Danny Cohen and Peter Kirstein:
While these experiments used special-purpose hardware and low-bit-rate encodings such as LPC, new experimental interest for transmitting voice over packet networks arose after Sun Microsystems introduced the SPARCstation 1 with a built-in mu-law audio codec.
Versions of software for Sun workstations from the early 1990's include vt and vtalk.
Experiments leading to the MBONE tools and RTP were conducted on the ARPA test network called DARTnet, a transcontinental IP network of about a dozen research sites currently connected by T1 trunks:
Date: Tue 12 Feb 91 00:43:28-PST From: Stephen Casner
Subject: Packet voice over DARTnet To: DARTNET Cc: email@example.com, deschon I would like to report a successful test with packet voice carried across DARTnet today. The experiment was set up by Erik Perkins at UDel and Annette DeSchon at ISI, using the VT (Voice Terminal) program recently released by ISI. Generally the sound quality was quite good and it was easy to carry on a normal conversation. Occasional dropouts were heard, but most of these were apparently caused by effects other than packet loss or delay. Unexpected activity (e.g., compilation) on the test host at the ISI end was correlated with some of the dropouts. Other dropouts heard were actually the result of noise from the listening end causing the "speakerphone function" to momentarily cut off the talker's microphone to prevent echo. A third source of dropouts was observed during a segment of continuous music; the music came through uninterupted until the dynamic silence threshold eclipsed the music level just as it is supposed to do to cancel background noise. We concluded that it would help to add commands to defeat the silence threshold and echo suppression functions for test purposes. Bob Braden told me there was also a test of packet video last week by the SRI folks connecting to ISI using one of the new Sun boards. Let's hear more about it! -- Steve To: braden Cc: Dartnet Subject: Re: Audio teleconference Date: Mon, 12 Aug 91 14:49:05 PDT From: Van Jacobson Bob, If it would be agreeable to all the participants, I'd like to try using LBL's new audio tool, vat, instead of vt for the teleconference. Vat is functionally equivalent to vt (except the vat speakerphone mode works better) but has [what I consider] an easier-to-use, window-based user interface. The only problem I could see is that vat requires whatever host you're using for conferencing understand X. But, since I think all the hosts we use run either X or Sunview (which can speak both X and NeWS), this shouldn't be an obstacle. - Van ps- unfortunately, the current version of vat won't interoperate with vt so it's an either/or choice of which to use.