It’s amazing what some think HD-R will deliver over analog in the way of advantages and better performance – equally amazing that others believe the same digitally inspired innovations will instead be disadvantages and degrade radio listening everywhere. Even topics of new technology become political footballs passionately tossed around with spin and misinformation.
An engineer recently suggested on a popular list-server that after the hybrid conversion phase, we’d be able to turn off the analog modulation and replace it with all digital at the same analog power level.
Coverage would improve, he argued, and performance would be bulletproof. Early in this game, it’s easy to overlook the details and misunderstand the reality of HD-R.
Truth and Consequences
Let’s get this straight. The digital carriers on both AM and FM HD-R have to be approximately 20 to 23 dB below the level of the existing analog carrier. It’s got to stay that way to preserve the existing coverage areas of all existing stations, otherwise massive interference would result.
It’s hard to accept, but digital modulation is really that much more efficient. Two hundred watts of digital covers like 25,000 watts of analog. Station power listings will likely disappear from logos, letterhead and advertising one-sheeters in the coming era of HD-R; after all, “200,000 milliwatts” just doesn’t hack it.
During the hybrid phase, existing receivers need to be able to reproduce un-degraded analog audio with the presence of digital noise in the passband. Suppressing the digital carrier 20 dB helps that considerably. When the last analog carrier is turned off years from now, the FCC may revisit this limitation in light of changing service contours vs. interference.
We can’t dispute the fact that weaker adjacent channel signals on AM will be harder to hear for many when HD-R gets rolling. Radio engineers and DXers love to listen to weak signals, well beyond protected contours.
The average consumer almost never does and could care less. Don’t expect the FCC to have much sympathy or relief for interference complaints coming from the fringes. As with most existing interference issues in our analog world, buying a filter, a better receiver or antenna will remain the standard remedies.
Smoke and Mirrors
Interference is a hot topic as HD-R rolls out. The issue often is clouded by smoke and mirrors. Too many analog receivers using sloppy IF and decoder designs can give the impression an HD station is splattering digital noise beyond its legal channel limits.
Don’t be fooled by one receiver. The NRSC-specified occupied bandwidth RF mask for HD-R can easily be met by the new transmission gear out there. Marginally performing receivers are nothing new in radio. We can only hope that the Consumer Electronics Association will urge all radio manufacturers to build HD Radio-friendly designs from now on.
HD-R must employ audio bit-rate reduction to work in existing bandwidth-limited allocations. The debate over bit-rate reduced or compressed audio quality will never go away. Purists just won’t let that happen.
Too many of us remember when audio codecs were first used to conserve bandwidth and recording space. SEDAT gave the process a bad name. But codec technology has advanced light-years and continues to improve almost continuously.
Dueling algorithms are a real issue, to be sure; yet in a world where consumers define high-quality audio as a downloaded MP3, we are forced to face a bar that has been lowered on a different set of goal posts.
Audio quality has been subjected to an escalating and more insidious form of degradation right out of the recording studios. Almost all CDs now come “pre-processed,” complicating the mission of well-meaning audio processing aficionados eager to deliver that knockout sound for their PDs.
Gain leveling, limiting, equalization tweaks and even clipping have already been burned into the source material. Digital sampling and compression schemes just don’t like clipped waveforms. Running your air product through another round of dynamic molestation before it hits the PAC codec creates yet another new challenge for HD-R – and a new realization.
Almost every engineer who has test-driven IBOC and HD-R on their stations has told us that the task of processing the digital audio quickly becomes the need to unprocess. Otherwise the audio coming out the other end easily turns into a pile of dog doo-doo.
The more you adjust an Optimod 6200 or Omnia-HD to make it louder, the more you keep coming back to the old “less is more” imperative. Sure, you need to provide some automatic level control for noisy environments like moving automobiles. But beyond that, plenty of headroom and a little protection limiting are about all you really need.
Without the nemesis of pre-emphasis, processing HD-R for both AM and FM becomes a completely different challenge. The emperor’s clothes are little more than a see-though cape in this age-old battle.
The more important processing controls you will want to optimize are the ones that clean up or conceal codec artifacts. HD-R on AM uses a compressed datarate of 36 kbps. That delivers a scant 3 percent of the original audio information in stereo; 97 percent is cleverly ignored.
FM HD-R uses 96 kbps. Just a few years back, anything below 192 kbps was considered “marginal” for broadcast quality. Without question, MPEG and PAC have gotten better with every passing month, but innovative new digital processing tricks and products will mask many of the sins of compression and help make the air sound even better. Those who attend the NAB show and seek this out will be rewarded.
Even before your audio hits the processor, engineers and programmers preparing for HD-R conversion will help themselves the most by eliminating compression stages and reducing chances for dueling algorithms. Convert your stored music library and STL to all linear.
Fast workstations and big hard drives are now affordable. Insist on maintaining quality standards and closely monitor all imported audio sources, especially MP3s that arrive as e-mail attachments. Many that sneak by on analog will be hideous tune-outs on HD-R. As we’re finding out all too often, all MP3s are not created equal.
A special plea to program directors: Don’t embarrass yourself or your station by pounding HD-R with the same heavy processing you’ve become addicted to on analog. HD-R is your golden opportunity to make a clean break from the bad habits of the past.
Even the most unsophisticated listeners will be expecting digital radio to sound better. That means cleaner… like their CDs and (gulp) their MP3s. Insisting on the tired, old, highly processed “radio sound” on HD-R will not impress your present listeners or win over any new ones. If you’re having trouble with this, listen to XM or Sirius for some clues.
If you are lucky enough to attend NAB, look closely at all the new HD-R related products being shown.
This is truly the showcase year for HD-R. New antenna combining solutions to be shown will mean many master or panel antenna users will be able to reduce their transmitter and operating costs substantially.
Hang onto that backup or auxiliary antenna near your main. It might come in handy for HD-R. Ibiquity now says that closely maintaining the same analog and HD-R coverage patterns to support the 20-23 dB differential is turning out to be not as important as originally assumed.
As more field data comes in, the reality of existing pattern distortions caused by mounting structures and performance variations delivered by different radiator types already makes FM coverage a sloppy game of horseshoes or hand grenades. HD-R is proving to hold up well in pattern nulls and is impressively tolerant and robust. The FCC should be watching this closely and do all it can to establish technical specifications that enable a responsible yet easier and more rapid conversion to HD-R.
Easier done than said
For covering yourself legally in the rollout period, STA requests are no longer needed. Simply send the FCC a letter within 10 days of turning it on, listing the basic transmitting specs and a contact name and number staff can call to follow-up on interference complaints (RW April 7, page 2). Curiously, antenna information is not required, at least not for this exercise.
Sounds like the commish is taking the path of least resistance here and will let listeners decide if they need help with interference, a new twist on “let the marketplace decide.” There’s no evaluation of specific station interference situations we thought might be undertaken before HD-R authority is granted, even for the more problematic AM band.
Leonard Kahn must be having a heart attack. It’s a wide-open digital highway ahead for HD-R folks. Let’s get busy.