The IBOC system, branded HD Radio by its developer, Ibiquity
Digital, got a shot in the arm in August when a new audio encoding
algorithm tailored for the terrestrial digital system was introduced.
The new algorithm, called HDC, was developed jointly with Coding
Technologies. Coding Technologies is the creator of Spectral Band
Replication (SBR), which enables improved audio quality at extremely
low bit-rates. You may know Coding Technologies as the creator of the
“Plus” in AAC Plus, the AAC encoding enhancement.
The announcement came at an opportune time. The NRSC had suspended
its evaluation of the IBOC system because of the audio quality. It has
since reactivated its work. With the ongoing delays with a widespread
IBOC rollout, rumors abounded about the future of an IBOC system. For
now, the work appears to be moving forward again.
The coding issue has always been a sore spot for IBOC. Lucent's PAC
algorithm was showcased as the system to use, but it had always been
under fire. It was part of the IBOC mix from the merger between USA
Digital Radio and Lucent Digital Radio in 2000. Previously, USADR had
used AAC. AAC was also used in the tests that were in the reports
submitted to the NRSC. PAC had no track record and no submitted test
data to back the claim that it was ready. However, broadcasters knew
the score and made their voices heard.
According to Ibiquity, HDC had been under development for some time,
which was a well-kept secret. The payoff looks good, as reports from
listening demos show that HDC makes the grade overall.
Some demo CDs were made available to NPR personnel for evaluation. I
also received a demo CD and was able to play it in several
environments, which allowed me to explore the nuances in the
The demo CD contains 10 audio source samples. Each source is
presented in seven formats: the original source, FM analog, HDC at
96kb/s, HDC at 64kb/s, AM analog, HDC at 36kb/s and HDC at 20kb/s. Two
of the samples are spoken voice (male and female), one is classical,
one is a typical promo/commercial and the remaining six are various
styles of rock and pop.
The comparison between the source material and FM and AM analog
broadcast quality provides a baseline reference to current technology.
The analog samples are listenable and have the conventional sound of
our current technology. In my own listening tests, which were not
blind, I was able to hear differences between some of the encoding
schemes, but the differences were not troublesome.
Obviously, the highest bit-rate (96kb/s) sounds the best and most
like the original. Even with careful listening between the two, it was
difficult to tell the difference if I could hear one at all. As I
switched to the lower bit-rates, some subtle differences began to
At 64kb/s, I could sometimes hear a slightly metallic edge to the
audio, particularly in the high frequencies. The 36kb/s samples had a
reduced stereo separation and the high frequencies did not have the
same sparkle (slight high-frequency roll off) as the original. At
20kb/s, there was no stereo separation and the high-frequency roll off
was more pronounced, but it was still listenable and a marked
improvement over current mono AM.
All the differences I detected were subtle. Unless a listener was
able to directly compare the original source to the encoded version, I
don't think that he would be able to tell a difference. It appears that
the Ibiquity team has found a good solution to the audio encoding
problem, and I congratulate them on their accomplishment. Now that this
major obstacle is out of the way, work can continue in the other
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