Introduction to Wi-Fi radios

BBC now testing AAC/AAC+ Internet radio streams at higher quality than DAB

After a seven-month delay, the BBC has finally begun testing its live Internet radio streams using AAC/AAC+, and the BBC has launched AAC+ streams for the on-demand radio streams.

The on-demand streams are available to everyone, but the live test streams are only available to BBC iPlayer Labs testers, so if you want to listen to them you first need to go to the BBC iPlayer Labs sign-up page, then just click on the pink button that says "I want to be a Labs tester", then instead of the receiving the poxy Real Player streams you'll receive the new AAC/AAC+ streams.

The following table shows the bit rates and codecs the live test streams and the on-demand streams were using yesterday. Bear in mind that because the live streams are currently testing, the bit rates and which codec is used are likely to change over time. If you right-click on the Flash window it tells you what the bit rate and codec is, although it shows "aac" for both AAC and AAC+.

The World Service is the only "national" BBC station that isn't using AAC/AAC+ yet, which is because of some arcane rule that stops the BBC from providing higher quality to listeners in the UK compared to what people around the world receive, but the World Service will also start using AAC+ within the next few months.

Higher quality than DAB

All of the BBC test live streams and on-demand streams are now at higher quality than on DAB. The reason why the Internet streams are at higher quality than on DAB is because they're using the AAC/AAC+ audio codec, which is twice as efficient as the MP2 codec used on DAB, so the following applies for the AAC/AAC+ bit rates being used at the moment:

• 128 kbps AAC provides the same level of audio quality as 256 kbps MP2 on DAB
• 96 kbps AAC+ provides the same level of audio quality as 192 kbps MP2 on DAB
• 64 kbps AAC+ provides the same level of audio quality as 128 kbps MP2 on DAB (this is for mono -- it's different for stereo)

The following table compares the audio quality on DAB with the equivalent MP2 bit rates used for the live Internet streams. As you can see, the equivalent MP2 bit rate that the live Internet radio streams are all higher than the actual MP2 bit rate used on DAB, so the audio quality of the Internet streams is also higher. In addition, the stations that are always or sometimes in mono on DAB (in the table, s = stereo, m = mono) are all in stereo on the Internet streams.

1 - Radio 3 is reduced to 160 kbps on DAB in the daytime when Radio 5 Sports Extra is on-air before 5pm, and Radio 4 is reduced to 80 kbps mono on DAB when Radio 5 Sports Extra is on-air after 5pm

BBC is testing 128 kbps AAC for the live streams

As the table at the top of the page shows, the BBC is testing 128 kbps for the live test streams for  Radios 2, 3 and 4. If they use 128 kbps AAC once the live streams actually launch, that would obviously disprove my claim that they're trying to avoid using 128 kbps AAC! No-one likes to be wrong, but in this case I'd be very happy to be proved wrong.

However, they are only testing 128 kbps AAC at the moment, so it still remains to be seen whether they will use 128 kbps AAC for all of the live and on-demand stereo streams; and if they don't, I would be proved right.

BBC has reduced the bit rates of the on-demand streams

The BBC has been using 128 kbps MP3 and 192 kbps MP3 for the stereo on-demand radio streams since last June when radio was integrated into the BBC iPlayer, but the BBC has reduced the bit rate levels of the stereo on-demand streams from using 128 and 192 kbps MP3 to using 96 kbps AAC+ and 128 kbps AAC respectively!

The BBC has tried to justify doing this by saying that AAC+ is more efficient than MP3, so reducing the bit rates won't reduce the audio quality compared to what they've been providing with MP3. But looking at this from an alternative perspective, if they had just changed the streams from using MP3 to AAC/AAC+ and left the bit rate levels unchanged at 128 and 192 kbps, the audio quality would obviously have been higher than it is at 96 kbps. So the BBC has effectively deliberately reduced the audio quality compared to what it would have been if they hadn't reduced the bit rate levels.

My opinion on this is simple:

All BBC stereo Internet streams should use 128 kbps AAC

BBC iPlayer TV streams are consuming an enormous amount of bandwidth

The following figure shows how much bandwidth the BBC Internet radio streams and the BBC iPlayer TV streams have been consuming up to August last year. The iPlayer TV streams were already consuming over 7 times as much bandwidth as the Internet radio streams by August, which was just 8 months after the iPlayer TV streams had launched.

The second very steep jump in bandwidth for the iPlayer TV streams following a small dip in the summer was due to the launch of the higher quality 800 kbps H.264 iPlayer TV streams in August. The BBC is also planning on launching 1.6 Mbps SD and 4 Mbps HD iPlayer TV streams this year, which will lead to further massive increases in the bandwidth required, and that doesn't even take into account the exponentially increasing usage of the iPlayer, which also requires the bandwidth to increase exponentially.

The BBC also wants to make the iPlayer available on new Freeview and Freesat set-top boxes that have an Ethernet connection so that people can watch the iPlayer streams on their TV sets rather than on computers, which Anthony Rose, who's in charge of the BBC iPlayer, said could lead to the bandwidth required increasing ten-fold over the next year or two -- that would make the bandwidth for the iPlayer TV streams over 70 times higher than the BBC's radio streams were using last August.

I completely support the BBC launching new higher quality iPlayer TV streams, but it is hypocritical beyond belief to limit the bit rates to just 96 kbps for any of the stereo Internet radio streams -- especially in the same year that the BBC is planning on launching HD iPlayer TV streams that will use a bit rate that's 40 times higher than the 96 kbps that most of the radio streams are using!

All of the BBC's stereo Internet streams should use 128 kbps AAC without exception.

BBC on-demand streams can be made very reliable by using a very large buffer size

Both the TV and radio on-demand streams on the BBC iPlayer use what's called a 'dual-threshold' buffering scheme.

Dual-threshold buffering works as follows: When a user clicks play, the buffer size is set to be a small size (the lower threshold) so taht the stream starts playing quickly. Then once the stream has started playing, software increases the buffer size to the upper threshold value, and the server sends data to fill the buffer up, and the server continues to top the large buffer up as the stream continues playing.

The advantage of dual-threshold buffering is that the streams simultaneously begin playing very quickly and the streams can use an arbitrarily large buffer size. The advantage of using a very large buffer size is that the larger the buffer size is the lower the probability will be that the buffer will every empty, and buffering (i.e. the stream pausing) only happens when a buffer has run out of data.

In other words, the on-demand streams can be made arbitrarily reliable by using a very large buffer size. But this also means that the BBC cannot claim that it needs to reduce the bit rates of the on-demand streams due to reliability reasons, which the BBC has been using as a justification for limiting the bit rate levels of the Internet streams -- or at least the BBC was claiming this until earlier this week, but it seems to have quietly dropped this claim

BBC live streams will use 'automatic bandwidth detection' and 'dynamic streaming'

The iPlayer TV and radio streams are going to start using 'automatic bandwidth detection' in March (although given that the live streams were 7 months late in launching, this will probably be late as well), and they're going to start using 'dynamic streaming' by the summer, once the new Flash Media Server 3.5 has been released.

Automatic bandwidth detection: As its name suggests, this consists of a user's available bandwidth being automatically detected by the streaming servers -- it works in a similar way to how broadband speed tests do, where the user automatically downloads a file, and the time taken determines how fast the connection speed is to the streaming servers. The server will then deliver a higher bit rate stream to users that have a high enough bandwidth, and other users will receive a lower bit rate stream. The vast majority of users listening at home should receive the higher bit rate radio streams, though.

Dynamic streaming: This consists of the streaming server switching between lower and higher bit rate streams on-the-fly according to the speed of the connection from the user's computer or device to the streaming server -- the bandwidth can be determined by monitoring the level of data in the buffer, so for example if the buffer starts to empty quickly that means the connection speed to the server will have fallen, probably due to Internet congestion, and the server will switch to delivering a lower bit rate stream -- and vice versa for switching from lower to higher bit rate streams. Dynamic streaming also performs automatic bandwidth detection when a user first connects.

Switching between streams at different bit rate levels can be done without introducing a gap in playback between one stream finishing and the other stream starting (unless the buffer completely empties in the meantime, of course), but if the stream switches to a lower bit rate the audio quality would obviously be reduced.

For the vast majority of people who're listening at home, though, streams shouldn't need to reduce in bit rate level, because the Internet is sufficiently fast these days such that 128 kbps streams are basically small trickles of data in comparison to all the video streams that are being delivered -- the days when 128 kbps Internet radio streams were unreliable are long gone.

Microsoft has actually been offering its Intelligent Streaming (aka Intellistream) technology for years, which works identically to dynamic streaming on Flash, and the WMA streams for stations owned by the UK's largest commercial radio group, Global Radio, all use Intellistream.

As both automatic bandwidth detection and dynamic streaming are specifically designed to ensure that streams don't suffer from buffering, this means that the BBC cannot claim that reliability is an excuse not to use 128 kbps AAC for the live streams either.

Overall, then:

The BBC simply has no justification whatsoever to avoid using 128 kbps AAC for all of the live and on-demand stereo Internet radio streams.

Comments

Bookmark with:
Digg	del.icio.us	Reddit	Facebook
Google	Stumbleupon	Slashdot