Audio support in HP MPower - computer network communications software - includes related article on analog-to-digital signal conversion

Hewlett-Packard Journal, April, 1994 by Ellen N. Brandt, Thomas G. Fincher, Monish S. Shah

Multimedia capability promises to enhance the communication and presentation of information through the use of real-world data types such as audio and video. Compact-disk-quality audio is the first of such data types to be offered as a standard feature on all of HP's new workstations.

HP MPower provides the hardware and software to allow recording and playing of audio files over a network, incorporating audio in email, adding audio annotations to system files, and recording and playing to external devices like tape recorders, CD players, and VCRs. With these capabilities audio-enabled applications can add voice annotation to documents ranging from spreadsheet rows and columns to CAD drawings. Programmers might add audio comments to theft programs. Error messages could take the form of spoken messages, or even distinctive sounds that convey more information than a simple beep. Finally, background music could be added to presentations.

This article describes HP MPower's audio functionality, application development tools, and audio hardware and software architecture.

Audio Tools

The audio tools provided in HP MPower allow users to record, edit, and play audio data in a variety of file and data formats. HP MPower also provides tools for converting between audio formats. All of these tools are built on the audio library, which defines the application program interface to HP MPower's client/server audio implementation. The application program interface, libraries, widgets, and header |es are available to third-party software developers who wish to use audio in their applications.

The Audio Editor. The audio editor is based on OSF/Motif widgets and audio library (alib) functions. The audio editor enables the user to record, play, and edit audio files in a variety of file and data formats. It displays a waveform representation of the data to make editing easy and supports basic editing tasks like selecting, cutting, and pasting. The main screen for the audio editor is shown in Fig. 1a.

The audio editor can be invoked or redisplayed by clicking on the audio icon on the HP MPower media bar. Dropping a file from the HP VUE file manager onto the audio icon will bring up the audio editor with the file already loaded and displayed. A file can be loaded into an already visible audio editor by dropping the file icon onto the editor screen.

Audio Control Panel. The audio control panel is an OSF/Motif interface to global audio parameters such as volume and output device selection (see Fig. lb). All cooperating audio applications can use the control panel so the user always knows where to go to control these attributes. The audio control panel also provides a Stop button that will stop the current play operation from any cooperating application.

The audio control panel allows the user to turn monitoring on or off. Monitoring involves listening to the audio input signal. In a conventional tape recorder, monitoring allows the user to listen to the audio signal being recorded. On a workstation that has HP MPower, monitoring can be used whether or not anything is being recorded. For example, a user can monitor the workstation's line inputs from a CD player or VCR even when recording is not in progress without using the CPU or other system resources.

Audio Playback. An audio file can be played by simply double-clicking on the file's icon in the file manager or in an audio-enriched mail message. The file will begin playing according to the settings of the audio control panel. A file can also be played by dragging its icon from the file manager to the speaker icon on the HP VUE front panel.

Other Functionaly. In addition to the graphical interfaces to audio functionality mentioned above, there are some other capabilities shipped with HP MPower that are accessible from a command line. In the directory/usr/audi0/bin executables for the audio editor (audio_editor), the audio control panel (Audi0CP), the double-click function (send_sound), and the convert and attributes programs are provided. The convert program converts audio files from any supported file format, data format, and rate and number of channels to any other format and rate. The attributes program tells everything that it can determine or guess about any audio file including file format, data format, sampling rate, number of channels, data length, and header length.

Audio Data and File Types

A number of audio data and file types exist in the industry today, making it difficult for audio flies to be shared in a heterogeneous environment.

The lack of standards for audio is currently being addressed by groups such as the IMA (Interactive Multimedia Association). This organization and others are trying to develop standards so that someday there will be a clearer picture as to how to store audio information in a format that is accessible to everyone and can be easily incorporated with other aspects of multimedia.

In the meantime, we chose to support two of the existing file formats and to develop conversion utilities that allow us to support sampling rates, data formats, and byte ordering methods that are not supported directly by our audio hardware.