RFC 3190 (rfc3190) - Page 2 of 17


RTP Payload Format for 12-bit DAT Audio and 20- and 24-bit Linear Sampled Audio



Alternative Format: Original Text Document



RFC 3190                  RTP Payload Format                January 2002


   sample ordering and channel interleaving specified in [2] plus
   extensions specified here.  This document also specifies out-of-band
   negotiation methods for the extended channel interleaving rules and
   for use when an analog preemphasis technique is applied to the audio
   data.

1.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [6]

2. The need for RTP encapsulation of 12-, 20- and 24-bit audio

   Many high-quality digital audio and visual systems, such as DAT and
   DV, adopt sample-based audio encodings.  Different audio formats are
   used in various situations.  To transport the audio data using RTP,
   an encapsulation needs to be defined for each specific format.  Only
   16-bit linear audio encapsulation (L16) has thus far been defined.
   Other encoding formats have already appeared, such as the 12-bit
   nonlinear, 20-bit linear and 24-bit linear encodings used in the DAT
   and DV video world.  This specification defines the RTP payload
   encapsulation format in order to use the new encodings in the RTP
   environment.

3. 12-bit nonlinear audio encapsulation

   IEC 61119 [3] specifies the 12-bit nonlinear audio format in DAT and
   DV, called LP (Long Play) audio.  It would be easy to convert 12-bit
   nonlinear audio into 16-bit linear form at the RTP sender and
   transmit it using the L16 audio format already defined.  However,
   this would consume 33% more network bandwidth than necessary.  This
   payload format is specified as a more efficient alternative.

   The 12-bit nonlinear encoding is the same as for 16-bit linear audio
   except for the packing of each sampled data element.  Each sample of
   12-bit nonlinear audio is derived from a single sample of 16-bit
   linear audio by a nonlinear compression.  Table 1 shows the details
   of the conversion from 16 to 12 bits.  The result is a 12-bit signed
   value ranging from -2048 to 2047 and it is represented in two's
   complement notation.  The 12-bit samples are packed contiguously into
   payload octets starting with the most significant bit.  When the
   payload contains an odd number of samples, the four LSBs of the last
   octet are unused.  Parameters other than quantization, e.g., sampling
   frequency and audio channel assignment, are the same as in the L16
   payload format.  In particular, samples are packed into the packet in
   time sequence beginning with the oldest sample.




Kobayashi, et al.           Standards Track