I don't understand the technology. If I play a HD-DVD, video by HDMI, and audio by SPDIF, why does the video take space from the audio?
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I think he meant space on the DVD itself. It has a finite storage capacity. This gets divided between video and audio. Standard DVDs had the standard audio as DD5.1 max, then added the 6th ch. DVDs have limits on data storage.
HD/BD-DVD came with lots more capacity, so more video bandwidth and audio bandwidth. Now, if a movie is long, like Transformers, to get a good video at 1080p without much data losses, audio storage space was not able to support the True HDDD.