LPAC Archiver

Version 1.4

Contents

Introduction

• Feature Highlights
• How does it work?
• History of Modifications
• Playing LPAC Files

Encoder Mode

• Type of Compression
• Joint Stereo
• Random Access
• Backward compatible File Format

General Settings

• Target Folder
• Overwrite
• Delete Source Files
• Create Report

Information

• Legal Notice
• Contact

Introduction

LPAC is a codec (coder / decoder) for lossless compression of digital audio files. "Lossless" means that any compressed file can be decompressed in a way it will be bit-wise identical with the original. This is the main advantage of LPAC compared to lossy formats like mp3 or realaudio. On the other hand, lossy codecs can achieve higher compression ratios. For example, mp3 at 128 kbit/s achieves a (fixed) compression ratio of 11, whereas LPAC's compression ratios range from 1.5 to 4, strongly depending on the audio material. Typically they are around 2 for pop music and 2.5 for classical music. This may not seem much, but remember you will get back every single bit, no matter how often you subsequently compress and decompress a file. It is true that general archivers (Zip, LZH, gzip) are lossless, too, but they often achieve nearly no compression on audio files.

LPAC is your choice if you want to save disk space without any quality drawbacks introduced by lossy codecs like mp3. The LPAC file format (*.PAC) is designed to be compatible and lossless across different platforms and processors. There are versions of LPAC for Windows, Linux and Solaris. LPAC Archiver is the user interface of the LPAC codec for Win32 (Windows95/98/Me/NT/2000). The codec itself is supplied as a DLL (lpaccodec.dll). For usage with LPAC Archiver 1.3, the codec version must be 3.00 or higher.

Further information on the latest versions of LPAC Archiver and the LPAC codec can be found on the LPAC Homepage.

Feature Highlights

• Supports the compression of PCM wave files (*.WAV) with either 8, 16, 20 or 24 bit resolution at any sampling frequency.
• Several files can be compressed in a single run.
• Six different compression modes from highest speed to highest compression.
• Graphical user interface with several options.
• Backward and forward compatible with former versions.

A detailed description of all features is given in the following sections.

How does it work?

LPAC stands for Lossless Predictive Audio Compression. It uses a method called Linear Prediction to calculate an estimate for the audio signal's current sample value from previous samples. If the prediction is good, the difference between the estimate and the actual sample will be very small. These difference values are coded using so-called Huffman coding, which exploits that the possible values appear with different probabilities.

More information on lossless coding can be found in the document Verlustlose Audiocodierung (PDF, in German).

History of Modifications

• 1.40: Provides a check for accurate decoding and a priority selection. In addition, items in the file list can now be removed, and there should be no overflow anymore if if the total size of all files exceeds 2 GB.
• 1.30: Supports the LPAC codec 3.0 which offers improved compression efficiency and increased speed. The Custom Compression Options are adapted to the new codec's features.
• 1.20: Progress bar is updated more frequently. More detailed report file. Some internal modifications.
• 1.10: Supports the LPAC codec 2.0 which offers compression of 24 bit files, CRC checksum and some general compression enhancements. For compatibility reasons you can choose a backward compatible file format for 8 bit and 16 bit files. There is also a new predefined compression mode Medium Compression, whereas the former Normal Compression mode is now called Simple Compression. Furthermore, all compression options are now saved in the configuration file.
• 1.01: Some minor internal modifications.
• 1.00: New user interface. The File Options dialog was removed, but its overwrite selection was integrated in the main dialog. In addition, you can specify a target folder for all output files and delete source files automatically. These options are now saved to a configuration file. The codec itself is supplied as a DLL. The windows help file (.hlp) was replaced by this HTML document.

Playing LPAC Files

With the LPAC plug-in for Winamp you can use Winamp to play LPAC files. This gives you the opportunity to use features like playlists, equalizers and much more.

Encoder Mode

In the Encoder Mode section you can select the type of compression, enable Joint Stereo and Random Access, choose whether to use the backward compatible file format, and set special Custom Compression Options.

Type of Compression

• Fast Compression
  Uses fast polynomial prediction. Requires only integer operations, but achieves poorer compression results than the other modes.
• Simple Compression
  Uses adaptive linear prediction with a small prediction order. Not quite as fast as Fast Compression, but significantly more efficient.
• Medium Compression
  Uses adaptive linear prediction with a medium prediction order. Often a good compromise in terms of compression speed and efficiency.
• High Compression
  Uses adaptive linear prediction with a higher prediction order. Also performes a Fulls Search for Order.
• Extra High Compression
  Uses adaptive linear prediction with the highest prediction order. Performes a Fulls Search for Order and uses Adaptive Entropy Coding. Slow but most efficient.
• Custom Compression
  This mode offers the possibility to set some options manually. These options are discribed in the section Custom Compression Options.

Joint Stereo

Tries to make use of the dependencies between the two channels of a stereo signal, which leads to better compression if the audio file mainly contains components which lay in the center, e.g. dominant vocal parts. This feature is not available in Fast Compression mode. In other compression modes, it should always be enabled.

Random Access

Generates a compressed file whose blocks can be accessed without decoding previous blocks. Such files are supported by the LPAC plug-in for Winamp to allow faster forward and rewind during playback. This feature slightly increases the file size and is not available in Fast Compression mode. If you just want to archive files in order to save disk space, you should not use this feature.

Check for accurate Decoding

Decodes the compressed file after encoding to ensure that accurate decoding is possible. The total encoding time will increase by approx. 30%. You should use this option if you enable Delete Source Files. If the decoding check fails, the wave file is not deleted.

Use backward compatible File Format for 8 bit and 16 bit Files

With this option, the codec is forced to use the file format of codec 1.5x for 8 / 16 bit wave files and the file format of codec 2.0x for 20 / 24 bit files. This option should be disabled unless you want to use an older codec (version < 3.0) for decompression.

Custom Compression Options

When the type of compression is set to Custom Compression, the following options can be accessed via the Options button.

• Adaptive Entropy Coding
  Enhanced adaptation of the entropy coding stage. Improves compression, but slightly slows down speed.
• Full Search for Order
  Searches all possible prediction orders for an optimum. Improves compression, but slows down speed more and more if the value of Prediction Order is increased.
• Block length
  Number of samples which are grouped in a block for processing. Choosing Auto adapts this parameter to the sampling frequency (1024 for sampling frequencies < 44.1 kHz, otherwise 2048).
• Prediction order
  Maximum Order of the prediction filter. Higher values improve compression, but also slow down speed, especially if Full Search for Order is set.

Please note that you can only set these compression options if you are using the current file format (codec 3.0). If the type of compression is set to Custom Compression, the option backward compatible file format will be automatically disabled.

General Settings

These settings mainly apply to file handling.

Target Folder

Selects the folder where output files are to be saved. Choose the folder via the Select button. If no target folder is chosen (clear input line by pushing the Clear button), each file is created in the folder of the according input file. Applies to compression and decompression.

Overwrite

Defines what to do if a file to be written alredy exists.

• Always overwrites the old file,
• Never leaves it unchanged but doesn't write a new file either,
• Prompt displays a dialog to choose individually between the two possibilities.

Delete Source Files

Deletes the WAV / PAC source files after they have been encoded / decoded. When you use this option for compression, you should also enable Check for accurate Decoding. If decoding is not possible, the source file is not deleted.

Create report

Creates a report file "report.txt" in the target folder of the first file in the file list. Only available for encoding. Doesn’t affect compression.

Priority

Sets the priority class of the program. Normal is recommended.

Information

Legal Notice

LPAC Archiver is freeware. The software may be copied and used for private and nonprofit purposes only. You are not allowed to modify this software or to use it for any commercial purpose.

The author shall not be held liable for any problems caused by the use of LPAC Archiver or LPAC files.

Contact

I would appreciate suggestions and comments. Please send them to

Tilman Liebchen
Technical University Berlin
Institute for Telecommunications
Einsteinufer 25
D-10587 Berlin, Germany

Phone: +49-30-314-24034
Fax: +49-30-314-22514
E-mail: liebchen@ft.ee.tu-berlin.de
WWW: http://www-ft.ee.tu-berlin.de/~liebchen

LPAC Homepage: http://www-ft.ee.tu-berlin.de/~liebchen/lpac.html

© Tilman Liebchen, 8 November 2001