DAB+: One Step Forward, Two Steps Back

Since the release of the Compact Disc in 1982, there has been a notable trend for multimedia to go digital. To be honest, I’m surprised it has taken this long for radio to follow suit. As much as the idea of digital radio was perfectly logical, its implementation is lousy, and I would have done things differently.

What Was Done Right

Separate frequency allocations for national and local stations

This was a brilliant move. Have you ever been listening to JJJ or SBS on a long car journey, only to have the signal drop out – leaving you aimlessly scanning for it on a different transmission tower? That won’t happen anymore, because national broadcasts no longer need to duck and weave around the local channel allocations.

Running at VHF Band III

The national broadcasts on VHF band III run at 206.352MHz, this at double the frequency of FM radio. The genius behind this: a quarter-wave FM antenna can operate happily as a half-wave DAB+ antenna. You can upgrade the radio in your car without having to replace the existing aerial.

Using a Modern Codec

As of late, new and better codecs have been released quite frequently. When DVB was launched in 2000, the audio codec of choice was MPEG-1 Audio Layer II, which has been around since 1993 and requires 192Kbps of bandwidth before it sounds half-decent. DAB+ uses the HE-AAC+ codec, which is the 1997-spec codec with some modern enhancements such as Spectral Band Replication (introduced in 2002 as part of the MP3Pro codec) and Parametric Stereo (2004). At the time DAB+ was drafted, it was one of the most efficient codecs available.

What I Would Have Done Differently

SMB Connectors

Since cars had radios, the car antenna has been connected with a Motorola connector. For whatever reason, car stereos with DAB+ have two antenna sockets. The classic Motorola connector for AM/FM, and a separate SMB connector for DAB+. Despite a 73cm fibreglass whip antenna being perfectly suited to FM and DAB+, manufacturers ship head units with a coil antenna that needs to be adhered to the front windscreen, forever in the driver’s field of view. It looks ridiculous, and is totally unnecessary.

I purchased an SMB to Motorola adapter, which is a significantly simpler, better and more elegant solution. The signal is so strong, that it the signal floods the unit when I’m within 10km of the transmitter with line-of-sight. This is a a common problem that also plagues Digital TV – too much signal causes problems. I would have thought my receiver would have some form of automatic attenuation, but it appears not.

Used a Royalty-Free, Unpatented Codec

Vorbis has been around for much longer than HE-AAC+, it sounds better and is completely open. Opus would probably be my choice these days, but it wasn’t around when the DAB+ spec was drafted.

The use of patented codecs means that royalties need to be paid, which just bumps up the cost of each unit, and makes the standard generally more cumbersome to work with. Because of this, I think it’s going to be a while before my DAB+ tuner card works on Linux (if ever).

Normalised the Output Levels

This drives me nuts. Just like on DVB, DAB+ is less than half the volume it should be, which means that my amplifiers need to work twice as hard as they should. I’m sure this can be fixed at a broadcast level, but for some reason everyone turns the volume down. On DVB, I’ve captured audio to confirm that the volume peaks at around 33%. DAB is probably about the same, but it’s hard to tell when you can’t record the raw bitstream.

Broadcast the Digital Feed Straight off the Mixer

You know how AM radio has a bandpass filter that gives it the frequency response of a telephone? Well, you’ll get that on digital radio, even on stations broadcasting at 80kbps. It’s almost like they’re capturing the compressed output, transmitting it, running it through an ADC then compressing to HE-AAC+ and broadcasting it digitally. It’s so far from raw that AM radio actually sounds better. FM has the same problem, but it’s not as bad because well… FM isn’t as bad. That said, it’s bad enough to negate any gain in sound quality you’d expect from DAB+. It combines the worst of both.

Conclusion

There were two things I was looking forward to when I upgraded moved to DAB+: Sound clarity and response that reached parity with compact disc, and a display which showed what song was playing. Both are theoretically possible on the technology, but neither are true due to a half-arsed implementation. I’d say to go DAB+ if you were offered the choice,  but considering there’s not much to gain, you’d only opt for this if you didn’t have much to lose. There’s always comfort in future-proofing your gear, but the early adopter’s tax often exceeds the price of re-buying later on. Remember: the first DVB (Digital TV) set top boxes in Australia were $600, were the size of a VCR and only did SD resolution. Five years later – they’re $50, one quarter the size, and analogue TV is still being broadcast. If DAB+ is modeled by this rollout, the winners will be those who hold off.