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Buffer Underrun and Overrun Scenarios
By Stephen Bucaro
Buffer underrun and buffer overrun are occurrences that can result in some very frustrating errors. This is not a "how-to" article about fixing buffer underrun and buffer overrun errors, but a basic description of what a buffer is, why we need buffers, and what causes buffer underrun and buffer overrun.
Buffer Underrun
The most common occurrence of buffer underrun is CD recorders. Let's imagine an example of a CD recording session. The computer has an ATA hard drive capable of transferring data at a rate of 8 MBps (Mega Bytes per second). The CD recorder has a recording rate of 8 MBps. Everything should work fine, right?
Note: The data transfer rates mentioned in this article do not apply to any specific device. They're just for purposes of discussion.
The 8 MBps specification for the hard drive is for "burst" mode. In other words, it can transfer data at a rate of 8 MBps for only a few seconds. Then the transfer rate drops much lower, and if the hard drive hasn't been maintained, for example it has not been defragmented recently, the transfer rate can drop even lower.
Whereas a hard drive can skip from cluster to cluster while reading and writing, a CD recorder must burn the data track in a continuous stream without stopping. The design of a CD recorder requires a "sustained" transfer rate.
When two devices that operate at different transfer rates must communicate, we can make them work together by placing a buffer between them. A buffer is a block of memory, like a bucket for bytes. When you start the CD recording session, the hard drive begins filling the buffer. When the buffer is almost full, the CD recorder begins drawing bytes out of the buffer.
If everything goes smoothly, the hard drive will be able to keep enough bytes in the buffer so that the speedy CD recorder won't empty the buffer. If the buffer runs dry, the CD recorder has no data to burn into the CD, so it stops. Buffer underrun error.
We can reduce the chances of buffer underrun by configuring a larger buffer. Then the hard drive will be able to put more bytes in the bucket before the CD recorder starts drawing them out. However, sometimes you can't increase the size of the buffer because the computer doesn't have a large amount of RAM installed. When the computer needs more RAM, it uses "virtual" RAM. That is, it allocates part of the hard disk and pretends like that's RAM. Now, even though you've increased the size of the buffer, you have caused the hard drive to work even slower.
Buffer Underrun and Overrun Scenarios
Buffer Overrun
The most common occurrence of buffer overrun is video recorders. Let's imagine an example of a video camera connected to a computer. The video camera records at a data rate of 168 MBps. The computer monitor is capable of displaying data at a rate of only 60 MBps. We have a big problem, right?
Thanks to MPEG compression, we might not have as big a problem as first appears. With MPEG compression, the video camera does not have to send the entire image for every frame. It sends only the data for the part of the image that changed, and it compresses that part.
If the image doesn't change much, and the part that changed compresses well, the video camera might need to transfer at a rate of only a few MBps. But if the entire image changes every frame and the image does not compress well, the video camera might transfer data at a higher rate than the computer monitor is capable of displaying.
Again, we have two devices that operate at different transfer rates that must communicate. We can make them work together by placing a buffer between them. When you start recording video, the video recorder starts filing the buffer. The computer display immediately begins pulling data out of the buffer to compose display frames.
If everything goes smoothly, the computer display will be pulling data out of the buffer fast enough so that the buffer never completely fills. If the buffer fills up, the video camera can't put any more data in, so it stops. Buffer overrun error.
We can reduce the chances of buffer overrun by defining a larger buffer. Then the video camera will be able to put more bytes in the bucket before it fills up. Hopefully, the video camera will run into a few frames where the entire image doesn't change, reducing its data transfer rate enough so the computer display can catch up.
Underrun, Overrun Protection
Today, 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 and reduce the recording speed if the buffer starts to run low.
Video camera buffer overrun is also less common. Video uses a program called a "codec" (for encode/decode). A smart codec can monitor the buffer and reconfigure itself when the buffer gets too full. It might for example automatically reduce the color depth of the video, or drop frames, until the computer display catches up.
Underrun and overrun Protection doesn't completely solve the problem. If underrun protection activates, a CD recording session will take much longer. If overrun protection activates, the video quality will be reduced. The only way to solve underrun and overrun problems, after increasing the size of the buffer, is to match the data transfer rates of the devices that need to communicate. You can upgrade to a faster hard drive, or install to a high performance video card.
Now, if you need to troubleshoot a buffer underrun or buffer overrun errors, at least you know what a buffer is, why we need buffers, and what causes buffer underrun and buffer overrun errors.
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