Building a Digital Video Capture System - Part I


The commercials make it look easy. Get a video camera, shoot some scenes, capture the video into your computer, and with a few clicks and drags you turn those disjointed shots into a cinematic masterpiece ready to put on a CD, email to grandma, or stream off your Web site. No problem. But capturing and manipulating digital video in a computer is a little trickier than that.

NLE on a PC - Image courtesy of Pinnacle

In part one of this two-part article I'll give you a crash course in digital video capture basics, what kinds of system issues there are, outline the types of capture hardware available (and costs), and what kinds of results you can expect. In part 2 we'll roll up the sleeves and review and compare a number of capture systems that won't send you to the poor house.

Compression Is King

There is no other way to say it. Video takes up a lot of everything; bandwidth, storage space, time, and money. We might like to think that all things digital are preferable to all things analog but the brutal truth is that while analog formats like video might not be exact or precise they are remarkably efficient when it comes to storing and transmitting vast amounts of information. For example, a typical NTSC television program that you can receive with a pair of rabbit ears on a $60 TV set is streaming data at about 26 MB/s (that's megaBytes per second not megabits). If you strip out some of the carrier information and extra baggage inherent in NTSC signals (and convert to megabits for the sake of later arguments) you end up with about 124 Mbps. But that's still about 1.5 gigabytes per minute or 93.5 gigabytes per hour. If you want to do something simple like a dissolve from one scene to another then you have to have two video streams running at the same time so double those numbers. If you wanted to do HDTV-quality then you're closing in on 5 Gb/s.

You can see that we're beginning to talk about teraBytes of storage and transfer rates approaching gigabytes per-second. These days you can buy a 30 GB EIDE hard drive for about $150. While the EIDE specifications list a maximum transfer rate of 33 MB/s that's only theoretical. Under ideal conditions an EIDE drive might have a read/write rate of around 12-15 MB/s but most hard disk drives can only achieve those rates using caching techniques and only hit those numbers in very short bursts. In reality EIDE sustainable read/write rates are usually anywhere from 3 to 6 MB/s. Wide Ultra SCSI claims a theoretical transfer rate of 40 MB/s and SCSI Wide Ultra II claims maximum throughput rates of about 80 MB/s. But even these SCSI numbers are only theoretical while the reality is most SCSI buses and hard drives can only achieve sustainable read/write rates of about 6 to 13 MB/s.

We're still only halfway toward that 26 MB/s single-stream of uncompressed video. Now there are ways around this bottleneck (terribly expensive ways that is), such as using fibre-channel and specially configured RAID systems but even then there are other problems I'll touch on in a bit.