Brooktree Bt878

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A PCI interface chipset from Brooktree that provides video decoder functionality.

It is supported under Linux by the bttv kernel driver module.


The Bt87x chip family were the direct successors of the popular Bt848 family of ICs, and were used by many analog cards produced in the later part of the 1990's; which collectively help constitute the Bttv devices categorization.

Brooktree Heritage

When Brooktree was purchased by Rockwell Semiconductor Systems, the IC family continued to be referred to simply as Bt87x. However, Rockwell Semiconductor Systems was subsequently spun off from its parent company a few years later, and, under the new Conexant Systems, Inc. banner, the ICs would begin to be rebranded in marketing literature as being part of Conexant's Fusion family -- i.e. they started to become referred to as the Fusion Bt878 and Fusion Bt879.

When Conexant would later expand its Fusion lineup with an updated version of the Bt878, the "Bt" part was no longer included in the newer product's nomenclature, and, instead, was released simply as the Fusion 878A.

Hardware Features

The Bt87x chip's design is largely based upon the Bt848A video decoder but are upgraded to also include several audio capture capabilities (summarized in the table below and discussed in further detail later on).

Bt87x decoder family comparison
Featured Bt878 Bt849A
All of Bt848A's features (i.e. Composite, S-Video multi-standard
Video Decoder and PCI bus master)
Yes Yes
Teletext support Yes Yes
Audio in Yes
TV Audio Yes
FM Radio - Yes

The Bt878 A/V decoder is a multi-function PCI device; having both a video and audio function that feature direct memory access (DMA)/PCI bus master support. ('function' is a PCI term ... most PCI devices have only one of them...)

Note: In Linux, the two available PCI functions for the chip are illustrated by the output of the lspci command, whereby one should be returned something like:
0000:03:04.0 Multimedia video controller: Brooktree Corporation Bt878 Video Capture (rev 11)
0000:03:04.1 Multimedia controller: Brooktree Corporation Bt878 Audio Capture (rev 11)

The DMA controller architecture design could be described as a small and pretty nice programmable RISC engine. Essenitally, you basically build into the device driver software a very simple RISC program (very RISC - has about 8 instructions) that copies data between the BT848 and your RAM. The two functions have different memory mappings although they have many similar registers - e.g. the I²C bus is only available via the video function, while the I²S bus is only available via the audio function.

The chip supports both 'software' and 'hardware' I²C Protocols. Software means that you write to the SCL & SDA lines directly and the CPU/driver has to time everything itself. Hardware means you give it a byte or two to read/write and it goes away and does that and gets back to you with an interrupt when it's done. Unfortunately the hardware I²C does not support writing just one byte (the address byte but no data) so you cannot do safe write or read probes of I²C client addresses (it could confuse an I²C client that does not have sub addresses - e.g. a PLL).

Audio Support

The bt878 and bt879 (the latter no longer in production) include a high-frequency Analog to Digital Converter, supporting both 8 and 16 bits audio at 448000 samples per second, for capturing broadcast audio, as well as line level and mic input sources. In addition, the chips also feature a digital audio interface.

The chips have a digital audio packetizer and FIFO buffer for DMA audio transport across the PCI bus to the host PC.

Analog Audio

The bt878 audio ADC has three inputs - STV (Sound TV), SML (Sound Mic/Line), and SFM (Sound FM), though the FM input would be relevant only to the 879 chip.

On the 878 datasheet, the relevant pins are 94, 98, and 100. These ADC pins serve the following functions:

Pin Signal I/O Description
100 STV I TV sound input from TV tuner.
98 SFM I FM sound input from FM tuner.
94 SML I MIC/line input.

Digital Audio

The digital audio interface consists of three input pins: ADATA, ALRCK, and ASCLK. Pins 87-89 This interface can be used to capture 16-bit I²S style digital audio or generic non-continuous packet synchronized data

Pin Signal I/O Description
89 ASCLK I/O (Audio Serial Clock) Bit serial clock.
88 ALRCK I/O (Audio Clock) Left/right framing clock.
87 ADATA I/O (Audio data) Bit serial data.

External links