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Screenshot of BBC iplayer: television and radio through the Internet.

Streaming media

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by Chris Woodford. Last updated: March 7, 2018.

A decade or two ago, the telephone wire heading into your home was a quaint way to chat with your family and friends when you couldn't speak to them in person. The basic idea hadn't changed much since the 1870s, when Alexander Graham Bell (1847–1922) and others pioneered telephone technology. But in the 21st century, people have started to see telephone lines a different way: now they're broadband Internet connections, piping music downloads, YouTube videos, news, and information—as well as telephone calls—into our homes 24 hours a day. Streaming media (a way of playing files as they download) has been a central part of this information revolution. What is it, exactly, and how does it work? Let's take a closer look!

Photo: BBC iPlayer is a pioneering "on-demand" television and radio player that streams programs down your Internet connection in "real time." That means programs play at the same rate they download, and each program takes roughly the same time to watch via streaming as it would if viewed live on your television.

What is streaming media?

If a picture is a worth a thousand words, a moving picture is worth a million. But how do you cram all that information down a telephone? The trouble is that a couple of copper wires—the basic technology behind our home phone lines—cannot, ordinarily carry information quickly enough to bring things like radio and TV into our homes. If you've ever watched a fax machine chugging along, sending or receiving a printed document at a grindingly slow speed, you'll know just how slow telephone lines can be at carrying anything other than a person's voice (the only job they were ever designed to do).

In the days when most people had dial-up Internet connections (where you make a connection to your Internet Service Provider using a modem to enable what is essentially just a normal telephone call), slow speeds were a major limitation on what could be done online. If you wanted to listen to an MP3 music track (typically about 5 megabytes in size), you could spend half an hour waiting for the entire file to download onto your hard drive, then open it up and play it back. Video files (more likely to be 50 megabytes) would take several hours to download this way, so they were not generally available on the Net. In those days, it was impossible to listen to a music or movie file of any size without a long and tedious wait. The problem was essentially a matter of bandwidth: the speed of an Internet connection (how quickly it can download information) sets a limit to how quickly you can transfer a file.

In the mid-1990s, in the early days of the Web, Rob Glaser and his Real company (originally called Progressive Networks) pioneered streaming media as the solution to this problem. The basic idea is simple. Suppose you want to watch a large video file on your PC. You install a media player (a streaming-media-playing program) on your computer that plays the file while it downloads. So it downloads maybe the first 10 seconds of the file, stores or buffers it, then immediately starts to play it. As the media player starts playing the first part of the file, it's also downloading the next 10 seconds ready for when you come to that bit. The media player never actually stores more than a little bit of the entire file: once it's played part of the file, it deletes it to make way for the next bit. If the media player can download the file as fast as you're watching or listening to it, you'll see no interruptions; if there are delays in downloading for any reason, there will be occasional pauses while the player downloads and "buffers" the next bit of the file.

Real Player screenshot

Photo: Streaming media as it used to be. This is an early version of RealPlayer (the program that kick-started the streaming media revolution) playing here over a 56 Kbps dial-up Internet connection. The status bar at the bottom shows the radio station is being played at 44.1 Kbps (over 44,000 binary digits per second), which is well within the limits of the connection. The trick with streaming media is for the player to adapt to the limits of your Internet connection to avoid excessive buffering.

Downloading and streaming compared

Before we go any further, we need to know more about how the Internet works.

Data (computerized information) moves efficiently across the Internet by being broken up into little bits known as packets. Each packet is independently addressed and travels separately, and different packets can travel by very different routes. Imagine yourself wanting to send a really heavy textbook to a friend in another country. Instead of sending the entire book, you tear it into separate pages, put each one in its own envelope with a separate stamp and address, and mail all those envelopes one after another. Your friend may receive them at slightly different times, in the wrong order, but she can easily reassemble them into the book. Why would you mail a book in such an odd way? It turns out the Internet works best this way with everything broken into small, similar-sized chunks. (We have a whole article about how the Internet works that explains all this in more detail.)

When you download a file in the traditional way, you're effectively asking another computer (a server that sends out files to many different people) to send you zillions of packets one after another and you have to wait for all of them to arrive before you can do anything with any of them. With streaming, you start to use the packets as soon as enough of them have arrived. That's the essential difference. You can think of streaming as playing during downloading, but in fact the two things are different in all sorts of ways:

Examples of streaming media

Genuine streaming

QuickTime screen shot

Photo: Genuine streaming: Apple's QuickTime offers a choice of streaming music and movies. It's only one of three rival streaming technologies.

Most Internet radio stations use genuine streaming, downloading and playing simultaneously with a program like RealPlayer, Apple's QuickTime, or the Microsoft Windows Media Player (the three rival types of streaming). With a broadband connection, you can enjoy audio quality that's not far off the quality you get from a downloaded MP3 file (though, as we discuss in our article on MP3, that's never quite as good as you'd get from a CD). As Internet connections have become faster, and more people have broadband, it's become possible to watch videos and TV programs this way too, though unless you stream in high definition over a really fast broadband line, quality is still far short of what you'd get from watching TV or a DVD. That's one reason why online movie stories still sometimes use downloads instead of streaming.

Pseudo-streaming: progressive downloading

Not all websites that appear to stream video work by streaming. Some (including YouTube) actually use an alternative approach called progressive downloading (fast-start streaming), which is like a cross between conventional downloading and streaming. It's popular because it's often quicker and easier to implement than genuine streaming. A large chunk (and sometimes all) of the file you're watching downloads into your web browser's cache (its internal working memory buffer) and your browser plays it simultaneously. Unlike with a truly streamed video, you can't always skip forward: generally you have to wait for the file to download to the point you want to see. Another key difference is that the file remains in your browser cache even when you've finished watching. You can tell when a website is working by progressive downloading because the video window will show two separate indicators on a progress bar, like the one below: one shows you how much of the file has downloaded, while the other shows how much you've played. Until recently, almost all progressive downloading used Macromedia Flash files (with SWF or FLV extensions), that are served from a conventional web server and played on a Flash plugin installed in your browser. Following the arrival of HTML5, modern browsers (including the slimmed-down ones on smartphones) can now do this themselves without using Flash at all.

Example of a progressive download progress bar.
Photo: A progressive downloading progress bar. When you see this, you're not actually streaming a video—you're playing it as you download the entire thing. If you know how to look in your browser cache, you'll probably find the file is sitting in there somewhere after you've played it (look for a very big file or one with a very new date and time, but don't expect to find a meaningful filename or extension).

How Real streaming works

The artwork below, taken from Rob Glaser's original patent, shows how streaming media was originally designed to work. The figure on the left shows how sound is packaged and sent as digital data to your computer (the purple box at the bottom); the one on the right shows how your computer receives, unpacks, and turns this data back into sound. Let's look at these two steps (transmission and reception) in a bit more depth.

Transmission

Schematic diagram showing how a Real Player packs and unpacks audio data streamed over a telephone line.

Artwork: How streaming works in a Real Player. From US Patent: 5,793,980: Audio-on-demand communication system by Rob Glaser et al, Real Networks, August 11, 1998, courtesy of US Patent and Trademark Office.

Live or recorded sound (red boxes) is made from analog (continuously varying) sound waves. So, at the transmitting end, the first step is to convert these waves into digital data using an analog-to-digital converter (green). If the source of sound is digital to begin with, we can miss this step out. Next, the digital data is compressed (orange) by over 95 percent (in the original patent, they quote a figure of 22:1) so it can be transmitted more quickly. The compressed digital data is stored (light blue) and then progressively transmitted by a server (dark blue) over the Internet (yellow) to your PC (purple). Of course, there's no reason why lots of programs can't be compressed and stored indefinitely in a library and then streamed whenever people want to listen to or watch them, but don't forget that Real Player was originally designed so it could stream things like live radio shows as they were transmitted from the studio. It's also worth noting that the original version of Real Player shown here was designed to be able to send real-time audio across grindingly slow dialup Internet connections (as slow as 14.4 Kbps); modern streaming can send high-definition video over broadband connections hundreds of times faster, but works in essentially the same way.

Reception

Inside your PC, a similar process runs in reverse to turn the digital data you receive back into sound. Packets of data are received down a phone line from the Internet (yellow) and stored in buffers (small bits of DRAM memory, blue). The buffers are designed to be as full with data as possible: if they're too empty, because the audio isn't being transmitted fast enough, there's a risk of the audio playback being interrupted (which is when you see the annoying "Buffering" message). The data from the buffers is decoded and decompressed (orange) and passed to a wave driver in a sound card (light gray), which generates streams of audio still in digital format. A digital-to-analog converter turns these streams into analog signals that power a loudspeaker, hopefully recreating a faithful version of the original sound (red).

What about errors?

Garbage in, garbage out—so goes the old saying in the computer business. So if the data from the "transmitting" server gets corrupted, won't you hear poor quality audio from your loudspeaker? The modems sending and receiving data use error control/correction protocols to ensure that the data that's received is the same as the data that's transmitted.

Where next?

Streaming media—bringing music and videos on demand—has advanced dramatically since I wrote the first version of this article in 2007. Back then, I wrote:

The audio and video information you download has to be compressed dramatically (substantially reduced in quality to make smaller files) so it downloads in a reasonable amount of time. Most streaming videos are still quite small and "pixelated" (full of obvious square blocks) when they play on something like YouTube. That's because even today's best broadband connections are not fast enough to download the huge amount of information in a fullscreen, high-definition TV picture. In a few years time, when broadband connections are many times faster, it will be possible to stream HD-quality video over the Net. Then the age of information-on-demand really will have arrived!

How times have changed! According to the ITU-T's ICT Facts and Figures 2017, 84.4 percent of households in developed countries (and 53.6 percent in the world as a whole) have Internet access and global Internet bandwidth grew by 32 percent in just the one year between 2015 and 2016. More users with more bandwidth, many with modern browsers that handle HTML5, have made possible ever greater use of streaming media. Collectively, the world now watches a billion hours of YouTube per day, while Facebook Video has rapidly become one of the most used features of the world's favorite social media platform since its launch in 2015. Netflix, the world's most popular streaming movie service, had reached 117.58 million subscribers by the end of 2017; many of its movies and TV shows are available in HD. Reliable, affordable, high-quality streaming of audio and video is now the norm; two decades ago, when Rob Glaser was granted his original streaming-media patent, it would have seemed like a very distant dream.

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Text copyright © Chris Woodford 2007, 2018. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2007/2018) Streaming Media. Retrieved from https://www.explainthatstuff.com/streamingmedia.html. [Accessed (Insert date here)]

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