Dolby Digital encodes PCM audio using AC-3 (Dolby audio codec #3) data reduction technology. In addition to the audio, the datastream includes embedded control information, called metadata, that provides details about the accompanying audio. These details include format, dynamic range, overall level and how to best playback or downmix the audio on a variety of systems. Both metadata and the audio datastream are created with the DP569 encoder.
Product PointsApplications: Recording/mixing studio; broadcast; DVD-Video and DVD-ROM authoring.
Key Features: DP569: RS-232 remote port; 1 kHz tone generator; DP562: tricolor LED meters; digital and analog XLR audio outputs; user-definable presets.
Price: DP562 – $3,600; DP569 – $5,000 (includes Dolby Remote Kit); Dolby Remote Kit – $300 if purchased separately
Contact: Dolby Laboratories, Inc. at 415-558-0200
+ Dolby Remote interface
– Tedious user interface
– Interface not intuitive
The Score: If you need to create AC-3 datastreams for broadcast or authoring, these boxes are the ticket.
Metadata parameters control many of the playback features – some more drastically than others. Setting these parameters correctly is vital for a mix to play back well on a variety of sound systems. While the mix may be created on a professional 5.1 monitoring system and can be heard in this pristine state by the consumer, it must also sound as good as possible on consumer playback systems. The metadata parameters help maintain compatibility with any consumer playback system, from 5.1 home theater to mono.
Features – DP569 Encoder
In 1998 the DP569 5.1 channel Dolby Digital AC-3 real-time encoder was introduced at a quarter of the price of the previous PC-based DP561 5.1 channel encoder. The DP569 offers 1 to 5.1 channels of encoding, as opposed to the earlier DP567. The DP569 includes broadcast features such as fault-monitoring circuits to warn of errors and a bypass mode with a switched output to let a redundant unit automatically take over in case of a systems fault or power failure – a must for broadcast applications.
Metadata and other parameters of the DP569 are accessed from a menu-driven LCD screen using cursor controls. The front panel has a 2.5″ x 0.5″ two-line, 16-character, backlit alpha/numeric LCD, eight menu-navigation keys, eight user preset recall keys, 14 tricolor LEDs (indicating various operational states, input status and channel activity) and an RS-232 eight-pin mini-DIN serial port connection (which can be used in conjunction with Dolby Remote). Dolby Remote is a Windows-based software application program that facilitates setting the parameters of the DP569.
The back panel has a power connector, three digital inputs for channels 1/2, 3/4 and 5/6 with loopthough connections – all on BNCs, a digital reference input and loopthrough connector on BNCs, a VITC input and loopthrough connector on BNCs, a bypass in and TTL delay input on BNCs, a main digital output and a switched digital output on BNCs, an LTC input on XLR, and four 9-pin D-sub connectors for: an auxiliary data input port, an RS-485 remote port, general-purpose inputs and general-purpose outputs.
From the top level of the menu, the channel mode, data rate, sampling rate, dialogue normalization value and the stream number ID are always displayed on the LCD. This provides quick at-a-glance feedback for these vital settings.
The channel mode refers to the channel format of the audio being encoded: mono, stereo, four-channel (such as unencoded LCRS or Quad, not necessarily “surround”) 5.1 surround, etc. The data rate is the amount of information the encoder spits out over time.
The DP569 supports sampling rates of 32 kHz, 44.1 kHz and 48 kHz. The sampling rate can be auto-set by having the unit refer to the digital input reference signal (digital input 1/2 or external word-sync via the digital reference input).
Dialogue normalization or Dialnormis a metadata parameter that has been the focus of many discussions. Itattempts to give a uniform playback loudness to all Dolby Digital encoded programs. In the AC-3 datastream Dialnorm is a 5-bit word that adjusts the output level of a Dolby Digital decoder in 1 dB increments from 0 dB to -30 dB. The Dialnorm word is repeated every 32 ms in the AC-3 datastream.
The measurement used for setting Dialnorm is based on a representative portion of dialogue at the average program level using LAeq, which is the long-term average of A-weighted sound pressure. (LAeq meters are available from Brüel & Kjær, CEL, Cirrus, Larson Davis and Quest Technologies.) The goal of Dialnorm is having all audio come out of a Dolby Digital decoder at the same relative loudness or, more specifically, to set all average program levels at -31 dB FS.
The Dialnorm value determines the threshold setting for dynrng (dynamic range) compression takes place. Dynamic range control is a metadata parameter that is carried in the bitstream by two words: dynrng and compr (compression). If a dialnorm value is set incorrectly, the dynamic range control won’t operate as intended. This can result in the program being played back with severe dynamic range compression applied.
All settings are accessed through a series of menus viewed on the LCD display via the menu navigation keys. The keys are familiar up/down/left/right cursors, accompanied by enter and escape keys.
Once in the menu structure, settings are made in an intuitive manner by hitting enter to go into a specific menu item or confirm a setting. Hit escape to exit a specific menu item or to revert to a previously enter value.
From the top level of the menu, the up/down cursor keys or the left/right cursor keys (up/down and left/right cursor keys are often used interchangeably: up = left, down = right) display the current values for: digital input status, reference input, timecode input, timecode control, clock source and coding delay, allowing quick access to these data.
Pressing the setup key takes you down one level into the menu structure that gives access to the menu titles: audio service, I/O control, encoder control, dynamic range, preprocessing, Bit Stream Information (BSI) parameters and operating mode.
Access each menu title by pressing enter when the title is displayed. Once in a menu title, the parameters available to the menu title are accessed. To change the value of a parameter, press enter when the desired parameter is displayed, use the cursor keys to change the value then either press enter to confirm the setting or escape to revert to the previous setting. To exit the values of a parameter hit escape. Using escape will bring the user one step up the menu tree.
Programming the encoding parameters in this one-at-a-time menu maze is akin to a nightmare. Given the interactive nature of the parameters, I found it needlessly time-consuming and impractical to have to jump from setting to setting and then back again – one at a time – to make the simplest of adjustments. I am not pleased that Dolby chose 15-year-old technology for its user interface. Programming all the parameters for AC-3 encoding, with their interactive nature, is an exercise in tedium. That said, there is another option.
Dolby has a been working on a Windows/PC-based software program called Dolby Remote, which acts as a remote control program for editing all the encoding parameters. I strongly recommend making metadata settings with Dolby Remote. It saves a lot of paging through menus, time and frustration.
Users with Macintosh computers are out of luck for the time being. The folks I spoke with at Dolby say they would like to develop a Mac version, but the software development department needs to be convinced of the need. Mac users, get those e-mails flying!
Features – DP562 Decoder
Sitting quietly at the end of all the data reduction and metadata control parameters is the DP562 Decoder. Its most basic function is returning the compressed AC-3 datastream back into 1 to 5.1 discrete channels of AES/EBU PCM digital or analogue audio.
In addition, the DP562 provides sophisticated monitoring controls, which aid in setting up a monitor system, various listening modes to check downmix compatibility between 5.1, four-channel, phantom center, stereo and mono playback and various output modes to check the results of Dialnorm and dynrng settings.
Monitor features include the ability to adjust the overall master level from +10 dB to ö90 dB in 0.5 dB increments. Each of the six outputs may be adjusted individually from +6 to ö18 dB in 0.125 dB increments. An internal pink-noise generator cycles through all six outputs to provide a test signal for aligning the SPL for each channel.
There are two delay settings – one for the surround channels and one for the center channel. The surround channel delay range is divided into two settings, one for 5.1 monitoring that is variable from 0 ms to +50 ms, and one for Pro Logic monitoring which is variable from +10 ms to +75 ms; both are adjusted in 1 ms steps. The center channel has a delay range of ö3 ms to +5 ms, also adjusted in 1 ms steps. If the center channel is set to a negative delay setting, a time warp is not required.
In reality the other five channels have a corresponding positive delay applied. The delay settings may be used in conjunction with speaker placement to help compensate for less than ideal monitor positioning. These adjustments and the ability to save the settings make monitor alignment very easy.
The output channels can be rerouted within the DP562 so that no matter what the channel configuration of the source AC-3 datastream, one can reroute the outputs to conform to the monitor’s setup.
There are also extensive bass management controls with a variable crossover that allow low-frequency information to be redirected to the speakers that best reproduce the frequencies. There are 12 bass redirection combinations and three crossover points to choose from.
Listening modes provide the ability to check how a 5.1 channel surround mix will play when it is downmixed to playback on a four-channel (may be LCRS or quad) system, a system without a center speaker, a stereo system or even a mono system. With these listening modes, the results of downmix settings made on the DP569 Encoder can be checked.
There are four output modes to check a mix’s interaction with dialnorm and dynrng settings – none, custom line and RF. With none selected, dialnorm and dynrng settings are ignored – this is how to listen to a mix exactly as it was created. Line mode gives the results of the Dialnorm setting and the dynrng setting.
The corresponding gain correction is applied, as indicated by the Dialnorm setting, and the appropriate dynamic range compression algorithm is monitored. RF mode is used to check the results of a mix being RF-modulated as will be the case for some TV inputs. RF mode incorporates additional compression, limiting and gain reduction in conjunction with the Over Modulation Protection (OMP) setting to ensure that an RF input is not over modulated
Unfortunately the DP562 incorporates a similar menu-driven user interface to the DP569. All settings are accessed through a series of menus viewed on a 2.5″ x 0.5″ two-line, 16-character backlit alpha/numeric LCD display via menu navigation keys – once again, 15-year-old technology.
The DP562’s navigation keys are even more limited than those of the DP569. There are only up/down cursor keys and an escape key. To make matters even more awkward, the parameter selection and confirmation paradigm differ from the DP569’s. Without an enter key, settings are confirmed by hitting the escape key. It’s actually counterintuitive!
Currently there is also no remote control software aid for the DP562 like there is with the Dolby Remote program for the DP569. With the Decoder, one must plod through the menu maze. Fortunately, the menu structure for the DP562 is not as extensive as the DP569. While Dolby has impressive technical engineering and scientific skills, I wish the company would put more effort into its user interface design.
The first step in creating an AC-3 datastream is determining the format for which the datastream will be used (DVD-Video, DVD-ROM, DTV, etc.). This determines the data rate and the sampling rate.
The coding or channel mode is set based on the channel configuration of the mix. In this case assume I’m encoding a 5.1 channel mix. My coding or channel mode will be set to 3/2, three front channels and two surround channels. There is an LFE track, so the LFE enable setting will be set to enable. When working with the Dolby Remote program, note that these settings allow an upper-frequency response of 18.05 kHz.
The next setting is the bitstream mode, which describes the type of mix being encoded. In all, there are nine different types of services. In my example I am creating a full mix so my setting will be Main Audio Service: Complete Main.
The dialogue normalization value is next. An LAeq meter can help to determine a Dialnorm setting by measuring an average section of the mix during a playback. Without an LAeq meter, the setting can be roughed in by either eyeballing a dB FS meter or by checking the gain/gain reduction meters on the dynamic compression page of the Dolby Remote program, or by comparing the program to a dialnorm reference disc (available from Dolby).
Coding delay is the next setting. Even though the DP569 is a real-time encoder, it takes some time to process the signal. This processing time is known as latency. The minimum encoding latency is 179ms. That’s 5:29 (frames: subframes) referenced to 29.97 video. The coding delay for the DP569 can be set internally anywhere between 179 ms and 450 ms or it can reference an external delay source via the TTL delay input.
It should be noted that the DP562 also has a decoding latency. The model I reviewed had the decode latency set to 15 ms or 36 subframes referenced to 29.97 video (about 1/2 a frame). This decode latency complies with the IEC61939 spec. A new IEC spec, however, calls for the decode latency to be set to one frame or 33 ms at 48 kHz for AC-3. In May 1999 Dolby released a software upgrade for the DP562 to comply with the newer IEC spec.
If you are mixing to picture and monitoring the audio through the DP569 and DP562, you will notice that the audio is a minimum of 5:65 (frames: subframes) or 194 ms late (179 ms + 15ms)! For the sake of the mixing session, you must either offset the picture to delay it by six frames or offset the audio to advance it by six frames to regain audio/video sync. Awkward, but doable.
The AC-3 datastream can be recorded as an AC-3 file on a hard drive, authored to a DVD-Video or DVD-ROM disc, broadcast as a part of an HDTV or a DTV signal or transmitted as part of a digital cable signal or digital satellite signal, to name a few.
AC-3 is not intended to be recorded as an AES/EBU signal. Instead, AC-3 data should be thought of as a final delivery format, much like an LP, cassette, CD or VHS tape. These formats are created at the end of the production or post production process for distribution to the masses. This is what AC-3 data is intended for. As a soundtrack on a DVD-Video, DVD-ROM or as the final step in an HDTV or DTV broadcast. Audio should only be encoded into this format when everything else that needs to be done to it, has been done to it (i.e., post production, mastering, etc.), as the AC-3 data compression algorithm will not stand up to repeated encode/decode operations.
With the Dolby E DP571 encoder and DP572 decoder currently available, it is hoped that some gaps left in the production and post production process using AC-3 encoding will be filled.
The data compression algorithm used in Dolby E is meant to be more robust than AC-3 and able to endure multiple encode and decode operations without significant audio degradation, as is found with the data compression of AC-3. Thus Dolby E encoded multichannel audio can be worked on in production and post production environments where audio is normally recorded and rerecorded several times before the final audio product is created.
With Dolby E now available should you buy a DP569 and DP562? Yes. If you need to create AC-3 datastreams for broadcast or authoring, these are your boxes; Dolby E will not do this.
If you mix audio for 5.1 but do not create AC-3 datastreams, the answer is not as easy. If you need to check your mixes through the encode/decode process and specify metadata settings, today you will need these boxes.
To learn more information about Dolby Digital, AC-3 and Dolby E, check out Dolby’s web site at www.dolby.com. The company has posted a vast collection of technical articles on all of the topics covered here.