Re: HTF HD-DVD Review: Transformers (Highly Recommended)
Quote:
| I'm not sure that it does use more space. |
Here's the deal on TrueHD vs. DD+...
DD+ is a constant bit rate codec. TrueHD is a variable bit rate codec. DD+ can have up to 24-bit resolution at whatever bit rate the audio is encoded at. If the bit rate drops too low, there would, obviously, be compression related artifacts in the sound. At the 1.5 mb/s rate used by Paramount and Universal, most experts agree that DD+ delivers a virtually transparent reproduction of a 24-bit uncompressed master. Warner uses mostly 16-bit encodes, hence the DD+ bit rate of their discs mostly at 640 kb/s.
As the resolution of TrueHD goes up, so does the bit rate. At 16-bit/48 kHz resolution, TrueHD has an average bit rate ~1.4 mb/s. While the overall file size of a 16-bit TrueHD track is similar to a DD+ track at 1.5 mb/s, there are bit rate peaks over 3 mb/s in the TrueHD track. When the bit budget is allocated for a given disc, the peak bit rate for the video encode will be set based on the peak bit rate of all other material running simultaneously. That means all audio tracks based on PEAK bit rate, not average, as well as IME or anything else running at the same time as the video.
The video compression codec will have some bearing on decision for audio. VC-1, being the most efficient of the three used on HD DVD and Blu-ray, will generally allow for more space allocated to audio. Of all the movies that I am aware of over 2 hours on HD DVD that also contain TrueHD, all are VC-1 video encodes. For those that don't know, Transformers is AVC. Whether or not that had anything to do with not having enough space for a lossless audio track is open to speculation.