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.