Audio Modes
Reference Guide: EN8000 MPEG-4 PArt 10 (H.264/AVC) Encoders
Page E-5
ST.RE.E10233.1
Dolby recommends stereo signals may be coded at 192 kbit/s, and 5.1 at 448 kbit/s, but other
rates are available if required.
E.2
Audio Coding Modes
E.2.1 Mono
This mode has a single audio channel that is encoded independently. It is seldom used in
broadcast as most viewing devices now have stereo speakers or headphones.
E.2.2 Stereo
This treats the incoming signal as separate left and right signals. Some coding algorithms will
look at the left and right signals and, depending on the correlation of these will combine the
signal into a mid and side channel to save on bits.
E.2.3 Joint
Stereo
This option is available for MPEG 1 layer 2 only. This applies a technique called intensity
coding. The human ear is not as good at locating higher frequencies as it is lower ones. The
use of this mode may introduce more artefacts than stereo.
E.2.4 Dual
Mono
Used if the left and right channels are carrying separate services. The encoder knows that it
must treat the two channels independently. The main use for this mode is for multilingual
transmission where decoder selects which language to decode on left or right.
E.2.5 Surround
Sound/5.1
A stereo signal produces a very focused audio field so unless the viewer is sitting in the
correct position, the audio reproduction suffers. More audio channels are required to generate
a larger audio field in which the viewer can listen.
The encoder expects the signal to arrive as:
•
AES 0: Left, Right (coded as a stereo pair)
•
AES 1: Centre, Low frequency Effect (coded as a mono channel with restricted frequency
on the LFE channel)
•
AES 2: Left surround, Right Surround.
E.2.6 Auto
Detect
This feature looks at the AES stream to determine if the stream is dual mono or other. If other
is indicated then the encoder assumes a stereo source.
E.3
Audio Coding Modules in Encoder
The EN8000 Standard Definition Encoder has two separate audio encoding modules:
•
Standard MPEG stereo audio encoding including MPEG-1 Layer II and Dolby Digital.
This module will be denoted as SA [Standard Audio] in the Figures.
•
Advanced audio encoding, which is currently restricted to the AAC algorithms. This
module will be denoted as AA [Advanced Audio] in the Figures.
•
Two sets of stereo audio services, A and B, can be sent to both modules independent of
the source whether it is analogue, digital or embedded on SDI.
