Browse Prior Art Database

TETRA AUDIO AND VIDEO DELAY MITIGATION USING SHORT DATA SERVICE

IP.com Disclosure Number: IPCOM000009655D
Original Publication Date: 2000-Jan-01
Included in the Prior Art Database: 2002-Sep-09
Document File: 6 page(s) / 299K

Publishing Venue

Motorola

Related People

John Hughes: AUTHOR [+2]

Abstract

TETRA includes an inherent audio delay of 53.333ms. A small one-off increase in the audio encoding time can result in this delay being further increased by 60ms. A solution is presented which reduces this 60ms incremental delay by as much as 49ms. The solution is also applicable to video delays.

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Page 1 of 6

MOTOROLA Technical Developments

@

TETRA AUDIO AND VIDEO DELAY MITIGATION USING SHORT DATA SERVICE

by John Hughes and Paola Hobson

ABSTRACT

TETRA includes an inherent audio delay of
53.333ms. A small one-off increase in the audio encoding time can result in this delay being further increased by 60ms. A solution is presented which reduces this 60ms incremental delay by as much as 49ms. The solution is also applicable to video delays.

INTRODUCTION

  This paper addresses the problem of audio delay and video delay in TETRA radio systems, and high- lights the potential problems associated with small increases in this delay due to frame slippage. A solution is presented, based on the use of the Short Data Service (SDS) in TETRA, which helps reduce

the impact of frame slippage and, in some cases, completely reverses the effects.

AUDIO AND VIDEO DELAY IN TETRA

  The TDMA framing of signals used within TETRA introduces an inherent delay, due to frame 18 being reserved for signaling information.

  Each audio frame in TETRA is 60 ms in dura- tion while a TDMA frame is 56.667 ms in duration. Each TDMA frame consists of four 14.167 ms timeslots. Every 18 TDMA frames, 17 audio frames are transmitted (60ms x 17 = 56.667 x 18). Referring to the diagram below, the minimum inher- ent delay can be shown to be 53.333ms (=56.667- (60 - 56.667)).

Fig. 1 Audio to TDMA frame mapping

0 Motaola, Inc. 20x'

50 January 2000

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Technical MOTOROLA @ Developments

  The total delay experienced over the transmis- sion path will be greater than this due to:

l infrastructure delay

  Of the above delays only the transmission delay and audio/video capture delays are fixed. The pro- cessing time in the transmitter may vary due to vari- ations in the ACELP search routines (i.e. time to search the codebook can vary), variations in the time to perform video encoding and also due to increased load on the DSP and/or Host. With regard to the infrasuncture, delays may vary due to queuing.

60ms

l processing delays at the transmitter and receiver

l subscriber audio/video capture delays

l transmission delay

  This variation in delay can lead to an audio and video frame being ready for transmission too late and hence missing the time slot where it was due for transmission. This can have serious implications for the transmission of delay sensitive real time infor- mation. The impact of this delay is assessed for both video and audio.

AUDIO

  If audio frame 15 is not ready to transmit in TDMA frame 17, it has to be delayed until TDMA frame 1 (TDMA frame 18 is reserved for signaling), thus incurring an extra delay of 60ms. This delay will then be present throughout the voice call result- ing in degraded speech quality. Figure 2 below rep- resents this increased delay schematically.

I 15 16 17 1

     I 15 : I 16 I 17 I 1 TX process
delay 16 i 17 18 L 1

I 13 : 14 No Audio 15

Ii 14 No Audio 15

4-

Rx process

delay Normal Audio Delay

. . I...