Audio Advice

Digital radio on the different platforms

Internet radio - higher quality than DAB

Internet radio has acquired an underservedly bad reputation for audio quality, which has mainly been due to the likes of the BBC providing their Internet radio streams at very low bit rate levels, and so the audio quality has been poor. However, the UK's largest commercial radio group, GCap Media, has been providing Internet streams using 128 kbps WMA for its radio stations for some time now, and these provide higher audio quality than they're at on DAB. And at the time of writing (March 2007) the BBC has recently announced that it will be using new audio formats for its live and on-demand (i.e. Listen Again) radio streams, which will almost certainly mean that AAC/AAC+ will be used, in which case the audio quality will overtake that provided on the BBC's stations on DAB -- and the bit rates on Internet radio streams are only likely to increase over time.

There are also around 5,700 Internet radio streams on the Shoutcast.com Internet radio portal using bit rate levels of 128 kbps or higher with the MP3 or AAC+ audio codecs, and in my experience, the large majority of 128 kbps+ Internet radio streams provide higher quality than DAB stations do, which is due to MP3 and AAC+ performing far better at a bit rate of 128 kbps, because MP3 was designed to be used at a bit rate of 128 kbps, whereas MP2, which is the codec used on DAB, was designed to be used at bit rate levels of 192 kbps or higher (the BBC uses MP2 at 256 kbps for its TV channels), so it's unsurprising that it sounds as poor as it does at the 128 kbps bit rate level that 98% of stereo stations are using it at on DAB.

Multicast radio streams - higher quality than DAB+

We will see the BBC launch multicast in the next few months (the time of writing is March 2007), and the following table shows the bit rates and audio codecs that the BBC and the commercial radio groups have been using for the BBC's multicast streams on the trial so far:

1 - The BBC was using 128 kbps AAC for its trial multicast streams and the BBC is likely to use 128 kbps AAC when the multicast streams launch

Multicast will basically always provide higher audio quality than DAB+, let alone DAB. The BBC may well use 128 kbps AAC for their music stations on DAB+, but by the time that happens the multicast streams will be using higher bit rates, because the cost of Internet bandwidth follows Moore's Law, so the cost of it halves every 18 months and there's twice as much of it every 18 months. And the commercial radio broadcasters will use the switch to using DAB+ to reduce the bit rate levels they're using to reduce their transmission costs, so although their stations will provide higher quality than they do on DAB+, they won't be anywhere near as high quality as the multicast streams.

BT looks set to support multicast, probably either later this year or early next year; Virgin Media will start supporting it later this year when it launches its 50 Mbps cable broadband package; and Tiscali's network is configured to support it as well, and between them these three ISPs account for 60% of the UK's broadband market.

When you'll receive support for multicast depends on which ISP you're on, so it's difficult to generalise, but most ADSL users still have their broadband supplied by BT even if you actually pay someone else for broadband, so most ADSL users will have to wait until BT has rolled out its 21CN (21st Century Network) to your area. BT has begun rolling its 21CN, and it will be rolled out to virtually every telephone exchange by the end of 2010, with just a few telephone exchanges in remote areas of the country that will get it in 2011. BT's 21CN will also enable "up to 24 Mbps" ADSL2+ at the telephone exchanges, so we're likely to switch from ADSL to ADSL2+ between now and 2010/11 as well.

Digital TV platforms - higher quality than DAB

The BBC's radio stations all use higher bit rates on the digital TV platforms than they do on DAB, and the digital TV platforms use the same MP2 audio codec that DAB uses, so the audio quality of the BBC's radio stations on the digital TV platforms is higher than on DAB (apart from the World Service on Freeview, which uses the same bit rate on DAB and Freeview).

1 - Due to the BBC cramming too many stations onto their DAB multiplex, whenever Radio 5 Sports Extra goes on-air other stations have to have their bit rates reduced in order to accommodate it, and the rule the BBC uses is: if R5 Sports Extra goes on-air before 5pm, Radio 3's bit rate is reduced to 160 kbps; or if R5 Sports Extra goes on-air after 5pm, Radio 4 is reduced to 80 kbps mono.

2 - The same bit rates are used for all of the BBC's stations on Freeview, satellite and cable, apart from the World Service on Freeview

Many of the commercial radio stations also use higher bit rates on satellite than the same stations are at on DAB, although on Freeview the bit rates of commercial radios stations is the same as is used on DAB -- see the digital radio bit rates page for more details.

DAB

As suggested above, DAB provides lower audio quality than the other methods of receiving digital radio. Some people suggest that DAB is okay for listening to on portable radios due to the fact that portable radios are incapable of reproducing high quality sound; but as you can now buy Wi-Fi Internet radios, I'd recommend that you buy one of those instead of a DAB radio -- assuming you've got Wi-Fi, of course. Wi-Fi Internet radios are excellent pieces of kit, and people who've bought one invariably love them, so I'd say they're worth getting Wi-Fi for anyway if you haven't got it already.

Sound cards

The quality of a sound card in a computer can make a huge difference to the audio quality your computer produces, so the best general advice is to simply avoid buying cheap sound cards. The performance of sound cards in terms of value-for-money has improved over time, so you can now buy a sound card that would have been described just two or three years ago as being a "pro-sumer" (i.e. professional/consumer) card for as little as about £50 today.

The main issue has been that lower-priced sound cards convert all incoming and outgoing audio to a common sampling frequency of 48 kHz (which was apparently chosen to be in-line with the DVD standard), and all internal processing is performed at the common 48 kHz sampling rate. The reason it's been done like this is that it reduces the amount of digital circuitry required, which makes the chips used cheaper, and as the entry-level sound card market is fiercely competitive on price, even penny counts.

The problem with doing this is that CDs, and therefore the vast majority of MP3 and other compressed audio files use a sampling frequency of 44.1 kHz, and because changing the sample rate is a relatively computationally expensive task to perform if you want to maintain high quality, this also means that quite a lot of digital circuitry is needed to do it well, so cheaper sound cards have tended to cut corners here as well, and the audio quality has suffered as a result.

But as Moore's Law allows the number of transistors to fit into a given size of silicon to double every 18 months or so, it's become more affordable for the sound card manufacturers to process the audio internally at the native sampling rate of the audio, which means that the audio quality won't be reduced by converting to a different sampling rate.

Also the quality of the ADCs (analogue-to-digital convertors) and DACs (digital-to-analogue convertor) used on cheap sound cards haven't been very good on cheap sound cards -- although this issue can be avoided altogether if you use S/PDIF digital input/output, because this bypasses the ADCs and DACs altogether, see below for more on this subject.

The following sound cards that cost around £50 at the time of writing, perform far better than entry-level sound cards, and are well worth the extra money they cost:

• E-MU 0404 24/96
• M-Audio Audiophile 2496
• Creative X-Fi Xtreme Music

Digital Audio Output (S/PDIF)

Digital audio output (S/PDIF = Sony/Philips Digital Interface) connectors on audio and video equipment are far more prevalent today than they used to be, and they can improve the audio quality significantly when compared with outputting audio via an analogue output on lower-priced equipment, such as on low-priced Freeview set-top boxes. 

There are two forms of S/PDIF connections: optical and electrical (otherwise known as 'coaxial'). Optical S/PDIF is frequently referred to as TOSlink and have leads with connectors that look like this:

The electrical S/PDIF connectors use standard phono leads/interconnects that are used on hi-fi systems (if you've got phono leads that have the left and right channel leads connected together then you can just use either the left or the right (usually coloured white and red) and leave the other half unconnected).

A good website to buy S/PDIF cables from is http://www.tvcables.co.uk/, although there's plenty of suppliers on eBay as well.

The reason why it is usually better to send signals via S/PDIF than via the phono analogue outputs on lower-priced equipment, is that in order to save money, manufacturers use cheap, and therefore lower quality, DACs (digital-to-analogue convertor) and analogue audio output electronic circuitry, so the audio quality isn't as high as it could be. And the advantage of using S/PDIF is that it allows you to bypass analogue output circuitry altogether and use the higher quality analogue output circuitry on equipment that's dedicated to hi-fi audio instead.

The following diagrams shows the two different situations of sending the audio output via the analogue and the S/PDIF digital connections: