Video capture card
Video capture cards are used to get the picture from the cable, aerial, or satellite television source into the PC.
They decode and capture the video signal from the channel you want to view or record. Most cards include a tuner to capture an specific TV signal (called 'tuner cards'). Video capture cards without tuners do exist, but these are not commonly used with MythTV. Sometimes people mistake video capture cards with video display cards which provide the output to the (tv)screen.
These cards (also called frame grabbers and software encoders) are usually based on a chipset like the Bt848/878 or Conexant cx2388x, and decode broadcast NTSC, PAL, or Secam television. These cards deliver an unencrypted data stream to MythTV, which then encodes to MPEG-4 or RTjpeg using software encoders. Advantages of this type of card are:
- Low cost
- High flexibility -- you can choose your encoding method and employ filters prior to encoding
- If you have the CPU power, MPEG-4 produces better quality with smaller file sizes than the MPEG-2 used with most hardware encoding cards
The main disadvantage of this type of card is the high CPU requirements. Depending on your encoding settings, you may need close to 1GHz of CPU speed to handle a single encoding stream, and more if you have multiple cards or want to play back while recording. MPEG-4 is particularly demanding; for systems with weaker CPUs, RTjpeg consumes less CPU time at the cost of larger file sizes and more digital video artifacts.
These cards, currently the Hauppauge PVR cards, the AVerMedia M179 and any other card which supports the V4L2 MPEG Encoder API, have a video processor on-board which compresses the tuned video signal into an RTjpeg, MPEG-2, or MPEG-4 program stream before sending it into the computer. The advantages of these cards are:
- Greatly reduced load on the entire machine (with certain technical caveats). This is particularly important for machines with weak CPUs or when using multiple tuners in one system.
- The reduced load means that your video stream is less likely to suffer from encoding "hiccups" if a process causes the system to become momentarily too busy to process the video.
- Most hardware-encoding cards output an MPEG-2 stream, which can simplify backing up to DVD.
The main drawback of this type of card is that you're limited to the video format and encoding options supported by the manufacturer (usually MPEG-2). You can transcode to MPEG-4 after recording to save space, if desired, but this requires post-record processing and will degrade quality slightly.
Most cards of this type are designed to encode standard definition analog (NTSC, PAL, or SECAM) video signals. At least one product, though, the Hauppauge HD PVR, encodes analog HD output, as produced by cable or satellite TV boxes. Such a product is currently the only way to record HD content from providers that encrypt their digital data streams. (Some cable boxes have FireWire outputs, but cable operators often encrypt the IEEE-1394 output for some of the channels tuned by these boxes.)
Digital Capture Cards
The only form of digital capture MythTV currently supports is that over FireWire from compatible set-top-boxes. This could actually be considered an external tuner connected over firewire. There are currently several HDMI capture cards and devices available on the market, however these will not work for capturing video. Nearly any device you may wish to record HDMI from will also be using HDCP to encrypt the stream, and as such cannot be used.
Digital broadcasting (for DVB and ATSC at least) is in the form of an MPEG-2 transport stream, so unlike analog capture cards, there is no need for any kind of on-board encoding engine, the required program stream is extracted and handed directly to the computer for viewing or saving. Some cards have a hardware Program ID filter (hardware pid) which means the card can extract the required program stream from the transport stream itself.
In either case, the computer power required to save a MPEG-2 transport stream (but not view it) is very small, being only what is required to shift data from the PCI/USB bus and save it to disk.
Confusingly, many useful tools for working with MPEG-2 transport streams have "dvb" in their names, even though they work just as well with MPEG-2 transport streams derived from "ATSC" broadcasts.
Some digital capture cards also support analog (NTSC or PAL) transmissions, usually via a frame grabber. If your digital capture card lacks such hardware and you want to record both digital and analog transmissions, you'll need to buy a separate analog capture card - either a frame grabber or a hardware-encoding card. If your card does support multiple different modes, it will often be a hybrid tuner, rather than a full dual tuner. If that is the case, you will have to set up both inputs in the same Input Group to tell MythTV that they cannot be used at the same time.
Some cable and satellite TV boxes include FireWire ports. You can use these ports, along with a FireWire port on your MythTV box, to record both analog and digital channels from the cable box. Essentially, the FireWire card and cable box function like a digital hardware MPEG card, although configuration details and capabilities differ. Depending on your cable operator, the channels tunable via the cable box may be the same as those that are tunable via a standard digital tuner card, or you may be able to record some or all of the cable system's encrypted channels, as well.
With the world moving towards digital broadcast standards, this type of card is likely to become dominant in the next few years.