Everything Dolby
 The
most outstanding feature of the DTT2500 is the ability to decode Dolby
Digital soundtracks found on DVDs and satellite broadcasts. Compared to
systems that provide Dolby Surround Pro Logic decoding, Dolby Digital
is much more superior as it encodes each channel separately, providing
the sound designer with more channels to work with to include directional
cues.
Dolby Digital (AC-3)
The Dolby Digital standard supports as little as a single
channel, and supports up to 6 channels. The 5 channels supply full-frequency
sounds for the left, right, center, left surround and right surround speakers.
The 6th channel only stores bass and low frequency information that is
sent to the subwoofer - perfect for bass-heavy action films.
Dolby prefers to refer to this 6-channel configuration as
5.1, because the low frequency effects (LFE) channel doesn't store
the full range of sound - just the bass! Confusing, but that's the way
things have gone. :)
The DTT2500 decodes Dolby Digital using Zoran's ZR38600
Dolby Digital surround sound decoder IC. You can find out more from this
press release.
Dolby Digital on the DTT2500 works like a charm! Once I
connected the S/PDIF Output from the Live!Drive to the Dolby Digital S/PDIF
In and loaded up WinDVD in S/PDIF Output mode, the DTT2500 automatically
kicked into Dolby Digital mode.
I have tested the system with some action films on DVDs
like The Matrix, The Mummy, and not forgetting Saving Private Ryan - one
of the best DVDs in the audio department. With its Dolby Digital encoded
soundtrack, I could hear bullets flying all around as if I was in the
heat of the battle. The DTT2500 handles the decoding job with aplomb.
An undocumented feature of the DTT2500 is that when
decoding a Dolby Digital sound source and CMSS is set to Stereo, the decoder
changes to a Dolby 3-Stereo-like decoding. The center channel is spread
out to the front left and front right speakers.
Dolby Surround Pro Logic
Dolby Surround is an older technology that is still prevalent
in most films and TV programs. All these media deliver sound with only
2 channels - left and right! So the decoder has to extract the
center and surround channels from just the left and right channels that
are provided, and send them to the correct speakers.
One disadvantage of Dolby Surround is that there is only
one surround channel, so both surround speakers behind will be reproducing
the same sound (in mono).
The DTT2500 handles Dolby Surround decoding just fine. It
is able to decode Dolby Surround from the following inputs:
- Analog Line-In (Front)
- Digital DIN - only the front left, and front right channels are used
for Dolby Surround ProLogic decoding
- S/PDIF In
The first 2 cases are considered as analog inputs,
so CMSS needs to be turned off, before setting the DTT2500 to the Pro
Logic Analog-In mode. This is quite a hassle for me as the unit powers
up in CMSS Music mode, and I have to pres the CMSS button 4 times to turn
it off, then press the Dolby Surround button twice.
On the other hand, if the sounds comes into the DTT2500
from the Dolby Digital S/PDIF In, and the audio is from a DVD or
any other Dolby Digital material that is digitally encoded, the DTT2500
will automatically switch to Pro Logic Digital-In mode. CMSS can be used
in this case.
But, if the signal passed into the Dolby Digital S/PDIF In
is a normal stereo PCM signal, the DTT2500 needs to be set to Pro
Logic Analog mode manually.
Sounds a little confusing here, but little can be done because
there is no way for any decoder to tell if an analog signal (stereo) is
encoded in Dolby Surround or not. Unless it is from a Dolby Digital source
(like DVDs) which sends information digitally to the DTT2500 to tell it
that the track is Dolby Surround encoded.
Once I got the hang of it, it was easy to switch modes,
although I personally would prefer that the process would be less convulted,
especially having to turn off the CMSS mode before being able to switch
to Pro Logic mode.
Links
Take a look at this diagram
of the evolution of Dolby Digital at Creative's site. This diagram is
also printed on the DTT2500's box.
Here are some interesting links to Dolby's web site if you
want to find out more!
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