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| 8
| 8
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| Maximum  
| Maximum  
| 5000
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|- align="left" style="color:#ffffff; background-color:#008833;"
|- align="left" style="color:#ffffff; background-color:#008833;"
-
! colspan="3" | Audio
+
! colspan="3" | Zone Query
-
|-
+
-
| 1
+
-
| AudioHeaderLength
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| 50
+
-
|-
+
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| 1
+
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| Flags
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| Bit 0 - halt
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-
|-
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-
|
+
-
|
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| Bit 1 - lossless
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|-
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|
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| Bit 2 - timestamped
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|-
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|
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| Bit 3 - 0
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|-
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| Bit 4 - 0
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|-
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| Bit 5 - 0
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| Bit 6 - 0
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|-
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| Bit 7 - 0
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|-
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| 2
+
-
| SampleCount
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-
| The number of samples in this frame
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|-
|-
| 4
| 4
-
| Frame
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| QueryZoneLength
-
| The frame number
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| The length in bytes of the zone
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|-
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| 4
+
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| NetworkTimestamp
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| The time when the previous frame was sent over the network (if the timestamped flag is on)
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|-
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| 4
+
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| MediaLatency
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| The length in milliseconds of the audio buffer
+
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|-
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| 4
+
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| MediaTimestamp
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-
| The time when this frame was placed into the audio buffer {if the timestamp flag is on)
+
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|-
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| 8
+
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| StartSample
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| The sample number within the current track of the first sample in this frame
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-
|-
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| 8
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| TotalSamples
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| The total number of samples in the current track
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|-
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| 4
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| SampleRate
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| The sample rate in samples per second
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|-
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| 4
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| BitRate
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| The bit rate in bits per second
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|-
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| 2
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| VolumeOffset
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| The volume offset in signed binary milli-dB (+/- 32dB)
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|-
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| 1
+
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| BItDepth
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| The bit depth (8, 16, or 24)
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-
|-
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| 1
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| Channels
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| The number of interleaved audio channels
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-
|-
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| 1
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| AudioReserved
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| 0
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|-
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| 1
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| CodecNameLength
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| The number of bytes in the name of the codec
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-
|-
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| m
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| CodecName
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| The codec name (in UTF-8), where m = CodecNameLength
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|-
|-
| n
| n
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| Pcm
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| QueryZone
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| Pcm samples, where n = Channels * BitDepth * SampleCount / 8
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| The queried zone, where n = QueryZoneLength
|- align="left" style="color:#ffffff; background-color:#008833;"
|- align="left" style="color:#ffffff; background-color:#008833;"
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! colspan="3" | Track
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! colspan="3" | ZoneUri
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|-
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| 4
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| TrackSequence
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| Sequence number for this track.
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|-
|-
| 4
| 4
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| TrackUriLength
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| ZoneLength
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| The length in bytes of the track uri
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| The length in bytes of the zone
|-
|-
| 4
| 4
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| TrackMetadataLength
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| UriLength
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| The length in bytes of the track metadata
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| The length in bytes of the URI
|-
|-
| p
| p
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| TrackUri
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| Zone
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| The track uri, where p = TrackUriLength
+
| The zone, where p = ZoneLength
|-
|-
| q
| q
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| TrackMetadata
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| Uri
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| The track metadata in DIDL-Lite format, where q = TrackMetadataLength
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| The URI, where q = UriLength
|- align="left" style="color:#ffffff; background-color:#008833;"
|- align="left" style="color:#ffffff; background-color:#008833;"
! colspan="3" | Metatext
! colspan="3" | Metatext

Revision as of 10:11, 10 May 2011

Contents

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
Zone Query
4 QueryZoneLength The length in bytes of the zone
n QueryZone The queried zone, where n = QueryZoneLength
ZoneUri
4 ZoneLength The length in bytes of the zone
4 UriLength The length in bytes of the URI
p Zone The zone, where p = ZoneLength
q Uri The URI, where q = UriLength
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
Retrieved from "http://wiki.openhome.org/wiki/Av:Developer:Songcast:Ohz"