(registered 2014-05-12, last updated 2014-07-31)

Type name: audio

Subtype name: aptx

Required parameters:

rate:
RTP timestamp clock rate, which is equal to the sampling rate
in Hz. RECOMMENDED values for rate are 8000, 11025, 16000,
22050, 24000, 32000, 44100, and 48000 samples per second. Other
values are permissible.

channels:
The number of logical audio channels that are present in the
audio stream.

variant:
The variant of apt-X (i.e., Standard or Enhanced) that is being
used. The following variants can be signaled:

variant=standard
variant=enhanced

bitresolution:
The number of bits used by the algorithm to encode four PCM
samples. This value MAY only be set to 16 for Standard apt-X
and 16 or 24 for Enhanced apt-X.

Optional parameters:

ptime:
The recommended length of time (in milliseconds) represented by
the media in a packet. Defaults to 4 milliseconds.
See Section 6 of RFC 8866.

maxptime:
The maximum amount of media that can be encapsulated in each
packet, expressed as time in milliseconds. See Section 6 of
RFC 8866.

stereo-channel-pairs:
Defines audio channels that are stereo paired in the stream.
See Section 3. Each pair of audio channels is defined as two
comma-separated values that correspond to channel numbers in
the range 1..channels. Each stereo channel pair is preceded
by a '{' and followed by a '}'. Pairs of audio channels are
separated by a comma. A channel MUST NOT be paired with more
than one other channel. The absence of this parameter signals
that each channel has been independently encoded.

embedded-autosync-channels:
Defines channels that carry embedded autosync.
Embedded-autosync-channels is defined as a list of
comma-separated values that correspond to channel numbers in
the range 1..channels. When a channel is stereo paired, embedded
autosync is shared across channels in the pair. The first channel
as defined in stereo-channel-pairs MUST be specified in the
embedded-autosync-channels list.

embedded-aux-channels:
Defines channels that carry embedded auxiliary data.
Embedded-aux-channels is defined as a list of comma-separated
values that correspond to channel numbers in the range
1..channels. When a channel is stereo paired, embedded auxiliary
data is shared across channels in the pair. The second channel
as defined in stereo-channel-pairs MUST be specified in the
embedded-aux-channels list.

Encoding considerations: This media type is framed in RTP and
contains binary data; see Section 4.8 of [RFC6838].

Security considerations: See Section 5 of [RFC4855] and Section 4
of [RFC4856].

Interoperability considerations: none

Published specification: RFC7310    

Applications which use this media type: Audio streaming

Fragment identifier considerations: None

Additional information: none

Person & email address to contact for further information:
John Lindsay <Lindsay&worldcastsystems.com>

Intended usage: COMMON

Restrictions on usage: This media type depends on RTP framing,
and hence is only defined for transfer via RTP [RFC3550].

Author/Change controller: IETF Payload Working Group delegated
from the IESG.