Prototype of a Wireless PC2TV solution. Extending your PC/laptop screen to a digital television or a projector at your home, office or an exhibition center.

1. Low cost wireless laptop link to
TV
Anish Sane
Deepak Malani
Guided by : TVP, NVCR
Sponsored by: Philips

2. User Scenario
• Aim - Display laptop screen contents onto Television, with
wireless connectivity
• Applications
– Home use (one source, one destination)
– Conference room with Digital projector
(n sources, one destination)
– Multicast streaming at gatherings
(one source, n destinations)

3. Target Specifications
Features Specifications
Video resolution 1280x720
Frames per second 30
Colour Resolution 24-bit True Colour
Video Interfaces Digital: DVI
OS on Laptop Linux
OS on Board Linux
Distance between Laptop & TV 10m (30ft)
Power Supply 5V Adapter

4. System Overview
Television
Video Memory DVI Encoder IC
Grabbing Decoding
Streaming
USB Port USB Wi-Fi dongle USB Wi-Fi dongle Beagle Board
Laptop Side (Tx) TV Side (Rx)

5. Part I
Content Generation (Laptop side)
Content Display (TV side)

6. Video data rate
• Video frame grabbing
• Raw data rate
(1280x720) x 30 fps x 24bits/frame = 663.5 Mb/s
• Throughput offered by 802.11g devices =
22Mbps
• Hence necessity for video compression
– Compression ratio (>30)

7. 1. Content Generation
• Initial Approach – Framebuffer (/dev/fb)
– Grab RGB pixel data
– RGB to YUV transcode with 4:2:0 subsampling
– YUV to MPEG-2 stream using libavcodec (open-
source)
• Performance
– 640x480@3fps
• Evaluation
– Device file read takes 200ms/frame

8. Video display mechanism (Laptop)
Video Grabbing
Application
Frame Buffer
/dev/fb
Graphics Display XShared
Application Memory
Display
hardware
No
Hardware Graphics
Video Display Acceleration
Application Adapter
Required ? Yes

9. Content Generation
• Approach 2:
– Grab from Xshared Memory (XShm)
• APIs from open source
– XShmGetImage()
• Performance
– 1280x720@30fps
– MPEG-2 compression using ffmpeg (based on
libavcodec)

10. Compression schemes
MPEG-2 MPEG-4 H.264 (MPEG-4/AVC)
Compression Ratio x 1.2x 2x
Computation Time y 1-1.4y 4y-6y
(for various video
bit-rates)
• Compression Ratio
x = 60-300
• Computation time (PC) for encoding
y = 1 second for 2.6 seconds of video

11. Bottleneck Diagram
PC/Laptop
TV side
side
11Mbps 6Mbps 2 Mbps 5.5 Mbps
Content Generation Content Streaming Content Displaying
HD Quality HD720p HD, 1280x720
1280x720@30fps framedrop if > 2 Mbps

12. Generated Data Rate (after compression)
• DCT Quantization scale v/s bitrate
12000
11036 (2)
10000
8000
Bitrate (kbps)
6000 5531.6 (4)
4000
3139.5 (8)
2000 2100.3 (14)
984.7 (31)
0
0 5 10 15 20 25 30 35
Qscale (2-31)

13. Qscale=4

14. Qscale=14

15. 2. Content Decoding on TV side
• BeagleBoard
• Processor – OMAP3530
– 600MHz ARM Cortex core
– NEON SIMD Coprocessor (advantageous for video
applications)
– C64x+ DSP
• Interfaces
– USB for Wifi device
– Digital Video Interface (DVI) for TV
– SDIO for filesystem and storage

16. Software on Board
• 2.6.28 kernel
• Video Decoder
• Mplayer
– Uses libavcodec
– Uses NEON acceleration

17. DSP Acceleration
• Gstreamer multimedia framework which uses
DSP/BIOS Link
• Pipeline based application
– filesrc | demux | codec | sink
• Codec
– ffdec_mpeg2video
– TI evaluation codecs
– No HD codecs for 720p yet for DSP

18. Video Decode Performance on Board
• Resolution: 1280x720
• Frame rate: 24 (streaming videos)
30 (captured videos)
• Video fidelity: 2 Mbps
– At higher bit rates, frame rate decreases
• Comparison of video fidelity (pictures)
• Power consumption=2W (0.4amp, 5VDC)

19. True HD

20. On OMAP

21. System Overview
XSharedMemory
Television
Grab continuously
DVI Encoder IC
Transcoding framebuffer
software to stitch Grabbing Decoding
into raw video
Mpeg decode with
Compression DSP Accerlation
software using
ffmpeg video codec Streaming
libraries compressed_video.mpeg Buffer memory
USB Port USB Wi-Fi dongle USB Wi-Fi dongle Beagle Board
Laptop Side (Tx) TV Side (Rx)

22. Part II
Content Streaming
and Automation

23. Objectives
• Wireless connectivity between laptop and
TV
• Automatic detection and pairing of two
devices
• Selectivity among multiple streams

24. Wireless Solutions
• 802.11g Netgear WG111GE (Realtek)
• Super-G TP-link (Atheros)
• 802.11n Star-king (Atheros)
• D-link DWA110 (Ralink)

25. Wireless Connectivity
• Wi-Fi Dongle (802.11g)
D-link DWA110 (rt73 based):
– Driver support : kernel.org
• rt73usb
– Modes supported: infrastructure(managed), ad-
hoc, monitor
– We are using ad-hoc mode for point-to-point
communication
– Data rate: 5Mbps-16Mbps
(More data rate when better S/N ratio)

26. Ad-hoc mode
• For point-to-point connection, no necessity of
having an access point
• No time/bandwidth overhead for data going
via an AP (as in infrastructure mode)
• No consumption of bandwidth due to beacons
from AP
• The laptop and the board can use channels
with mutual understanding & independent of
AP's channel

27. Alternate to Ad-hoc mode
• Using one side in managed mode &
attaching an AP on other (over wired link)
• Was thought to be useful when the
dongles/driver do not support ad-hoc
mode
– Netgear WG111GE (RTL8187B)
• Gives about same speed as ad-hoc

28. Protocol used
• TCP:
– Error control & retransmission on error
• UDP:
– Assumes error control at the application
layer
– No implicit retransmission at protocol
level

29. Protocol used
• We are using HTTP+TCP
• Packet drops are severe when UDP is used
(over Wi-Fi)
• Motion vector errors
• Retransmission can increase the lag between
captured & played video
• Data rate requirement can be satisfied, even
with retransmission
• Over wired link UDP packet drops are not
so significant

30. Bottleneck Diagram
PC/Laptop
TV side
side
11Mbps 6Mbps 2 Mbps 5.5 Mbps
Content Generation Content Streaming Content Displaying
HD Quality HD720p HD, 1280x720
1280x720@30fps framedrop if > 2 Mbps

31. Automation - Networking
Create Network on Laptop
Board Boots up
Start DHCP Server on board
Search For network No Yes
Laptop asks for IP,
Board provides IP
mode,essid,cell
Network Found? match?
No
Yes
Verify connectionNo
Connect to the network (mode,essid,cell) Connection Established

32. Automation – Streaming
• Laptop side
Streaming server
Grabbing application (ffmpeg)
• Board side
– Streaming client
– Media Player (mplayer or gstreamer)
• Data transfer between applications
– This is done using named pipes i.e. FIFOs

33. User interface
• Reconnect
– If the network gets disconnected, the user can use
this button to reconnect
• Select Stream
– When there are multiple sources, this control can
be used to switch between them
• Reset
– If the device freezes completely

34. Demo Plan
• Real-time screen capture on laptop and
stream to television, over Wi-Fi
• Video file streaming from laptop to TV on Wi-
Fi
• Resolution: 1280x720

35. Appendix

36. User-space & kernel space switching

37. Gstreamer framework (using DSP/BIOS Link)

38. BeagleBoard

39. Screen Capture
With and without acceleration

40. Product Comparison
Features Our Product Hauppauge Addlogix
InternetVue
Resolution 1280x720 720x576 1024x768 (graphics)
(video) 640x480 (video)
Video Ports DVI/HDMI S-video, DVI, VGA
C-video
File Support Any type Limited to MPEG, Any type
JPEG, MP3, DivX
Screen Capture Yes No Yes
Mode
Cost $150 (BOM) $149 $229
Wireless 802.11b/g/n 802.11g 802.11g
Connectivity
Ethernet Yes Yes Yes
Connectivity