DAB DYNAMIC

I note with interest the enthusiasm for the sound quality of the Leak Troughline II and III tuners (“arguably the best-sounding tuners ever”), but having owned Accuphase and Quad (FM4) tuners in the past, and as a convert now to DAB, I have been unable to ignore the obvious shortcomings of FM stereo.

To begin with, the stereo separation is frequency-dependent, being significantly reduced at the frequency extremes.  We might also remind ourselves of the rapid roll-off above 15kHz to filter out the 19kHz pilot tone.  The signal-to-noise ratio is easily compromised by sub-optimal signal strength, but even at its best it falls well short of DAB’s silent background, while at its worst it can exhibit a needlessly distracting hiss, along with other forms of interference.

This is all well known, but the dynamic range compression (DRC) applied by Radio 3 to most FM transmissions has been less extensively publicised.   I took the opportunity to analyse three identical clips of the same Radio 3 broadcast: from FM, DAB and Freeview.  I found that the dynamic range on FM was some 6 to 8 dB less than that on DAB and Freeview.

The effect of this dynamic range compression is clearly audible.  When an acoustic musical instrument is played more forcefully, it is not only the loudness that changes: the tone also changes because a different mix of harmonics is excited.  This is especially true of the piano.  Strike a note hard and the sound is both loud and brilliant, but when you play it gently, the hammer rebounds more slowly off the strings, thus allowing the hammer felt time to damp some of the higher harmonics, and in consequence yielding a smoother, more velvety tone.

If you take 8 dB out of the dynamic range of a recording, you change the relationship between volume and tone quality in a way that was neither intended nor engineered by the performing musicians.

We may further observe that the crackle from an original 78 rpm record can be acceptably suppressed in the DAB broadcast, but raised to the level of being intrusive on FM.  Once you’re aware of DRC, there’s no going back.

Classic FM provided the most extreme example for me some years ago when they compressed Emil Gilels’ recording of Beethoven’s Pathetique Sonata to such an extent that melody and accompaniment were brought up to the same level in the slow second movement.  The third movement sounded as though some highly inappropriate pop music processing techniques had been applied.

Classic FM’s range compression had mangled the music beyond anything I could recognise from my own LP of that recording, and Gilels must have turned in his grave.  I wrote to complain, but was simply told that as the compression was routinely applied at the FM transmitter, there was nothing anyone could do.  For my part I could not accept their FM broadcasts as a source of high quality sound.

In 2009 Tony Brown, of BBC Digital Information, confirmed my findings: “We do not use range compression for Radio 3 on DAB radio. DRC if selected gives a suggested setting and a few DAB sets even offer twice the figure if you select it. If DRC is not mentioned, then DAB sets don’t apply it. So for DAB sets with DRC if the switch is set to “off”, DRC was never applied in the first place.  DRC is a DAB term, and there is a small amount of audio range compression on FM. So small though that quite a few listeners don’t realise we apply it. The reason this is done is that it makes car radio and portable sets better able to cope with noisy surroundings as there is no DRC equivalent with FM.”

I suspect that rather more than “quite a few listeners” don’t realise that DRC is applied to Radio 3 on FM, but direct comparison of DAB and FM is difficult because you cannot consistently set each source to the same level.  However, the time delay between the sources does at least allow you to listen for a sudden fortissimo section on FM and then switch quickly to DAB to hear it repeated with the original wide dynamic range.  I noted this particularly on a live daytime Prom concert broadcast, and it was very obvious indeed.  Although the BBC neglected to confirm this point when I asked, it appears likely from other sources that compression is bypassed for Radio 3’s evening transmissions.

Whatever our individual aesthetic judgements of the relative merits of FM and DAB, Radio 3’s range compression is in my view a final and compelling argument against FM stereo, regardless of the merits of specific models of tuner.

Mike Thomson

Yateley

Hampshire

North London's DAB transmitter at Alexandra Palace has limited

reach; it doesn't usefully serve much of West London.

Thanks for your informed observations on dynamic range compression Mike. There is a little more that concerns people than just this bit of signal processing, however. Where FM may have dynamic range compression applied, large or small, DAB suffers severe data reduction - always large - from stone age MP2. Massed violin strings merge into a crude generalised representation, for example, and there is no sense of space, inner detail or stage depth, a tell-tale sign that ambient cues have been stripped out. This is what VHF/FM and a Leak Troughline, in all its simplicity, but purity, avoids – and you can hear it. That is why it’s sound is appreciated.

At the end of the day you should let your ears be the judge. And for me the Troughline transports me into the studio during live talks, where DAB just paints up a barren canvas.

The Leak Troughline cannot sound as good as a DAB tuner, says Mike Thomson.

As a technology DAB is neither entertaining, nor clever quite frankly. Everything about DAB was wrong, from its lack of extensibility to its crude data compression system. Its high transmission frequency of around 240MHz causes hills to block it, and concrete buildings and basements are no-go areas, so urban reception is poor. Hi-Fi World is just a few miles from North London’s main DAB transmitter at Alexandra Palace and we can’t receive it without burbling interference that eclipses the hiss of VHF/FM – and this is a common experience with DAB. Transmitter powers need to be increased, but that is now not going to happen because internet radio has pushed DAB into the technological long grass, where it belongs. NK