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All information in these pages is copyright (c) 1989-2003 by Roger Nichols. All rights reserved. Permission for personal reference only, and may not be reproduced by any method without written permission.

I Feel Disc-Connected, Part-2
By Roger Nichols

This month I get to talk about DVD-A. I just finished re-mixing Donald Fagen’s Kamakiriad in surround for DVD-A release. The plan was to do a “universal” or “hybrid” disc that will play on a DVD-Audio player and a DVD video player. But first, the mixes.

Transfers and Re-mix.

The original Kamakiriad tapes were recorded on a Sony 3348 48-track digital recorder at 48k/16bit. The Sony tape was transferred digitally into a 48k/24bit Pro Tools session. There was no reason to dump the audio into a 96k session, but there are advantages to dumping 16bit data into a 24bit session.

Since the tape was recorded at 48kHz, nothing above 22.5kHz made it onto the tape. Nothing you do in a 96k session will add anything above 22.5kHz. Even if you crank up 24dB of EQ boost at 30kHz, nothing will show up because there was nothing there to boost. Remember, if you multiply 24 times zero, the answer is still zero (in most U.S. public schools).

Now, there are plenty of reasons to take advantage of a 24bit session even though the original sounds were 16bit. First we look at fader levels. If you change the level of a 16bit signal by any amount there is a multiplication performed. Except for multiples of 6.02dB, which generate an exact 16bit answer, there will be fractions of a bit in the answer. 24bit gives you 256 steps between each bit change at the 16bit level. Any level changes, EQ, effects, reverbs, delays, panning, compression, gating, expanding or limiting performs mathematical computations that will produce 24bit results from the original 16bit audio.

You also must take into consideration that the final mixes will remain at the 48k sample rate because DVD-A allows any sample rate (44.1, 48, 88.2, 96, 176.4, 192), bit depth (16, 20, 24), and track number (1, 2, 3, 4, 5, 5.1) combination. All you have to be aware of is the space available, and the total available bit rate (9.6 megabits per second) of the DVD-A disc. This, however, may me the hardest part to overcome.

At first glance, the DVD disc seems to have plenty of room available. But after you start adding up the space necessary for six tracks in the surround version, 2 more tracks in the stereo version, and then the same space again for the video zone so someone can play your disc if they only have a regular DVD player, you quickly run out of room. Your bit budget must be monitored throughout the process to make sure that everything will fit. This is where encoding comes in.

Encoding

There are two reasons for encoding audio. The first reason is to make the files smaller so that they will all fit onto the disc. The second reason is to reduce the data rate. Since the files are smaller, the data rate is lower. If a five-minute song is only 50% of its original size, then during those five minutes of playing time only half as much data needs to be transferred to the decoder.

If you want to place 6 channels of audio at 96k/24bits onto a DVD without compression, the data rate would be 13.8 Megabits per second. This exceeds the maximum data rate of 9.6 Megabits per second for DVD-A. It just won’t come off of the DVD fast enough.

There are three different encoding (compression) schemes used for audio:

1) Dolby Digital, or AC-3 compresses the 5.1 channels of audio into a continuous stream of bits that contain information about all 6 channels. The data rate can be set between 224k and 640k bits per second. The reason you have a choice is so that you can decide whether fidelity or disc space is more important. An AC-3 data stream can contain from one to 6 channels of audio. Any configuration more than two channels include a LFE or Low Frequency Effects channel. The LFE channel is sampled at a lower frequency because it only needs to reproduce frequencies below 120Hz. There is some loss in quality when using AC-3. Only 48k is supported. AC-3 is used for surround audio tracks on DVD. All DVD players and receivers contain AC-3 decoders.

2) DTS also compresses 5.1 channels of audio into a continuous stream of bits. DTS uses a higher data rate. For 48k the choices are 1,536k or 768k bits per second. At 44.1k the data rate is fixed at 1,234k bits per second. DTS 96/24 allows even higher quality audio to be compressed. DTS is used for surround tracks on DVD and on DTS-CDs. DTS is available on newer DVD players and receivers. 96/24 DTS will playback as 48k on decoders that do not support 96/24 DTS.

3) MLP is a lossless compression scheme. That means there is no loss in audio quality when compressed with MLP. MLP is used for any channel configuration from mono to 5.1 surround. Audio files can be 16, 20, or 24bit with sample rates from 44.1k to 96k. MLP is used on DVD-A discs. All DVD-A players will decode MLP.

The final audio choice is no compression. DVD discs can contain stereo audio at 48k with a bit depth from 16 to 24 bits. DVD-A can accept un-encoded audio files from 16bits to 24bits, and from 44.1kHz to 192kHz. 192kHz is reserved for stereo audio only as anything else will exceed the maximum bit rate.

Make sure you always play back the audio after you have encoded it. One reason is to make sure that you didn’t place an audio file in the wrong channel. When you are encoding 20 songs each with six audio files, it can get pretty hairy. If you don’t have a professional decoder you can connect a digital audio from your DAW to the digital in of a surround receiver. This will work for everything except MLP. For MLP you will need to burn a DVD-A and play it back in a DVD-A player. So far there is no other choice.

Now that we have the audio straightened out, it is time to author a DVD-A with a companion video zone that will play on a regular DVD player. Last month I reviewed Disc Welder CHROME from Minnetonka Audio. CHROME will take care of all your DVD-A needs, and will even import the files for the video zone. The video zone files must be authored in a DVD authoring program. The problem is that DVD authoring programs and DVD players are designed primarily for movies with sound, not for sound with some picture menus. It becomes a giant pain to get everything just right on the DVD side.

You want to be able to have still pictures during the songs to show you which song you are playing, allow you to select another song and skip to it, switch between DTS and PCM and AC-3 while the song is playing, and not have any clicks between songs if you are playing the DTS track. This is such an ugly job that I am going to have to tell you how to do it next month, after I figure it out!Roger has been performing too long without a safety net. One too many lines of DVD scripting could plunge him into the abyss.