Lossless Coding On Your PC

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ct0402With the first releases from a major record label and hardware introduced at reasonable prices, DVD-Audio is finally starting to make its way into the consumer market. The surround sound approach to mixing and mastering for music has been proliferating over the past few years. With these in mind, this is perhaps an appropriate time to look at the tools downstream of a multichannel music production that are essential to the creation of the final DVD-Audio product. Tools for authoring that are specific to the needs of DVD-Audio should certainly come to mind, as well as measures for digital audio watermarking.

Another important tool is the encoder for MLP, or Meridian Lossless Packing. MLP, of course, is the standard DVD-Audio format for lossless audio compression. While it isn�t mandatory for a DVD-Audio title, it will be necessary in cases whenever the maximum DVD-Audio bit rate would be exceeded (9.6 Mbps), and also when storage capacity does not allow for at least 74 minutes of playing time. For what will likely become the most widely adopted multichannel digital audio configuration for DVD-Audio � six channels (5.1 channels) at 24-bit and 96 kHz � MLP will certainly be needed.

Implementation of MLP encoders in manufacturer hardware is forthcoming, and the licensing is handled by Dolby. However, with just a PC running Windows (95, NT 4.0, or later) that has a competent CPU (100 MHz Pentium at the minimum), you can also opt for the software MLP Encoder, available directly from Meridian�s Web site (www.meridian-audio.com) for $7500. As you will soon discover, this features a fairly simple, user-friendly interface that is straightforward to operate.

Choose Your Inputs
The interface consists of a single program window, with navigation occurring essentially via six tabbed pages. The first page is for specifying the input files containing the PCM digital audio to be compressed. The file formats accepted by the MLP Encoder are WAV, AIFF, Sonic HD 1.5, and DVD-Audio LPCM. These files can consist of single or multiple channels (up to six), and one to six files can be input, depending on the number of channels within each of them. The LPCM file must be of uniform sample rate for all channels, so if you have a file with channels consisting of various rates, it is recommended that you break the file into individual WAV files. This is also the necessary approach if you want to use Sonic Solutions HD 1.4 soundfiles. Special programs are included with the MLP Encoder package for these purposes.

For the LPCM file, you will need to specify the sample rate, the number of channels for Group 1 and Group 2 (if it exists), and the word lengths for each of these groups. The DVD-Audio specification allows for two groups of channel assignments, such as front channels for Group 1 and surrounds and LFE for Group 2, so that separate sample rates and word lengths can be used for each of them. Such grouping arrangements must conform to the DVD-Audio standard.

Specify the Encoding Parameters
After inputting your uncompressed audio files, you then need to select the options for the encoder on the next page. In addition to specifying the number of channels, the choice for the encoded data rate should be the DVD-Audio maximum of 9.6 Mbps. There is also the option to add a 2-channel downmix. Additionally, the MLP data rate can be set constant, though this is not normally recommended as variable data rate gives more compression. Other options on this page allow you to encode only a subset of our input data, select the number of samples to be read into the encoder at one time, and specify advanced encoder commands.

Assign Your Channels
The next step is to map the channels from the input files to the DVD-Audio channels. This is based on the number of channels selected and the permissible channel assignments from the DVD-Audio specification. A total of 21 such assignments are permitted for one to six channels. For DVD-Audio, the channels are given sequential numerical designations, starting from 0 and ending with up to 5. Channel assignments are labeled from 0 to 20, and each specifies the allocation of a channel to its output (i.e., left surround, center, LFE etc.), as well as Group 1 and Group 2 arrangements. For example, all of the permitted assignments for six channels are shown in the table.

Because the grouping configuration is relevant only to preparation of the input files, for which you may want to have groups of differing sample rates and/or resolutions, for six channels there will be only two assignments to select from on the Channel Assignment page. On the same page, you accordingly map the input file and the appropriate channel to the designated DVD-Audio channel. It should be noted that, for the input files, the MLP Encoder indexes channels starting from 0.

As an example, in fig. 1, the channel assignment has been selected for six channels. These have been mapped to one input WAV file (master_1.wav) with four channels and another (master_2.wav) with two.

Tweak the Downmix
If you opted for a 2-channel downmix, then an opportunity exists to specify the downmix coefficients. Specifically, you can customize the phase and gain for each channel�s contribution to the downmix. The page devoted to the downmix options allows you to invert the phase for a given channel, if so desired, as well as specify the gain in dB (between +6 and �24 dB). Alternatively, you can use a linear factor with positive or negative polarity, and values between �2 and +2 are accepted. Similar adjustments can be made globally. Each of the two downmixed channels are configured separately.

Output and Check
For the MLP-encoded output file, the conventional file extension is .mlp. Along with the encoding, there is an output file checking stage, in which the MLP Encoder internally tests the results of the encoding to ensure that the compressed audio fully complies with the DVD-Audio standard.

There is also the option to listen to the results of MLP compression and the MLP downmix yourself (strongly recommended). To this end, the MLP Encoder performs the bitstream checking and also outputs the decoded audio into WAV files. There are a range of options for how you would like the output files to be created � a single multichannel file, Group 1 and Group 2 files, a 2-channel downmixed file, and separate files for each channel. Additionally, there is the opportunity to generate files consisting of down-sampled, stereo pairs of channels for cursory monitoring on a computer (for which the soundcard may only handle up to 48 kHz.)

When you have finished inputting your source files and specified all of the necessary options, you can then choose to encode and check in a single step, or select the encode and check processes separately. A separate run-time window will pop up, which displays the status of the encode or checking (such as time until completion), and the MS-DOS output of internal and relevant parameters and statements, which are also optionally written into log files. The statements that indicate success for encode and check are �Encoding completed3/4 and �PASS,3/4 respectively.

Some Final Notes
While the performance of the MLP Encoder is CPU clock speed-dependent, its use isn1/4t necessarily limited to the latest and fastest PCs. To achieve real-time encoding capability, you need use at least a Pentium III 850 MHz CPU or equivalent.

Verification of the 2-channel downmix has its own special considerations. Besides the importance of listening to the decoded, downmixed output to gauge the relative blend of the input channels, it is also necessary to check for any undesirable clipping resulting from signal amplification (specifying a positive dB gain in the downmix options page). This clipping occurs when the device playing back the decoded output has been overloaded. A specialized tool for checking downmixes is included with the software package.

The MLP Encoder can be run in MS-DOS mode as separate executable files for compression and proofing, with all of the options specified on the command line. In addition, the parameters and options specified for encoding can be saved into a configuration file for future use.

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