Let's start of with some specs. The base card supports full D1-quad compression of video into MPEG-4 video format. What this means is that it can encode 704x480 sized video at a rate of 120fps for NTSC, or 704x576 at a rate of 100fps for PAL. This breaks down to 4 full streams at 30fps and 25fps respectively. Alternately it can do CIF encoding (1/4 size of D1) at 4 times that frame rate, or for the math-lazy, 16 channels at 30fps at 320x240 frame size.
The card can be purchased in 4, 8 or 16 channel input models. So to take advantage of all 16 channels on the top model, you would either have to record in CIF mode (320x240) or reduce the frame interval to get 7.5fps per channel for full D1 mode (704x480). I will be speaking mostly in NTSC, but the card does support PAL, so do the conversions as we go.
The card allows for the usual MPEG encoding settings including GOP (Group of Pictures), Quantization and Intervals. Intervals are sort of the opposite of frames-per-second, but correlate the same way. An interval of 1 means that the encoder captures every frame, while an interval of 3 means it skips 2 frames between every frame it encodes. The video muxer on the card performs at 30fps, so the interval setting will decide how many of these frames get encoded.
The encoder itself performs quite well. It performs all encoding to an on-board SDRAM chip, and can DMA the frames directly to the host memory, which is great for performance. The original driver did not take advantage of this since it copied the frames to user space. The new driver I've written makes use of v4l2 and it's videobuf-dma-contiguous framework and thus allows for memory mapped buffers with userspace. This gives us zero-copy to userspace.
The encoder also supports side-by-side MJPEG compression of video frames. So while you can be recording the compressed MPEG-4 to disk, you can also frame grab JPEG images. This is useful for tools that want to do such frame grabbing for video analytics, or for live viewing over a web server (it's very easy to send frame grabs via an MJPEG cgi script).
All of this is built properly now on top of Linux's v4l2 API. Unfortunately the API does not expect compression cards to pipe MPEG-4 video, so most clients using v4l2 expect compressed video to be either MJPEG or MPEG-1/2 streams of some sort.
Currently the only drawback from the MPEG encoder is that the frames are self-standing MPEG-4 video frames. I have to add a header to the key frames for them to be usable by most decoders.
Overall the video capture is great. I've run 44 simultaneous records (16, 16, 8, 4 channel cards) on a Core2Duo with a system load average of 1.65, and only about 10% CPU usage. Most of the load is disk I/O.
Each encoder input also supports a graphical overlay that can be programmed at pixel level with varying colors. This is great for textual overlays. Currently we use it to place a descriptive name on the recording along with a timestamp.
In addition to the encoders, the card supports one uncompressed display port. It's currently exposed via v4l2 as a standard analog YUV device. It can be configured to show any of the inputs ports in tons of configurations. So you can do things like a 4-up display. This live display also supports a graphical overlay.
The display is sent to the video-out port on the card (hard-wired), so it can be hooked to a monitor as well (good for surveillance applications such as what Bluecherry offers).
Finally we'll discuss my least favorite part of this card. While it's not a killer, it is just odd that the card supports sound only in G.723 format. For surveillance applications this is just fine. Delivering 3-bit samples at 8Khz sample rate, which is a 24kbs. While this is good for bandwidth, it's bad for anything that needs better audio quality. Not to mention that storing the audio and video together in any sane format requires converting G.723 to linear PCM.
However, the G.723 to linear PCM conversion isn't much overhead on performance, and neither is the encoding to 16Khz MP2 audio, which is how we store it for our surveillance products. Overall, our format is MPEG-4/Video and MP2/Audio in a Matroska/mkv container. This is exactly how it was stored in my 44 stream example above.
- Fast and efficient
- Can handle multiple inputs easily
- The new driver works well with v4l2 and alsa
- Perfect for security applications
- Nice OSD capabilities
- Motion detection supported per input
- Side-by-side MPEG-4 and JPEG capture modes per input
- MPEG-4 video. The SOLO-6110 will support H.264
- Low quality audio is not great for anything other than special applications (no TV DVR)
- G.723 audio format has been obsoleted twice since it was introduced. Nothing uses it so you must always re-encode it.