![background image](http://html1.mh-extra.com/html/tandberg/en8000/en8000_reference-manual_3498818187.webp)
Audio Modes
Reference Guide: EN8000 MPEG-4 PArt 10 (H.264/AVC) Encoders
Page E-3
ST.RE.E10233.1
E.1
A Brief Introduction to Coding Standards
Where appropriate, the output transport stream can be made compliant with ATSC A53(E)
ATSC Digital Television Standard and DVB 101-154 v1.7.7.
E.1.1 MPEG
Standards
Background
The Moving Pictures Experts Group (MPEG) was formed in 1988 to generate compression
techniques for audio and video. Since then various flavours of MPEG audio have emerged.
For present broadcast purposes MPEG-1 layers II, and III are deemed suitable for broadcast.
MPEG-1 Layer II
This perceptual encoder is almost identical to MUSICAM, apart from headers. It differs from
Layer I by splitting the signal into 3 parts, and then filters each group as Layer I. This means
that each subband contains 3 groups of 12 samples, so a frame is split into 1,152 samples, a
few more than Layer I. The coder takes advantage of the three groups by comparing the
values and if found to be similar, or if temporal masking occurs the number of bits can be
reduced accordingly.
This is selectable from the Audio A and B menu.
MPEG-2 AAC–LC (Advanced Audio Coding)
It was designed to be non-backwards compatible to be able to achieve high audio quality at a
rate of 64 kbit/s/channel for 5.1 systems.
AAC consists of several tools other than those shown in the basic model:
Pre-processing – Signal split into 4 equally sized frequency bands and their level adjusted.
Filter bank – MDCT filter is used.
Temporal Noise Shaping (TNS) – pre-echo removal
Intensity stereo coding / coupling stereo coding -
Prediction – intensity difference between the previous and current frames coding.
There are 3 profiles available main(MP), low complexity(LC), and scalable sampling rate
(SSR).
The low complexity profile is used for broadcast, which allows the pre-processing and
prediction tools to be discarded and the TNS complexity to be reduced.
There are 3 profiles or versions available:
•
Main (MP): includes all of the tools that improve encoding efficiency.
•
Low complexity (LC): some tools are not allowed and others are restricted to enable this
algorithm to fit into the broadcast space.
•
Scalable Sample Rate (SSR): maximises temporal resolution (getting the high frequency
sounds at the right time) at the expense of coding efficiency. This is similar to Sony’s
ATRAC.
AAC has specifically been designed to leave behind the baggage of Layers I and II so that it
can be more efficient. The time it takes to encode audio differs between the profiles, main
being the most intensive and SSR the least.
If licensed, this is selectable from Advanced Audio 4A – 4D menus. The minimum allowable
delay is 100ms.