What is bitrate
 

Professional cameras have specified bitrate as one of the technical attributes. Canon XF300 has a Multiple bitrate of 50Mbps. What effect does bitrate generally have on the video? Does it have something to do with the quality of the video?

When recording video, raster data (XxYxRxGxB) is taken from the sensor, compressed into a readable format, and saved to a tape, disk, or memory card.

Bitrate, measured in megabits-per-second, is the amount of data allocated to each second of footage. This means that a camera with 30mbps recording at 30 frames per second will have to divide it's 30mb up over 30 frames, giving each frame one megabit of data to be encoded in. Obviously, higher compression ratios (say, 5mbps) mean packing a lot of pixel data into a smaller space. This is done at the cost of image quality.

Some other info:

Different systems have different compression efficiency. Mpeg2 is extremely inefficient, and needs higher bitrates to produce a comparable image to say, Mpeg4. At the same time, mpeg4 typically stores less detailed keyframes on its own, and is frequently limited to only 24mbps. Mpeg4 may also be referred to as h.264 or AVC, and is known to handle complicated motion better than other compressions. There are also codecs that are built to store data on a frame-by-frame basis, like DVCPro or REDcode. These formats record at ridiculously high bitrates (100mps+) and basically store data as if they were still cameras taking thousands of separate pictures.

Do not confuse MBps with mbps. MBps is MegaBytes, while mbps is megabits. Megabytes contain 8 times as much information. A 10MBps SDHC card is actually capable of 80mbps.

What can you say about 50Mbps bitrate of Canon XF300? How much rate can you give for its quality on this amount of bitrate? How do you compare it with Sony XDCAM series?

As CODECs evolve, the actual bit-rate becomes, well, meaningless. Bit-rate is overshodowed in importance by the compression algorithms of the CODEC in question.

Said another way, trying to compare CODECs is sort of like trying to compare apples and oranges. You can do it, but you don't learn much -- about apples, oranges, or fruit in general.

The XF300 runs an MPEG-2 type CODEC. It doesn't compress as much, or as well, as your typically AVCHD CODEC (which is based on MPEG-4). This is why you can get more or less the same video quality from an XF300 as you can from, say, a Panny AF100. Even though the AF100 is recording at half the bit-rate.

I'm just sayin' that there's a lot more to video quality than just the bit-rate.

[Quote]What effect does bitrate generally have on the video? Does it have something to do with the quality of the video? [?Quote]

In general, and other things being equal, higher bit rate allows higher recording quality and/or lower compression. It also costs more in terms of memory and storage required for a given amount of recording time, and the added cost of the higher speed memory required.

As discussed above, it all boils down to how the vendor uses the higher bit rate in his implementation - and with professional gear there is good reason to expect higher bit rate gear to deliver higher overall performance.

The EBU did a series of highly involved codec trials a year or two back, and made approvals on the basis of these. In terms of camera codecs, the only two that got a completely green light were XDCAM 422 50Mbs, and AVC-Intra 100. (Higher end codecs - such as HDCAM-SR - are also obviously OK.)

That's not to say that the standard (35Mbs) XDCAM codec is "bad" - but it's not fully approved from the broadcasters perspective.

It's also not possible to uniquely define a given quality to any codec at a given bitrate. It will depend on the individual coder as well as codec and bitrate. The more complex the codec, the truer that is, especially for real time (such as in camera) encoding. That's why although AVC-HD (at 21Mbs average) may POTENTIALLY be able to rival MPEG2 at 35Mbs, no current hardware can. It is unlikely that AVC-HD will ever be fully approved for broadcast, not without a change in the spec, unlikely to ever rival the XF300 codec. (AVC-HD has no provision for 422 at the moment.)