.
 === How does a Songcast Sender use OHZ? ===
@@ Line 11: / Line 11: @@
 In order to use OHZ, a Songcast Sender must:
-* Publish an OHZ Uri instead of an OHM or OHU Uri
+* Publish an OHZ URI instead of an OHM or OHU URI
-* Broadcast the OHM or OHU Uri associated with this OHZ Uri whenever it is requested by a Receiver (solicited broadcast)
+* Broadcast the OHM or OHU URI associated with this OHZ URI whenever it is requested by a Receiver (solicited broadcast)
-* Broadcast the OHM or OHU Uri associated with this OHZ Uri whenever it changes (unsolicited broadcast)
+* Broadcast the OHM or OHU URI associated with this OHZ URI whenever it changes (unsolicited broadcast)
 === Why would a Songcast Sender use OHZ? ===

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