January 1, 2002
DVD Forum Issues Guidelines for DVD Digital Interfacing
By Richard Elen
Until now, a major gripe with DVD-Audio players among consumers and manufacturers alike has been the lack of a high-resolution digital output, in particular for audio. Copy protection issues have led to the majority of manufacturers (with a small number of exceptions) limiting their digital output capability to "CD-quality": 44.1 or 48 kHz sampling, while the DVD-Audio specification allows for superior quality sample rates, up to 192 kHz. As a result, consumers have had to rely on the digital converters built into the player, which are not always as good as they could be and are certainly not in the same league as external converters or those in a good receiver. Analog interfaces not always present on a receiver in this digital age have been required, with their attendant multiple cable runs and inconvenience. In addition, several features have often had to be duplicated between the player and the receiver, such as Dolby and DTS decoding, and bass management all of which potentially add to the likelihood of misadjustment in the living room.
When DVD-Audio was launched, it was suggested that while the first generation of players would not offer digital output capability, primarily due to the lack of agreement at the time on copy protection, later players would use IEEE 1394 (alternatively known as "FireWire" or "iLink") to deliver both audio and video to external processors, converters and other devices. In a 129-page document, published last September and revised the following month, entitled "Guideline of Transmission and Control for DVD-Video/Audio through IEEE1394 Bus", the DVD Forum has finally unveiled the protocols that will make possible a new generation of high-resolution digital converters and other digital signal processors. It also gives the DVD-Audio standard an edge over the competing Sony/Philips Super Audio CD format, where no plans have been announced to permit digital interfacing (although one manufacturer, Sharp, offers a very high-end separate SACD player and digital amplifier with a digital link between them).
The new interface guidelines make use of existing standards for video and audio transmission, notably the "MPEG2 Transport Stream" for video and associated audio content, and the "Audio and Music Data Transmission Protocol" (A&M Protocol for short) with its enhancements, designed to carry up to six channels of 24-bit digital audio content sampled at up to 192 kHz (plus MIDI data, interestingly) over FireWire. The DVD-Audio specification allows for six channels of 24-bit audio at up to 96 kHz sampling, or two channels at 192 kHz, so A&M allows room for future expansion.
MPEG-TS audio consideration in the document is limited to two channels of 16-bit linear PCM at 44.1 or 48 kHz sampling, ie "CD quality", but the guidelines note that "it will be necessary to define formats for higher quality audio", including higher sample rates, bit depth and number of channels (one is prompted to ask why, as multi-channel high-definition streams can be handled by the A&M protocol).
A player can use both protocols if necessary, transmitting multichannel audio via A&M while using MPEG-TS for video content. Ancillary data such as stereo downmix information, dynamic range control and emphasis, and control data can also be carried by the interface, optionally allowing these functions to be handled by the receiving device instead of the player. In addition to content, the system allows control signals to pass through one device to another, for example your TV accepting commands from a remote and passing them via FireWire to your DVD player, as well as status information to let your receiver know, for example, that the DVD drive is open so it should mute the audio.
On the subject of copy protection, the document is a little obscure. "When a DVD player transmits the video/audio data through IEEE1394 Bus," it says, "there are some rules of the copy protection system. Refer to the scheme and its compliance rules." It has been suggested that this means copy protection data will be carried across the link as-is, but there is likely to be more to it than that. Certainly, provision is made for audio copy permission, permitted number of copies, and relevant quality information to be carried in accordance with the DVD-Audio spec.
The A&M protocol allows for the transmission of both linear PCM (Pulse Code Modulation the standard format for the majority of digital audio other than SACD) and compressed audio. In the latter case, the guidelines note, the destination device "needs to have the capability to decode that data". This would allow not only for Dolby AC-3 and DTS encoded data, but also MLP (Meridian Lossless Packing, used by DVD-Audio) encoded material to be transmitted as-is, and decoded at the receiving end, in a preamp or receiver. However, elsewhere in the document provision is also made for the player to decode the content to PCM and to transmit that to the destination device via the A&M protocol. In this case, the destination amplifier or receiver would not need to be able to decode the MLP stream from a DVD-Audio player, for example, and decoding would be performed in the player. Potentially this would mean that an external device such as a receiver would only need to derive linear PCM audio from the A&M data stream without having to handle the decoding of multiple audio formats such as AC-3, DTS and MLP.
The implementation of high-resolution digital outputs on the next generation of DVD players is likely to have a significant positive impact on the consumer audio industry, especially at the high end. Simpler "DVD decks" employing the minimum of technology required to get a FireWire data stream out to an external processor will be one outcome, while at the other end of the interface cable, high-end manufacturers will be encouraged to develop high quality stand-alone converters, processors and receivers that will be better equipped to deliver the super-quality surround-sound promised by the DVD-Audio format.
Not to mention the fact that everything will be a whole lot easier to plug in!
Download the paper in PDF format here...