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A PC with a digital TV card such as those made by Technotrend and sold by Hauppage
can be turned into a set-top box using freely available software such
as Linux and vdr.
At the heart of this set-top box one often finds a full-featured DVB-S
card. These cards combine a satellite tv tuner and a hardware MPEG2
decoder. The tuner receives the satellite TV signal and the MPEG2
decoder converts it to a signal a normal TV set can understand, such as
PAL. One only needs to add an infrared remote control receiver such as LIRC
to sit back and enjoy TV.
The board I happen to own is a Technotrend/Hauppage DVB-S 1.3. Using a small circuit one can improve the quality of the TV image, add a digital sound output and a remote control receiver. This document describes such a circuit, should you wish to build one.
The extension board works equally well under Linux as under Windows. I refer to Linux throughout this page merely because that is what I feel comfortable with.
Also, the board has to be easy to solder. Designs which use
surface-mount devices (SMD) are nice but too difficult for me to solder.
The DVB-S card TV outputs are fragile and easily damaged; the board should protect them against short-circuits and overvoltage.
And, of course, it should provide great video quality. Enjoying an excellent TV image is what we're building it for!
After having looked at all available designs, I chose to implement
the circuit as a small daughterboard which can be clipped on the back
of the DVB-S card. This way the distance between J2 input and SCART
output connectors is minimal. There is no need for long flat cables and
their connectors. Picture quality can only benefit from this.
Optical SPDIF output (Toslink) was chosen instead of coax to have galvanic isolation between the set-top box and any audio equipment you might want to connect. This avoids ground loops.
Although it is possible to route everything on a single-sided board, I prefer a double-sided PCB: allows for better shielding and a nice ground plane, resulting in less noise.
Let's see what signals the DVB-S card provides us with.
|Pin||CVBS+RGB OUT vidmode=1 (default)||CVBS+YC OUT vidmode=2||YC OUT vidmode=3|
|1||Composite Video output||Composite Video output||Y - Luminance|
|3||Blue Video output
|4||SPDIF output||SPDIF output||SPDIF output|
|5||Green Video output||Y - Luminance||-|
|6||Right Audio output||Right Audio output||Right Audio output|
|7||Red Video output||C - Chroma||C - Chroma|
|8||Left Audio output||Left Audio output||Left Audio output|
|9||Infrared receiver||Infrared receiver||Infrared receiver>|
The default is just fine for us. This provides us with PAL
composite (pin 1) and component video (red, green and blue: pins 7, 5,
3), analogue (pins 6, 8) and digital (pin 4) sound and a place to
connect our remote control receiver (pin 9).
The extension board uses the "auxiliary connector" as mechanical support only.
If your TV has more than one SCART connector chances are the connectors differ slightly in the video signals they support. Usually the first connector is for composite and RGB component video while the second connector is for composite and S-Video signals. Careful reading of your TV's manual may be necessary.
|Female SCART at TV/VCR/DVD||Male SCART at the cable|
|Pin||Playback Composite/RGB||Playback S-Video||Signal|
|1||Audio right out||Audio right out|
|3||Audio left (or mono) out||Audio left (or mono) out|
|4||Common Audio ground||Common Audio ground|
|8||Source select||Source select||High (9.5-12V): Input from SCART, 4:3 aspect
Mid (5-8V): Input from SCART, 16:9 aspect ratio
Low (0-2V): Input from TV tuner
|13||Red ground||Chrominance ground|
|15||Red output||Chrominance output|
|16||Insert control||-||High (1-3V): RGB
Low (0-0.4V): Composite
|17||Composite video ground||Luminance ground|
|18||RGB Switching control ground||-|
|19||Composite video output||Luminance output|
|21||Common shield ground||Common shield ground|
Note composite video and luminance (black-and-white information) share the same pin. If you get a black and white image the most probable cause is that the DVB-S card is sending S-Video, but the TV thinks it is composite video, or the other way round.
A more complete description of these signals can be found at Eric-Paul Rebel's site.
When in doubt which video signal to use, avoid composite video if you can as signal quality is lowest. When having to choose between RGB and S-Video, I prefer RGB: colors seem more vibrant.
Additionally, you can connect the DVB-S card to your home theatre
sound system using the SPDIF (Sony Philips Digital InterFace)
connector, just like you would connect a DVD player. This allows
listening to TV sound from your home theatre speakers. Two kinds of
SPDIF cable exist; one is a copper coaxial cable with RCA connectors;
the other is Toslink optical fibre. The extension board has an optical
sound out connector for Toslink optical fibre.
Digital mono or stereo sound should work out of the box.
If you wish to listen to Dolby Digital (also called AC3) or DTS sound using vdr take a look at the AC3-over-DVB patch. The AC3-over-DVB patch encapsulates the AC3 or DTS data into special headers and sends them via the SPDIF connector of the card. Note the AC3-over-DVB patch does not decode AC3 nor DTS; it merely sends it. Most, but not all, home theatre systems then correctly interpret the encapsulated stream. Here is a list of home theatre systems that are reported to work OK.
The board was designed using the Eagle software package. You
can download and use Eagle for free for small projects like this.
Note the SPDIF digital audio output is routed away from and perpendicular to the video signals. This is to avoid interference between the digital sound and the analogue video signals.
Also, if you choose to make the board yourself you probably don't have plated-through vias. This means you will have to solder some components on both solder and component side and insert wires in those vias which don't have components in them.
This is what the various connectors are used for:
Don't forget to check you haven't shorted any of the auxiliary connector pins when soldering.
And this is how it looks when clipped on the back of the DVB-S card:
The red wire running from the "test point" of the extension board to the sheet metal of the tuner ensures a good ground. The four video signals (Composite+RGB) from four J2 pins each drive a 75 ohm load. The return current, however, has got to go through a single J2 pin, so the impedance of this one ground pin might become an issue. Connecting the ground of the extension board with the metal of the tuner solves the problem. The connector which clips on the tuners' legs is called a "2.8 mm female Faston" connector, BTW.
Maybe you can just see the plastic foil which I have put between the daughterboard and the DVB-S card. The two don't touch, but adding a little bit of extra isolation just to be on the safe side doesn't hurt.
|Source Select Jumper (JP4)|
|TV switches automatically to SCART
connector when PC turned on.
Screen format 4:3. (default)
|TV does not switch automatically to SCART
when PC turned on. Switch to SCART
connector using the remote control.
|TV switches automatically to SCART
connector when PC turned on.
Screen format 16:9.
|Insert Jumper (JP2)
|Use RGB component video signal. (default)|
|Use composite video signal.|
Try looking at a test card once using composite video, and once using RGB. You'll see the difference.
The infrared unit of the DVB-S card only understands RC-5 and RC-MM
codes. TV sets, DVD players and video recorders made by Philips usually
use these codes, so if you have a universal remote control you might
want to try to set the remote to Philips equipment. When choosing a
sensor, remember the RC-5 specification states the carrier frequency to
be 36 kHz. A TSOP
1736 or 1836 works fine.
I am using a TSOP 1736 mounted in the front panel of the PC, with a small 10µF electrolytic capacitor soldered in parallel with its power leads.
Note: the infrared receiver will only work if you do not have a Common Interface/Conditional Access Module installed.
On the software side, using the DVB-S cards' infrared receiver means you don't need
lirc_serial kernel drivers nor a lirc
daemon to run vdr. The vdr-remote
plugin is all you need. Of course, if you prefer, you can keep
I have experimented a bit using both a serial LIRC receiver and the DVB-S card infrared receiver, and both work fine. There are situations, however, where the DVB-S infrared is preferable. For a vdr set-top box, it is preferable to use the slowest CPU which does the job. A slower CPU generates less heat and needs less cooling. Less cooling means fans run slower and generate less noise. Noise only distracts from the movie you're watching. However, the speed requirements of vdr may vary wildly. Just watching TV needs very little - say, 256 MHz - while scanning a recording for commercials runs smoother on a fast 2 GHz CPU. I am experimenting with Pentium IV clock modulation, using programs such as speedfreqd to dynamically change CPU frequency as needed. The lirc_serial kernel module, however, seems to have difficulties with CPU clock frequency dropping from 2GHz to 256MHz; it becomes unresponsive to the remote control. This seems to be a driver problem (as of kernel 2.6.0). The DVB-S cards' infrared receiver does not have this problem.
The latest Eagle schematics, board layout and Gerber/Excellon files can be found here: Eagle design files (500 kb download). The dvbj2.brd file in this archive can be sent to PCB manufacturing services such as www.pcb-pool.com.
The main cost is the printed circuit board; the components only cost about 16€. The cost of the printed circuit board depends upon whether you make it yourself or have it made for you, and how many.
Last update page: February 21, 2005