_default _132_pages.indd - TELE-satellite International Magazine

FEATURE
The New Dolby
Dolby Digital Plus
Sound Format
Jacek Pawlowski
The race for better and better compression methods
seems to have no end. Once we start to feel comfortable
with the terms DVB-S, DVB-T, MPEG-2, we learn that
now DVB-S2, DVB-T2 and MPEG-4 (H.264/AVC) rule
the world. And now familiar Dolby Digital turns into
something else. In 2005, the newer and better format
had been standardized both by DVB and ATSC. The new
format is called Enhanced AC-3 (E-AC-3) or Dolby Digital
multiplex has started to transmit its audio exclusively
in Dolby Digital Plus format. This is French TNT HD
multiplex R5 with the following HDTV channels:
TF1 HD, France 2 HD, and M6 HD.
Dolby Digital Plus is a new coding tech-
nology allowing to save at least 33% (and
up to 50%) of the bandwidth used for
regular Dolby Digital. This leads to either
lower cost of transmission or give the
operator extra bandwidth for HD video.
Typical AC-3 bitrates are: 128~192 kbps
for stereo and 384~448 kbps for 5.1
Fig. 1. Bitrate required for top quality MPEG stereo, Dolby Digital (5.1 channels)
and Dolby Digital Plus (5.1 channels).
10 TELE-satellite — Global Digital TV Magazine — 04-05/2010 — www.TELE-satellite.com
channel content. Classical MPEG stereo
can occupy 160~256 kbps. Dolby Digital
Plus requires only 96 kbps for stereo and
224 kbps for 5.1 content. This relationship
is presented in Figure 1.
You do not have to be an expert to
notice that in place of a traditional MPEG
stereo, you can have 5.1 channels in the
same bandwidth if only you use DD+
coding.
But does better compression ratio
compromise audio quality? By no means!
DD+ codec is based on the original Dolby
Digital codec but it has several improvements
aimed at reduction of the prenoise
artifacts before sharp transients,
maintenance of phase relationships
between channels, as well as bit allocation
and quantization algorithm. As a
result, when DD+ signal is output from
the set-top-box, its quality is better than
that of classical DD despite the lower
bandwidth required for transmission.
Compatibility
Transmissions in Dolby Digital Plus
require a set-top-box (or “a receiver” as
we call it in TELE-satellite reports) to be
equipped with a DD+ decoder, otherwise
you will hear nothing. French viewers of
TNT HD multiplex R5 must use new DD+
compatible set-top-boxes.
But what about the output? Do you
need to upgrade to the latest A/V
receiver? No, not necessarily. If you have
one which is DD+ compliant, that’s great.
You should connect it to the set-top-box