Managing stored voice in the etherphone system
Authors: Terry D. , Swinehart D. 
Published: October 2004

The Etherphone™ system was developed at Xerox PARC to explore methods of integrating voice into existing distributed personal computing environments. An important component of the Etherphone system, the
voice manager
, provides operations for recording, playing, editing, and otherwise manipulating digitized voice based on an abstraction that we call
voice ropes
. It was designed to allow:

unrestricted use of voice in client applications,

sharing among various clients,

editing of voice by programs,

integration of diverse workstations into the system,

security at least as good as that of conventional file servers, and

automatic reclamation of the storage occupied by unneeded voice.

As with text, we want the ability to incorporate voice easily into electronic mail messages, voice-annotated documents, user interfaces, and other interactive applications. Because the characteristics of voice differ greatly from those of text, special mechanisms are required for managing and sharing stored voice. The voice manager reduces the work generally associated with building voice applications by providing a convenient set of application-independent abstractions for stored voice.

Clients view
voice ropes
as immutable sequences of voice samples referenced by unique identifiers. In actuality, a voice rope consists of a list of intervals within
voice files
that are stored on a special voice file server. A database stores the many-to-many relationships that exist between voice ropes and files. Maintaining voice on a publicly accessible server facilitates sharing among various clients.

These facilities for managing stored voice in the Etherphone system were designed with the intent of moving voice data as little as possible. Once recorded in the voice file server, voice is never copied until a workstation sends a play request; at this point the voice is transmitted directly to an Etherphone, a microprocessor-based telephone instrument. In particular, although workstations initiate most of the operations in the Etherphone system, there is little reason for them to receive the actual voice data since they have no way of playing it.

Adding such voice facilities to a diverse and complex software base presents challenging problems to the systems builder since much of the existing workstation and server software cannot be changed or extended. Manipulating stored voice solely by textual references, besides allowing efficient sharing and resource management, has made it easy to integrate voice into documents. The only requirements placed on a workstation in order to make use of the voice services are that it have an associated Etherphone and an RPC implementation.

The Etherphone system uses secure RPC for all control functions and DES encryption for transmitted voice. These ensure the privacy of voice communication, which is important even in a research environment, although the network is inherently vulnerable to interception of information. Storing the voice in its encrypted form protects the voice on the server and also means that the voice need not be reencrypted when played. All in all, the voice manager provides better security than most conventional file servers.

The performance of operations for editing and managing recorded voice must be compatible with human response times: sub-second response at a peak rate of several operations per second is more than adequate. Performance measurements confirm that the voice manager easily meets these requirements.

In conclusion, the major technical contributions presented in this paper involve the use of simple databases to:

describe the results of editing operations such that existing voice passages need not be moved, copied, or decrypted, and

provide a modified style of reference counting that allows the automatic reclamation of obsolete voice.

Approximately 50 Etherphones are in daily use in the Computer Science Laboratory at Xerox PARC. We have had a voice mail system running since 1984 and a prototype voice editor available for demonstrations and experimental use since the spring of 1986.