Echo Cancelling - Digital Audio Processing Example

Transcription

1 Videoconference Videoconference Videoconference Videoconference AVNet K.U.Leuven

2 What is it? Two-way communication Real-time (latency < 1second) 2

3 Sound: echo-cancelation 0,5 sec echo cancellor echo 0,5 sec Near End Far End 3

4 Image: eye contact! camera lecturer image far-end, (computer) 4

5 Audio signal Sampling Rate: 44,1 khz Bit Depth: 16 bit Stereo: x 2 16 bit x 2 x = bps 1,4 Mbit/sec 5

6 Video signal (RGB) 768 pixels 576 pixels 25 frames/second Each pixel has 3 values: (3 bytes) Red dot: 256 steps (1 byte) Green dot: 256 steps (1 byte) Blue dot: 256 steps (1 byte) Datarate for the uncompressed video signal: 768 x 576 x 3 x 25 = 32 MByte/sec 256 Mbit/sec 6

7 Video on the network? Ethernet 100 Mbit/sec (shared) LAN compression = 1/2.5 Ethernet 10 Mbit/sec (shared!) LAN compression = 1/25 ISDN line ( 2B + D) : 128Kbit/sec compression = 1/2000 Modem connection : 56 Kbit/sec compression = 1/4600 7

8 Network systems circuit switched network packet switched network 8

9 ISDN IP The network H320 protocol 128 kbps (2 lines) 384 kbps (6 lines) Bandwidth (quality is guaranteed) H323 protocol Up to 4 Mbps Caution: Firewall! Normal connection speeds kbps Quality depends on available bandwidth 9

10 Videoconference resolution 1280x720 HD kb/sec 384x288 CIF h.264 (AVC) h.261, h.263, h264 10

11 Audio processing (MP3) (32-320kbit/sec, 16 to 48kHz) G.711 (64kbit/sec, 8kHz) G.722 (32-64kbit/sec) G (24-32kbit/sec Polycom Siren) G.728 (16kbit/sec) AAC (arbitrary bitrates, 8 to 96 khz) 11

12 Video setup Put the main camera for recording the lecturer directly above the monitor showing the far end image. Extra cameras for audience, technical setup,, speaker panel,... Document camera to show documents and paperwork to local and remote audience.. (slides(?) h.239 for computer presentation and/or document camera Videorecorder/DVD player. Videorecorder(s) to record far and near-end images 12

13 Audio Presenter microphone should be as close as possible to presenter. Speakers for local sound should only reach audience and should not interfere with presenter microphones. Presenter should be able to hear the sound from the remote site. What about the computer sound? What about the sound from tape or DVD? Record the near-end and far-end sound on local tape recorder. 13

14 Audio for interaction: problems Echo : (between two sites) The far-end microphone captures the near-end lecturer sound through the far-end speaker and sends it back (with delay) to the lecturer. Echo delay is around 1 second. Feedback : (one site amplified) The sound of the lecturer is localy amplified. Sound from speaker and microphone are re-inserted in the circuits. This produces a high pitch sound. Reverberation : (one site with or without amplification) The sound from the lecturer or sound system is reflected on the walls, ceiling or floor and will influence the sound quality. Bad speaker placement will produce the same effect. Background noise : (one site with or without amplification) Sound from air conducts, traffic, computers, will normally be disregarded by the intelligent human hearing system. Automatic microphone mixers have more difficulty in differentiating between noice and genuine sound. 14

15 Audio for interaction Full duplex - Half duplex Full duplex: All sites can speak at the same time. It feels as if all speakers are in the same room. No operator or automatic mixer action is necessary. Half duplex: The sound of the remote microphone is muted when the local microphone becomes active through automatic or operator control. Echo cancelling - Echo suppression Echo cancelling: only the echo will be suppressed. All other sounds are still available. Echo suppression: all sound is suppressed to cancel the echo effect. This is an (automatic) duplex system. Automatic Conference Mixer - Automatic Microphone Mixer The automatic microphone mixer activates the different microphones in relation to the received volume. It adds the signals end sends them to the local amplifier. The conference mixer has additional (automatic) processing for background noice, constant gain mode (NOM), 15

16 Audio for interaction: solution Echo cancelation benefits the far-end but allows our side to raise the volume without echo problems. An automatic audio processing unit will be faster than human interaction to interact with changes. (but not always more intelligent) When using an automatic system all manual interference will degrade performance. Let the automatic system do the job! 16

17 Light Light for the presenter but not for the projection screens. Enough light to allow pictures from the audience. Allow them to see/take notes, Beware for reflections from/on computer screens, monitors,... Document camera should have quality light. There is a difference between illumination and scene lighting 17

18 Desktop system Polycom PVX software + webcam Over IP Compatible with Netmeeting (whiteboard, application sharing, document sharing) MS Windows + USB connection Usable for very small groups (maximum 2 to 3 participants for each endpoint) No echo-cancelation Xmeeting for Mac OSX videoconference webconference (Acrobat Connect, Skype,..) 18

19 IP and ISDN H.239 Roll About system Fast setup in meeting room or connected to existing setup in lecture theatre. Technical support (for setup) is needed 19

20 Room systeem IP en ISDN People & Content (DuoVideo) H.239 Integration in existing or new lecture theatre or room. Attention for illumination, camera positions, microphone placements and sound reinforcement. Easy user interface (Crestron, AMX, ) Integrates video, PC, document camera,.. Fast start up time (no installation needed) 20

21 Multipoint - MPS200 Media Processing System Connects up to 20 video sites (and 16 audio sites) Gateway between ISDN en IP sites Continuous presence possibility h.239 support 21

22 MCU = MPS200 IP Mixed ISDN/IP ISDN 22

23 Continuous Presence 23

24 H.239 Polycom People & Content Tandberg H.239 DuoVideo Standard for transport of second image information in high (XGA) resolution. (computer, document camera) Can also be used for second video resolution channel (VCR, DVD, document camera,.) 24

25 H-239 IP or ISDN 25

26 Recording the VC (or a lecture) Recording the VC content on server. Streaming to web browsers. h.239 support. Replay using VC endpoints. Replay using streaming. Export to other systems using MPEG-1 download. 26

27 VC at K.U.Leuven Pentalfa (Gasthuisberg): 90 seats AVNet Interactieruimte : 80 seats AVNet Greenroom : 5 seats AVNet mobile setup Tavernier Studio : 30 seats Mediacentrum, ICRI,? 27

28 Projects Pentalfa (multipoint-flanders) Venus (multipoint-europe) Industrieel beleid (Leuven, Gent, Kortrijk ) Notariaat (promotiezaal, Leuven and Kortrijk) Pedagogie (on location, Leuven and Kortrijk) Lessen van de 21ste eeuw (multipoint) Erasmus Mundus Nanotechnology (Leuven, Leiden, Chalmers and Dresden) Earth Observation (Leuven and Purdue) International Student Business Game (7 sites, Europe, Association) 28

29 250 videoconferences each year 70 multipoint 29

30 Course of action Early resource reservation A short introduction and competent support can solve problems and anxieties Provide references for contact Provide information on website 30