Encoder explicit signaling

* AAC-Encoder

   - Revise explicit signaling method which is required for downsampled SBR.
     The parameter is optional and default configuration is implicit signaling
     for dualrate SBR as before. In case of downsampled SBR and HE-AAC explicit
     signaling is configured as default.
     Modified file(s):
        documentation\aacEncoder.pdf
        libAACenc\include\aacenc_lib.h
        libAACenc\src\aacenc_lib.cpp
        libMpegTPEnc\src\tpenc_asc.cpp
        libMpegTPEnc\src\version
        libSYS\include\FDK_audio.h
        libSYS\src\genericStds.cpp

Bug 9428126

Change-Id: I35138140346f17384d109ee5829be9ea5bab80fc

1 files changed, 44 insertions, 8 deletions

diff --git a/libAACenc/include/aacenc_lib.h b/libAACenc/include/aacenc_lib.h
index 6002366..fcdaca5 100644
--- a/libAACenc/include/aacenc_lib.h
+++ b/libAACenc/include/aacenc_lib.h
@@ -938,14 +938,50 @@ typedef enum
                                                   - n: Frame count period. */
 
   AACENC_SIGNALING_MODE           = 0x0302,  /*!< Signaling mode of the extension AOT:
-                                                  - 0: Implicit backward compatible signaling. (default)
-                                                  - 1: Explicit SBR and implicit PS signaling.
-                                                  - 2: Explicit hierarchical signaling.
-
-                                                  The use of backward-compatible implicit signaling is recommended if the user specically
-                                                  aims at preserving compatibility with decoders only capable of decoding AAC-LC. Otherwise
-                                                  use non-backward-compatible explicit signaling.
-                                                  Bitstream formats ADTS and ADIF can only do implicit signaling. */
+                                                  - 0: Implicit backward compatible signaling (default for non-MPEG-4 based
+                                                       AOT's and for the transport formats ADIF and ADTS)
+                                                       - A stream that uses implicit signaling can be decoded by every AAC decoder, even AAC-LC-only decoders
+                                                       - An AAC-LC-only decoder will only decode the low-frequency part of the stream, resulting in a band-limited output
+                                                       - This method works with all transport formats
+                                                       - This method does not work with downsampled SBR
+                                                  - 1: Explicit backward compatible signaling
+                                                       - A stream that uses explicit backward compatible signaling can be decoded by every AAC decoder, even AAC-LC-only decoders
+                                                       - An AAC-LC-only decoder will only decode the low-frequency part of the stream, resulting in a band-limited output
+                                                       - A decoder not capable of decoding PS will only decode the AAC-LC+SBR part.
+                                                         If the stream contained PS, the result will be a a decoded mono downmix
+                                                       - This method does not work with ADIF or ADTS. For LOAS/LATM, it only works with AudioMuxVersion==1
+                                                       - This method does work with downsampled SBR
+                                                  - 2: Explicit hierarchical signaling (default for MPEG-4 based AOT's and for all transport formats excluding ADIF and ADTS)
+                                                       - A stream that uses explicit hierarchical signaling can be decoded only by HE-AAC decoders
+                                                       - An AAC-LC-only decoder will not decode a stream that uses explicit hierarchical signaling
+                                                       - A decoder not capable of decoding PS will not decode the stream at all if it contained PS
+                                                       - This method does not work with ADIF or ADTS. It works with LOAS/LATM and the MPEG-4 File format
+                                                       - This method does work with downsampled SBR
+
+                                                   For making sure that the listener always experiences the best audio quality,
+                                                   explicit hierarchical signaling should be used.
+                                                   This makes sure that only a full HE-AAC-capable decoder will decode those streams.
+                                                   The audio is played at full bandwidth.
+                                                   For best backwards compatibility, it is recommended to encode with implicit SBR signaling.
+                                                   A decoder capable of AAC-LC only will then only decode the AAC part, which means the decoded
+                                                   audio will sound band-limited.
+
+                                                   For MPEG-2 transport types (ADTS,ADIF), only implicit signaling is possible.
+
+                                                   For LOAS and LATM, explicit backwards compatible signaling only works together with AudioMuxVersion==1.
+                                                   The reason is that, for explicit backwards compatible signaling, additional information will be appended to the ASC.
+                                                   A decoder that is only capable of decoding AAC-LC will skip this part.
+                                                   Nevertheless, for jumping to the end of the ASC, it needs to know the ASC length.
+                                                   Transmitting the length of the ASC is a feature of AudioMuxVersion==1, it is not possible to transmit the
+                                                   length of the ASC with AudioMuxVersion==0, therefore an AAC-LC-only decoder will not be able to parse a
+                                                   LOAS/LATM stream that was being encoded with AudioMuxVersion==0.
+
+                                                   For downsampled SBR, explicit signaling is mandatory. The reason for this is that the
+                                                   extension sampling frequency (which is in case of SBR the sampling frequqncy of the SBR part)
+                                                   can only be signaled in explicit mode.
+
+                                                   For AAC-ELD, the SBR information is transmitted in the ELDSpecific Config, which is part of the
+                                                   AudioSpecificConfig. Therefore, the settings here will have no effect on AAC-ELD.*/
 
   AACENC_TPSUBFRAMES              = 0x0303,  /*!< Number of sub frames in a transport frame for LOAS/LATM or ADTS (default 1).
                                                   - ADTS: Maximum number of sub frames restricted to 4.