DVB-S Extension board

Go to DVB-S SCART extension board picture

Introduction

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.

Design Goals

The board should be complete but minimal.

The board should be a complete implementation of all features provided by the J2 connector:
- Composite and RGB output
- Stereo analogue and digital SPDIF audio output
- Infrared remote control input
The board should also be a minimal implementation. There is an endless list of features one might add, all of which are "nice to have" for a set-top box, but which add to the complexity and cost. I prefer simplicity.

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!

Design Choices

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.

The DVB-S card interface

Let's see what signals the DVB-S card provides us with.

The connector we're interested in is the one marked "J2". The pins of this connector are numbered as follows:

J2 pin numbering
10	9
8	7
6	5
4	3
2	1

where pin 1 is the pin in the first row on the the right column. On the DVB-S card pin 1 is marked with a small triangle. The pinout of the J2 connector depends upon a Linux kernel parameter vidmode:

J2 pinout versus kernel parameter
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
2	+5V	+5V	+5V
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>
10	Ground	Ground	Ground

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.

The TV interface

In Europe most consumer TV sets have SCART connectors at the back; they are the most common way of connecting a video, DVD player or set-top box to a TV. The connector carries stereo audio and video signals both from video to TV for playback and from TV to video for recording.
In the picture below, the female connector at the left is the one found at the back of TV sets, video recorders and DVD players; SCART cables usually are male-male and use the connector at the right.

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.

Here's the pin numbering of the SCART connector :

SCART connector pinout
Female SCART at TV/VCR/DVD	Male SCART at the cable

When only interested in playback the pinout of a SCART connector simplifies to:

Playback-only SCART connector pinout
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
5	Blue ground	-
7	Blue output	-
8	Source select	Source select	High (9.5-12V): Input from SCART, 4:3 aspect ratio Mid (5-8V): Input from SCART, 16:9 aspect ratio Low (0-2V): Input from TV tuner
9	Green ground	-
11	Green output	-
13	Red ground	Chrominance ground
14	Blanking 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.

Composite, RGB, S-Video: What to choose?

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.

The SPDIF interface

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 circuit

There are a number of circuits which do the job, but the circuit made by Thomas Breuer seems to be the reference, and is the one this work is based on. His design provides a nice balance between preserving excellent video quality while protecting the delicate video outputs of the DVB-S board.
While reading the schematic, note:

the +5V power supply for the analogue components is distinct from the +5V power supply for digital components. The 7805 avoids noise from the PC power supply getting to the infrared receiver or to the video circuit.
the J2 video outputs are best modeled as current sources.
the TV video inputs are best modeled as 75R resistances.
in normal operation, the BAT42 Schottky diodes do not conduct; they are there to protect against over- or undervoltage.
in normal operation, the 10R and 51R resistors have no influence, being in series with a current source. They are there in case of over- or undervoltage.
All four J2 video outputs - composite, red, green and blue - are 1V peak-to-peak into 75R. The composite signal carries 0.3V synchronisation and 0.7V video, whereas red, green and blue are 1.0V video only, no sync. Some TV's expect all RGB signals to be 0.7V and produce images which are too bright when fed 1.0V. Given that the J2 video outputs are current sources, the 180R resistor in parallel with the 75R of the TV video input reduces red, green and blue voltage to 0.7V peak-to-peak. The 22R resistor in series brings total impedance (22R + 180R||75R) back to 75R so filter characteristics don't change.
The circuit slightly emphasises higher frequencies; this corresponds to slightly accentuating picture detail.
J2 pin 1 provides the composite output. It is a current source which is loaded by two circuits:
1. the filter on the DVB-S PCI card between J2 pin 1 and the RCA/cinch video output on the backplate of the PCI card.
2. the filter on the extension board between J2 pin 1 and the composite output on the SCART connector.
The frequency response of the composite output suffers from being loaded by both circuits. However, this is not a problem if you are using the SCART output in RGB mode. In RGB mode the composite output is only used for sync, and sync gets through fine. It is also not a problem if you are using the extension board for S-video output with vidmode=2, as this does not use J2 pin 1 at all. (From information provided by Th. Breuer)

The board

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.

Finished board
Finished Board, Component side	Finished Board, Solder side

This is what the various connectors are used for:

Test before you plug in

Before trying the board it's a good idea to do a few test measurements. Verify there are no short circuits between any of the four (Composite+RGB) video inputs and ground, or between any of the four video inputs.

Next, let's try connecting the floppy power connector. You should measure

12V at the input of the 7805.
5V at the output of the 7805.
1.9V at the leg of R13 which is connected to the diodes.

Don't forget to check you haven't shorted any of the auxiliary connector pins when soldering.

Well grounded

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.

Isolated

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.

Jumpers

There are two jumpers on the board:

Source Select Jumper
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 connector 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
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.

Infrared remote control

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 using lirc.

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.

Installing the card

Of course, when the card is installed the SCART connector is inside the PC. Let me repeat this: the SCART connector will be inside the PC. This means you need to find a way to get the cable out. I chose to make a small hole in the shield around the motherboard connectors. There's also a strain relief around the SCART cable, so someone yanking the cable doesn't yank the innards of the PC out. An alternative would be installing a female SCART connector on the back panel, connecting it to the extension board with a short cable.

This PC has, from left to right, one budget DVB-S, one budget DVB-T and one full-featured DVB-S 1.3 card. On this picture you can see the big, black SCART cable and the much smaller lirc_serial cable.

If you connect one TV using the SCART connector please don't try to connect a second TV using the RCA video out connector at the back of the PCI card: SCART and RCA are not intended to be connected simultaneously. The DVB-S card may not be able to handle the load of two TV's in parallel. Connecting the audio output at the back of the PCI card to your stereo equipment while you're using the SCART connector is not a problem.

Picture gallery

Karim (kafifi at easyconnect dot fr) sent this picture of his extension board clipped on to a DVB-S 1.5:

The board also fits the (rare) full-featured DVB-T 1.2 cards:

Downloads

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.

Cost

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.

Acknowledgements

This design builds upon information from vdrportal, vdr-wiki and board designs from Thomas Breuer, Frank Hermann, fdm-ware and many others. Thanks to all those who provided feedback.

Last update page: February 21, 2005

Koen De Vleeschauwer

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