Telephone sound may productively support multimedia communication. This tool aims at enabling easy translation of data between various data formats related to the G.711 recommendation of the CCITT. The utility offers an easy way of preparing audio material for use in ISDN communication. On the other hand, the user can take benefit of the Microsoft CODECs when the G.711 raw data has become accessible for the Windows environment.
ISDN audio telecommunication may in principle be accomplished in many ways, but most regular calls are compressed according to the G.711 recommendation of the CCITT (Comité Consultatif International Téléphonique et Télégraphique, which nowadays has been integrated into ITU). G.711 allows compression by using logarithmic interpolation, which reduces 14 most significant bits into 8. As the sampling rate is 8 kHz, the transmission rate equals to the 64 kbps offered by one ISDN B-channel. There are two brands of G.711: A-law is dominant in Europe, whereas United States and Japan are commonly using u-law.
A few years ago Microsoft provided drivers, which perform audio encoding and decoding of G.711 files. The purpose was obviously to amplify the company's multimedia collaboration application suite NetMeeting. At the same time this gives the Windows users the ability to convert any other standard audio files into the Windows A-law or u-law formats. The G.711 Tool adds to the usability of these resources by performing a couple of simple, but critical functions.
The application primarily supports conversion between G.711 raw data and audio storage conforming to the standard Microsoft CCITT G.711 CODEC. In addition, it currently contains the following conversion options. Both brands of G.711 compression are available both ways: from raw data to a RIFF (Resource Interchange File Format) compatible wave file and vice versa. Software conversions based on reference routines produced by Sun Microsystems, Inc. are available for the following formats:
In case the user desires to utilize other formats supported by an installed sound card, this can be done with the Sound Recorder application, which is part of all Windows packages. The conversion dialog is started by manipulating the file properties or as part of a Save As operation.
The reader may want to consult the check sheets of two-way conversions between the formats made with Sound Recorder and the G.711 Converter Tool. The memory dumps demonstrate the benefit of using the program's codecs compared to the ones provided by Microsoft.
The application opens a Windows G.711 wave file, removes redundant header information, reverses the bit order and stores the result in a file of .alw or .ulw type (A-law and u-law). Similarly, raw data from an ISDN recording may be stuffed into a standard wave file and thus integrated into any multimedia oriented Windows application.
This is a program under construction and it doesn't yet have much of a user interface. However, the tool can also be used as a console oriented command line utility, e.g. in batch processing. Please consult the G.711 tool's command line syntax.
The conversions can be made without a sound card. In case the user does have a card, the application can be used to play telephone sounds and to perform some simple editing and recording.
The application is being developed and tested on a Windows NT 4.0 (SP6) machine with a SoundBlaster AudioPCI 64V sound card.