91av

DIGITAL radios costing many hundreds of pounds can make some types of music
sound worse than it does on FM radio, equipment reviews have revealed. Until
now, backers of Digital Audio Broadcasting (DAB) technology had promised
listeners “near CD-quality” sound.

The February issue of the British magazine Hi-Fi Choice compares the
DAB radio receivers now available from Arcam, Cymbol and Technics, which cost
from £500 to £1000. The reviewer was Richard Black, who is a concert
pianist and electronics engineer. He found that all three receivers produced
good sound with some material, but says “it all went horribly wrong” with music
that has what he calls “busy textures”. “It’s a kind of distortion that becomes
increasingly obvious, like metallic tinkling,” Black says. “It affects all kinds
of music, from rock group Aerosmith to Marta Argerisch playing Prokofiev’s 1st
Piano Concerto.”

91av learned the likely explanation at a meeting of the
Institute of Broadcast Sound at the BBC’s training centre near Evesham,
Worcestershire, last week. The problem will not be easy to cure: broadcasters
must either take much more care in producing their digital radio programmes,
reduce the number of channels they transmit, or change the transmission
standard.

The last option would make existing receivers obsolete overnight, and has
global implications because DAB is now an internationally agreed standard.

The sound problem appears to be buried deep in DAB’s workings. To prepare
material for digital transmission, a broadcaster uses a special encoder to chop
a continuously variable analogue signal into discrete “samples”, each of which
is assigned a digital code. The code is then compressed by throwing away audio
information which doesn’t affect the way it sounds.

This sampling is done at 48 kilohertz. This produces a range of audio
frequencies or “bandwidth” that spans more than 20 kilohertz, exceeding the
bounds of human hearing. The BBC originally planned to transmit DAB at 256
kilobits per second, but eventually chose to transmit 192 kilobits per second so
as to be able to pack more channels into its frequencies.

At the IBS meeting, BBC engineers showed why sound problems are occurring. If
the source of the music is an uncompressed digital recording, such as a CD or
DAT tape, DAB works near perfectly at 192 kilobits per second. But if the music
has already been copied onto a digital system that uses compression, such as
Mini Disc, or onto hissy analogue tape, the DAB encoder mistakes spurious noise
for music—and throws away musical detail while faithfully recording the
noise.

Alan Tutton of the BBC also demonstrated a clever, counter-intuitive
solution. If the sampling rate is reduced from 48 kilohertz to the “lower
quality” 32 kilohertz. The audio bandwith is reduced to 15 kilohertz—but
this is still a wider frequency range than most people can hear. The overall
result is much better sound over a DAB system than with 48-kilohertz
sampling.

Unfortunately, 48-kilohertz sampling is built into DAB chips and industry
insiders say it’s now too late to modify the international standard.