DVD players handle digital signals, but their output is now restricted to analog only. "The principal mission is to create a 1394 recommendation for playback-only DVDs," said Hideki Mimura, group manager at Toshiba Corp.'s data-storage development center, and chairman of a DVD Forum DVD-Video working group that is crafting the 1394 interface guideline, in tandem with a DVD-Audio working group.

"After we finish this step for playback-only DVD, the next step is discussions for recordable DVD," Mimura said.

The forum has proposed DVD-specific commands and expansions for addition to the IEEE-1394 standard. Some of these additional specs have already been folded into the standard, and some are under voting at the 1394TA group, which oversees the overall 1394 standard.

At the same time, Mimura's DVD Forum working group is preparing guidelines that clarify how to use the 1394 specifications to make 1394-compliant DVD products and to guarantee playback compatibility among appliances connected to DVD players. The ad hoc group has completed its discussions, and the forum is preparing the resulting document for upload to the DVD Forum Web site in late April.

The guidelines define two types of signal streams transmitted on the 1394 interface, video and audio. They also define how to control DVD players through a 1394 interface and detail the functions that need to be implemented in boxes such as digital TVs and audio amplifiers that control DVD products.

DVD handles various content — video, still images, standard audio and high-bit-rate, high-sampling audio. The video stream basically complies with MPEG-TS specifications. But since MPEG-TS is a broad standard, the forum has added some restrictions for the output from DVD players.

DVD video has a unique subpicture function that makes it ideal to output baseband video data in MPEG-TS, which is a mixture of decoded main video and subpicture video. But at present MPEG-TS does not have enough bus capacity to convey the baseband data as it is. If the signals are sent separately, an output appliance such as a digital TV set should have the capability to handle DVD-specific functions.

Re-encoding required

But the working-group members thought DVD-specific functions should be processed within DVD products, rather than by outside appliances. The guidelines therefore require DVD players to re-encode baseband video so that the data can be sent in MPEG-TS.

For the video stream, two-channel linear pulse-code modulation (PCM), at a sampling frequency of 44.1 or 48 kHz, is defined as the mandatory audio to be transmitted in the MPEG-TS stream, along with 16-bit quantization stereo.

DVD-video players, therefore, should have an MPEG-TS multiplexer as well as a re-encoder to multiplex video and audio signals for output through the 1394 interface as MPEG-TS data.

The A&M protocol was adopted to transmit high-bandwidth audio data such as that sampled at 96 kHz, and quantized in 24 bits in six channels; or at 192 kHz, 24 bits, two channels, enhanced. This protocol makes it possible to transmit these high-bit-rate audio formats and other audio formats supported by the DVD-Video and DVD-Audio specifications, such as linear PCM, packed PCM and compressed audio including AC-3, MPEG and DTS. When the output is directed to an audio amplifier, this stream will be used.

On another front, "Before the 1394 specifications and the guidelines even becomes available, however, a practical digital interface implementation for DVD products has to consider various copy-protection schemes adopted for DVD," said Mimura.

The DVD Forum is in discussions with related copy protection organizations to open a way to implement the 1394 digital interface on DVDs.