Bit rates and ballyhoo

July 9th, 2009 → 10:19 am @ josh

A dropped stream is the nemesis of any regular Public Radio Tuner user. Nothing is worse than being caught up in a great public radio program and have it suddenly cut out. Our goal here is to minimize the occurrences of dropped streams, and we’ve been able to improve streaming on the Tuner with each new release (and will do so again with the upcoming version 2.0). But that’s only part of the story. There are many variable that impact mobile streaming: the quality of the cellular infrastructure, the internet connections it can provide, the connection speed of your iPhone, and even the speed at which you travel.

As you can imagine, mobile streaming over an Edge or 3G connection requires that a signal reach your iPhone via the nearest cell tower. When you’re driving around town using your phone for a voice call (using a hands free earpiece, of course), cell networks are able to “pass” the call from one tower to the next without interruption. Right now, the same is not true of data connections. Stream dropouts may be inevitable if you’re on the move and leave the range of one cell tower and pick up the signal of another. At that point, your phone will be issued a new IP address and the stream connection is dropped.

There’s also the issue of the “size” of the stream and the speed of your mobile connection. This is the fun part where we get to talk about bit rates. Bit rate refers to amount of information (bits) that are conveyed or processed per second (“kbps”, or often shortened to “k”). Common bit rates for live audio streams are 32k, 64k, and 128k. A higher bit rate results in a higher sound quality for the stream. But it also means that a larger amount of information needs to be squeezed into the signal to your mobile device. So a slower Edge connection can have trouble processing a high bit rate stream. Too much information is trying to pass to your phone at one time. The result: a dropped stream.

Stations are faced with finding that perfect bit rate: one that provides a decent sound quality, but small enough to provide stable streaming over a variety of internet connections. The challenge is even greater for music stations, which want to provide high sound quality and a stereo stream (which double the bit rate demand to achieve same sound quality as a mono stream).

For streaming to the iPhone, Apple recommends a minimum bit rate of 48k. But during our testing for the Tuner, we found that an EDGE connection can be spotty for any stream above 32k. So, if a station wants to be reliable on all three iPhone connection speeds (EDGE, 3G, and Wi-fi), a 32k stream is your best bet. But that’s not going to provide great sound quality, especially for a music station.

To help users navigate these muddy waters, a new feature in the upcoming version 2.0 will provide some guidance based on a station’s bit rate. When you select a station, the app will display icons informing you which connection speeds best fit that stream. You can see them below the DUQ logo on the screenshot. (click image to enlarge)

• If a stream’s bit rate falls under 48k, the app will show icons for EDGE, 3G, & Wifi
• Above 48k, but under 128k, the app will show only 3G & Wifi
• 128k will show Wifi only

There may be some instances where you are able to stream high bit station on a lower quality connection. There’s no harm in trying a 128k stream over an Edge connection – but now you have a better understanding why it might drop out while listening.

Some stations offer multiple versions of their live stream at different bit rates to allow users to enjoy the best the sound quality available based on the speed their internet connection. We encourage stations to make those multiple streams available via the Public Radio Tuner. Stations who are interested in adding or updating their current stream can do so using Public Interactive’s stream submission page.