Anatomy of V4L-DVB devices

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Anatomy of a V4L device

Anatomy of a DVB device

Digital Video Broadcast receivers exist in many different flavours. This page lists some of the common receiver types, in order to give you an idea of what all is supported by the linuxtv dvb drivers and why some things within the linuxtv driver code look quite complicated.

Minimal Configuration

A minimal device for DVB reception at the PC consists of a frontend and some FIFO Memory. This FIFO samples the MPEG-2 Transport Stream delivered by the frontend and makes it accessible (e.g. by a port-interface, memory-mapped or a bus-interface like USB or IEEE1394) to the host processor.

All modern peripheral DVB devices follow this approach: common PCI cards, nearly all USB adapters and IEEE1394 DVB devices. Only they are supported directly by Microsoft's BDA architecture.

PCI Cards with onboard MPEG-2 Decoder

Here the frontend is connected to the AV711x hardware MPEG-2 Decoder. This decoder shares a dualported RAM with the PCI bridge where the MPEG-2 Decoder has its Video Framebuffer. (1)


The Onboard Decoder makes these cards somewhat expensive.

(1) With more detail:
The AV711x have his own framebufers on the onboard SDRAM, for MPEG I and P frames, whose contents are sent to the OSD unit for overlay windows; the OSD unit output is then encoded on PAL or NTSC display formats and output in CVBS (Composite+Video+Blanking+Synchronism) or Y/C (Luminance+Chrominance) or RGB (Red+Green+Blue) modes; the OSD unit output is also sent in digital format to the PCI bridge, which after optional transformations (scaling, cliping, colour space conversion, etc), is sent by PCI Bus Mastering to the Video Framebuffer, normaly on a VGA card or in main memory for a V4L application. The dualported RAM (DPRAM), which is connected to the AV711x and to the PCI bridge, is used for all the communications between the DVB drivers and the AV711x: To send the firmware and commands, to send MPEG-2 DVR streams (to be decoded and show as video/audio), to receive MPEG-2 TS packets, PES data or sections (demultiplexed from the stream), etc.

Standalone STB

In a SetTopBox the Frontend directly feeds the built-in hostprocessor (usually with onchip MPEG Decoder). These processors are mostly based on embedded ARM-, MIPS-, PowerPC-Cores with additional peripheral units like i2c-, IDE-, network-, Smartcardcontrollers, Demultiplexers, Decoders, Audio- and Videointerface.

Modern STB processor chipsets move more and more decoder tasks into Firmware or software, this allows implementation of new codecs like DivX, MPEG-4, WMV and even late updates of products in the field. We expect that this evolution continues.

STBs with interface to the PC

Have an integrated embedded processor with MPEG-2 Decoder and are able to work standalone. You can connect your PC, e.g. using the USB or FireWire port to record and play back DVB transmissions on your TV.

External Links