ChipVid project

General description

The Iosoft ChipVid project is a demonstration of high-speed low-cost networking; video images are captured using a 'frame grabber' added to the Microchip PICDEM.net board, as shown above. The raw image data is transmitted in UDP format over Ethernet at approx. 2 images/sec, the net data throughput being approx. 2 Mbit/s.

The frame grabber is compatible with both European (625-line) and US (525-line) composite video signals, and produces good-quality monochrome images from low-cost monochrome or colour cameras.

The UDP network data is received and displayed by a Windows utility written in Visual C v6. The above picture is an unretouched screen-grab from that Windows utility, and shows the frame grabber capturing a picture of itself.

The frame grabber hardware was designed by Iosoft Ltd. The PCB artwork was produced by Microchip Technology Inc., and has been released for general non-profit use.

Hardware construction

Important note. It is feasible for an experienced hobbyist or student constructor to undertake this project, though it does demand a significant level of electronics knowledge, skill and equipment. Although the design has been kept as simple as possible, it does include a mix of high-speed analog and digital circuitry, and both through-hole and surface-mount components. A faulty ChipVid board could damage the PICDEM.net board as well, so take great care when constructing and testing the board. Always use a current-limited supply, and remove power immediately if there is any sign of overcurrent.

Iosoft offers this design 'as-is', with no warranties of any kind, and regrettably can not provide assistance in the event of problems.

ChipVid assembly

ChipVid testing

• The input video signal is approx 1V p-p (if larger, check termination resistor R5 and JP1)
• U5 is detecting the horizontal & vertical syncs correctly (pins 1 and 3)
• U3 pin 19 (AOUT test point) has a buffered version of the video signal with DC offset. The amplitude should remain stable if the video signal is attenuated (AGC operation)
• U3 pin 1 (D7 test point) switches cleanly on when video level is above half-way (m.s. bit of ADC)
• With a colour video signal, adjust C4 to minimise the amplitude of the colour subcarrier, as measured on AOUT test point.

PICDEM.net modifications

• J1 (2 pins) is only used for mechanical support
• J3 (3 pins) must be connected to 9-volt power and 2 x ground - keep these connections as short as possible.
• J4 (27 pins) The I/O connections to the PICmicro - no additional connections
• OSC1 (1 pin, to the left of U2, linked to its pin 4) It is necessary to replace the existing PICmicro clock circuitry (crystal and capacitors) with the clock signal from the ChipVid board (U2 pin 4). This is achieved by cutting the two top-side tracks between the PICmicro pins 13 & 14 and the crystal, then wiring the OSC1 pin to pin 13

Modifying the design

• PCB layout. The PCB incorporates a large ground-plane area, and this is very important to maintain video quality. Any noise on the analogue circuitry will generally show up as 'wallpapering' (a background pattern on the video image) which can only be eliminated by careful layout and a good ground plane.
• Power supply. The ChipVid board has its own voltage regulator, which helps to eliminate noise carry-over from the microcontroller supply.
• Clock. A single clock source must used to drive both the ADC and microcontroller. The buffering of the clock line to the microcontroller is important; I have experienced ADC stability problems when using a simpler buffering scheme.
• Devices. Both 74HC04 and 74HCT04 devices have been used with success. Two devices have been used for convenience of layout; they could probably be combined into one.
• Filters. There are two LC filters in the design. The circuitry around L3 is supposed to eliminate high-frequency video components from the ADC input; it has been included as a precautionary measure, but probably isn't needed when using a low-cost solid-state camera as the video source. The filter around L1 removes the PAL/NTSC colour subcarrier, and may be omitted if using a monochrome video source.

Software

The PICmicro software is described in chapter 14 of TCP/IP Lean 2nd edition, which also includes the source code on CD-ROM (see file P16CAP.C)

There is a Win32 network client which receives and displays the images, and a Linux network client which creates an uncompressed GIF file. Click here for a zipfile containing the source code. The client software may only be used in non-commercial applications, and the Iosoft copyright notices must be retained. Contact Iosoft for commercial licensing information.