Buffer,Underrun,and,Overrun,Sc computer Buffer Underrun and Overrun Scenarios

----------------------------------------------------------Permission is granted for the below article to forward,reprint, distribute, use for ezine, newsletter, website,offer as free bonus or part of a product for sale as longas no changes a Gone are those times when the companies and the organisations didn't need a hi-tech system to handle them. Owing to the considerable increase in the business sector and thus, an enormous increase in the complexity of the organisational struc

----------------------------------------------------------Permission is granted for the below article to forward,reprint, distribute, use for ezine, newsletter, website,offer as free bonus or part of a product for sale as longas no changes are made and the byline, copyright, and theresource box below is included. ----------------------------------------------------------Buffer Underrun and Overrun ScenariosBy Stephen BucaroBuffer underrun and buffer overrun are occurrences thatcan result in some very frustrating errors. This is not a"how-to" article about fixing buffer underrun and bufferoverrun errors, but a basic description of what a bufferis, why we need buffers, and what causes buffer underrunand buffer overrun.Buffer UnderrunThe most common occurrence of buffer underrun is CDrecorders. Let's imagine an example of a CD recordingsession. The computer has an ATA hard drive capable oftransferring data at a rate of 8 MBps (Mega Bytes persecond). The CD recorder has a recording rate of 8 MBps.Everything should work fine, right?Note: The data transfer rates mentioned in this article donot apply to any specific device. They're just for purposesof discussion.The 8 MBps specification for the hard drive is for "burst"mode. In other words, it can transfer data at a rate of8 MBps for only a few seconds. Then the transfer rate dropsmuch lower, and if the hard drive hasn't been maintained,for example it has not been defragmented recently, thetransfer rate can drop even lower.Whereas a hard drive can skip from cluster to clusterwhile reading and writing, a CD recorder must burn the datatrack in a continuous stream without stopping. The designof a CD recorder requires a "sustained" transfer rate.When two devices that operate at different transfer ratesmust communicate, we can make them work together by placinga buffer between them. A buffer is a block of memory, likea bucket for bytes. When you start the CD recording session,the hard drive begins filling the buffer. When the bufferis almost full, the CD recorder begins drawing bytes out ofthe buffer.If everything goes smoothly, the hard drive will be ableto keep enough bytes in the buffer so that the speedy CDrecorder won't empty the buffer. If the buffer runs dry,the CD recorder has no data to burn into the CD, so itstops. Buffer underrun error.We can reduce the chances of buffer underrun by configuringa larger buffer. Then the hard drive will be able to putmore bytes in the bucket before the CD recorder startsdrawing them out. However, sometimes you can't increase thesize of the buffer because the computer doesn't have alarge amount of RAM installed. When the computer needs moreRAM, it uses "virtual" RAM. That is, it allocates part ofthe hard disk and pretends like that's RAM. Now, even thoughyou've increased the size of the buffer, you have causedthe hard drive to work even slower.Buffer Underrun and Overrun ScenariosBuffer OverrunThe most common occurrence of buffer overrun is videorecorders. Let's imagine an example of a video cameraconnected to a computer. The video camera records at a datarate of 168 MBps. The computer monitor is capable ofdisplaying data at a rate of only 60 MBps. We have a bigproblem, right?Thanks to MPEG compression, we might not have as big aproblem as first appears. With MPEG compression, the videocamera does not have to send the entire image for everyframe. It sends only the data for the part of the imagethat changed, and it compresses that part.If the image doesn't change much, and the part that changedcompresses well, the video camera might need to transfer ata rate of only a few MBps. But if the entire image changesevery frame and the image does not compress well, the videocamera might transfer data at a higher rate than thecomputer monitor is capable of displaying.Again, we have two devices that operate at differenttransfer rates that must communicate. We can make them worktogether by placing a buffer between them. When you startrecording video, the video recorder starts filing thebuffer. The computer display immediately begins pullingdata out of the buffer to compose display frames.If everything goes smoothly, the computer display will bepulling data out of the buffer fast enough so that thebuffer never completely fills. If the buffer fills up, thevideo camera can't put any more data in, so it stops.Buffer overrun error.We can reduce the chances of buffer overrun by defining alarger buffer. Then the video camera will be able to putmore bytes in the bucket before it fills up. Hopefully,the video camera will run into a few frames where theentire image doesn't change, reducing its data transferrate enough so the computer display can catch up.Underrun, Overrun ProtectionToday, CD recorder buffer underrun is much less common.Computers come with much more RAM than they did before,and CD recorders have learned to monitor the buffer andreduce the recording speed if the buffer starts to run low.Video camera buffer overrun is also less common. Video usesa program called a "codec" (for encode/decode). A smartcodec can monitor the buffer and reconfigure itself whenthe buffer gets too full. It might for example automaticallyreduce the color depth of the video, or drop frames, untilthe computer display catches up.Underrun and overrun Protection doesn't completely solvethe problem. If underrun protection activates, a CDrecording session will take much longer. If overrunprotection activates, the video quality will be reduced.The only way to solve underrun and overrun problems, afterincreasing the size of the buffer, is to match the datatransfer rates of the devices that need to communicate.You can upgrade to a faster hard drive, or install to ahigh performance video card.Now, if you need to troubleshoot a buffer underrun orbuffer overrun errors, at least you know what a buffer is,why we need buffers, and what causes buffer underrun andbuffer overrun errors.----------------------------------------------------------Resource Box:Copyright(C)2004 Bucaro TecHelp. To learn how to maintainyour computer and use it more effectively to design a Website and make money on the Web visit bucarotechelp.comTo subscribe to Bucaro TecHelp Newsletter Send a blankemail to subscribe@bucarotechelp.com---------------------------------------------------------- Article Tags: Buffer Underrun, Buffer Overrun, Hard Drive, Video Camera, Computer Display