Dolby’s DP564 decoder
Apr 1, 2004 12:00 PM, BY SEAN RICHARDSON
The newest Dolby Digital (AC-3) multichannel decoder, the DP564, features Prologic II, the latest in matrix surround decoding technology. The new decoder is twice the size of its predecessor, the DP562), and yet it is much lighter. The front-panel display is larger, and Dolby added soft gel keys to the front-panel interface. On the rear panel are 75 BNC connectors for AES input and output. An optical input and an Ethernet connector for streaming audio were added. Compared to its predecessor, the upgrades add significant, much-needed utility to the device.
The DP564 digital Dolby Digital (AC-3) multichannel decoder from Dolby offers Starz Encore improved functionality, and aids in decoding stereo tracks to 5.1. |
Capabilities for broadcast Although the decoder is widely used in DVD authoring suites, it has attributes that can also be useful in a broadcast facility. The unit can downmix a 5.1 stream in many different ways, including Dolby Pro Logic II, Dolby Pro Logic, stereo (Lo/Ro, LT/RT), 3-Stereo, mono and Dolby Digital Surround EX. The decoder has two 75 AES inputs, an optical input and an Ethernet port for streaming audio. Another feature is the dynamic-range-compression simulation, which allows users to simulate line, RF and custom dynamic-range compression modes on program material. The decoder also displays AES3 audio information such as sample rate, bit resolution and Dolby metadata information. These features let the user operate the decoder in many ways.
Transmission check
The audio staff at Starz Encore Group has been using the device for
about a year. One way we used the decoder was in a transmission path by
encoding Dolby Digital (AC-3) for air. We regularly monitor our
transmissions using Motorola home set-top boxes and the Dolby decoder.
When monitoring the audio levels of 5.1 and stereo interstitial
materials on the home set-top boxes, we discovered a problem: The 5.1
materials were airing at significantly lower audio levels. But the
source material downmixed fine and checked out as decoded Dolby E.
Knowing that metadata is not applied at the outputs of a Dolby E
decoder, we did a quick check at the Dolby Digital (AC-3) encoder prior
to uplink. Then, we used the DP564, along with a Dolby LM100 broadcast
loudness meter for dialnorm measurement, to analyze the emission stream
and to check the metadata. The DP564 graphic display showed
high-quality decoded information and good input. Therefore, no errors
were logged within the stream itself. Next, we switched the
decoder’s status display to show the amount of dynamic-range
compression information that was present in the encoded stream for both
line and RF-decoder operating modes. Having this information at hand
revealed a problem. There was a significant amount of RF-mode
compression information (i.e., metadata gain words that indicated
“cut”) and a fair amount of line-mode compression present
in the encoded bitstream as well. The dialnorm value indicated by the
LM100 was -21dB. The authored metadata (included in the Dolby Digital
bitstream) as displayed on the decoder was -27dB for the stereo
material and -31dB for the 5.1 material. We set the front-panel display
of the decoder to RF mode (the output most consumers use) to look at
the amount of compression the encoder was applying. The answer was
clear. Incorrect dialnorm settings on the encoder had led to a larger
amount of compression on the 5.1 spot versus the stereo material
because the set-top box decoder’s operating mode defaulted to RF
mode. (RF mode has an extra 11dB, so compression is heavier at certain
thresholds.) The amount of compression also depends on the settings of
the dynamic-range control (DRC). There was also a difference in encoded
dialnorm metadata. We had to re-encode both the 5.1 and stereo spots
correctly for on-air output.
Audio mixing
In the audio-edit suite, we ran the decoder through a different set of
tests. First, we inserted the DP564 in place of our existing Dolby
DP562. We normalized five analog outputs to a surround pre-mix input on
a Multimax EX monitor and made the AES input patchable on the AES
patchbay. Next, we loaded the decoder’s interface software onto a
PC located in the suite and connected the decoder to the PC through the
PC’s nine-pin port and the decoder’s front-panel RS-232
port. (The software interface is only available for PCs.) The remote
software puts all of the decoder’s functionality right on a
desktop. In the audio suite, one of the functions of the system is to
check the downmix of a 5.1 stream, but checking how stereo mixes
translate is a good idea also. In fact, it’s a must, considering
the innumerable Dolby Pro Logic decoders in the world. As an old mixing
trick, we often use the system for decoding stereo tracks (such as
music) to 5.1 to add to the envelopment of the mix. In recent days, the
audio staff at Starz has used the decoder quite frequently for
constructing 5.1 mixes. Of course, we double-check any music or effect
decoded with the system by downmixing it again. Checking the downmix of
a program should be a routine task in any part of the broadcast chain.
Sure, you can set up a system with no confidence checking and use it to
decode stereo material live to 5.1, but this might cause problems once
the material reaches the consumer. It is too easy to apply delay twice
for the Hass effect in the surround channel — once in the upmix
and once in the consumer’s decoder. Any time you are working in
this environment, you should use the Pro Logic II Music Mode and adjust
the delay in all of the channels to zero.
Optical input
We found the optical input on the decoder useful during an experiment
with an HD receiver. We connected the optical port from a
high-definition set-top box to the optical input on the decoder, making
it easy to check metadata information.
Almost perfect
After using the DP564 decoder for nearly a year, we found that it
offers substantial improvement in functionality over its predecessor.
It’s a good tool for 5.1 mixing projects, and a reliable source
both for checking Dolby Digital (AC-3) stream information, and for
checking downmix information for digital broadcast program material.
Its remote software offers an improved user interface and makes it
easier to use. The only thing the decoder doesn’t have is
dialnorm, which plays an extremely important role in digital broadcast
and DVD mastering. If it included a dialnorm meter, it would be
perfect.
Sean Richardson is the manager of post production for the Starz Encore Group.
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