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Eliminating Voice Truncation with the IMBE Vocoder Disclosure Number: IPCOM000006512D
Original Publication Date: 2002-Jan-11
Included in the Prior Art Database: 2002-Jan-11
Document File: 2 page(s) / 89K

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Erik S. Perrins: AUTHOR


Eliminating Voice Truncation with the IMBE Vocoder

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Eliminating Voice Truncation with the IMBE Vocoder

by Erik S. Perrins

A common problem in communication systems is the time it takes for voice information to begin passing through the system once the user presses push to talk (PTT).  This time interval is commonly referred to as access time, and can be as long as several hundred milliseconds.  If the transmitting user speaks during this interval their voice will be truncated and the user at the other end will not receive that portion of their speech.

One way to address this problem is to buffer the transmitting user’s speech until the access time has passed and then begin transmitting the buffered data.  This introduces an additional amount delay into the system, which is very noticeable and annoying in duplex communications.  This delay can be eliminated if the data in the buffer is emptied at a faster rate than it is filled.  This has to be done with some sort of pitch preservation technique or the user’s voice will be heard at a higher pitch (as with the cartoon characters “The Chipmunks”).  One method of preserving pitch is by using the IMBE vocoder.

The IMBE vocoder is designed to allow the pitch to change gradually from frame to frame (the frame length is 20ms).  If IMBE frames are deleted periodically the pitch will not be noticeably altered due to the design of the vocoder.  This allows for a simple method of emptying a buffer faster than it is being filled, while preserving pitch in the process.  It can be done in the transmitter without the receiver knowing about it.

Here is a description of how this can be accomplished.  Once the user presses PTT, the transmitting radio immediately begins to pass audio samples to the IMBE vocoder, and buffers the IMBE frames until the access time has passed, at which point it...