Buffer Underrun and Overrun Scenarios

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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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