TANDBERG EN8000, Справочное руководство

Страница: 188 / 196

Audio Modes
Page E-4 Reference Guide: EN8000 MPEG-4 PArt 10 (H.264/AVC) Encoders
ST.RE.E10233.1
MPEG-4 HE AAC
This standard builds on the MPEG-2 AAC. Some of the existing tools have been improved and
new ones added. All AAC objects (including the error robust modes) contain the Perceptual
Noise Substitution (PNS) tool. PNS detects noise in the signal and in the final bitstream a
noise flag is raised and the power level of the noise stored. The decoder generates a random
noise of the level indicated, and inserts this into the audio signal.
At lower bitrates this standard also includes a tool called Spectral Band Replication (SBR).
HE AAC is based around the aacPlus Codec, and
Figure E.1 shows Coding Technology’s
aacPlus coding system as used in Digital Radio Mondiale (DRM).
Figure E.1: CT-aacPlus Encoder for DRM
To enable the decoder to reproduce the high frequency content some additional information is
added to the bitstream, shown here as ancillary data. Using SBR can reduce the number of
bits by 30%.
The following quote comes from VIA licensing web-site.
‘HE AAC enables the delivery of 5.1 surround sound at 128 kbps, consumer-grade stereo at
48 kbit/s, and excellent quality stereo at 32 kbit/s or below.’
Any codec that uses SBR is going to be about 30% better than its base codec. This places HE
AAC very high on the agenda. The low complexity codec is still the one to consider for
broadcast purposes, as with today’s technology this is may be coded real-time and achieve
the required quality.
E.1.2 MPEG-4 Formats
MPEG-4 audio may be stored and streamed in many formats, which are described in the
standard. These include:
•
ADTS : This format places a header at the start of every audio frame, and provides
enough information for each frame to be independently decoded. It contains a fixed and a
variable header.
•
LATM/LOAS : As for ADTS this also places a header at the front of each audio frame in
such a way that each audio frame can be independently decoded. This is the required
format for compliant DVB streams (ETSI TS 101-154 v1.7.7), and is available on the
Encoder.
E.1.3 Dolby Digital
Dolby Digital is a proprietary algorithm from Dolby that forms part of both the ATSC and DVB
standard for digital broadcasting.
The encoder includes a psychoacoustic model to improve the quality. The signal is divided
into 32 multiple sub-bands, which correspond to the critical bands of the human ear. The basic
number of bits are set for each sub-band, but there is also a pool of additional bits that can be
allocated as required.
Audio
Out
SBR
Encoder
AAC
Encoder
Bit
Mux
Bit
de-
mux
AAC
Decoder
SBR
Decoder
Audio
in Filtered
audio
CT-aacPlus
Bitstream
Audio
data
Ancillary data Ancillary data