Audio Processing for HD Radio –
Getting the best out of bit-rate reduced transmission systems.
(Originally in Radio Guide’s Processing Guide magazine)
Part 1 – Changes
By Cornelius Gould
2005 will be remembered as the year digital transmission exploded onto the scene. Many engineers worked tight schedules modifying existing transmitter sites or in some cases completely rebuilding them in order implement this new technology.
As with any new enhancement to the broadcast service, there is always some learning curve to overcome to get the most reliable results from this new service.
With HD Radio technology, this learning curve is pretty intense as it involves a completely separate transmission system which makes completely separate radio waves that exist around your main AM or FM signal.
SOMETHING TO LEARN
Unlike previous advancements to the broadcast medium, this time around we are faced with a brand new technology whose inner workings are shrouded in mystery and some broadcast engineers are faced with a scenario where their best efforts fall short due to some mysterious process within their digital transmitters – and the resulting audio might not be very nice sounding.
Others are following the guidelines presented by audio processing manufacturers and are having good results, but they still want to have a better grasp as to what is going on “under the hood” to better understand the HD Radio beast they have to handle.
While driving around my section of the country with an HD Radio receiver, I find it interesting to listen to how HD Radio is being implemented by broadcasters. One thing that jumps out at me is how many systems that still need work in many basic areas.
THE OTHER DELAY
The most common problem is a lack of diversity delay on the analog channel. This is most apparent when a listener with an HD Radio drives in and out of the optimum reception conditions for the digital carriers.
Essentially what happens is they find them selves jumping back by as much as eight seconds in time when the HD carriers are decoded, only to shoot ahead as many seconds when the radio rolls back to analog service. As a result the listener can totally miss entire sentences in a conversation.
Other issues involve audio quality and audio consistency.
I have heard stations with as much as a 12 dB level difference between the digital and analog services. Another annoyance comes from stations whose digital transmitters are fed with the clipped, pre-emphasized FM analog processed audio.
Even if the de-emphasis is turned on (to make it flat again), the resulting audio heard from the digital service is still be very unpleasant to the ear.
FOCUS ON THE AUDIO
The above examples show there is a lot that needs to be learned by the broadcast engineering community about this technology. If this system is to be successful, proper adjustment and implementation is essential.
Of course, I am aware there are many people out there who feel HD Radio should not have been allowed to be used due to its use of the spectrum within what has been traditionally considered the “guard bands” of AM and FM signals. While there is considerable debate as to the validity of many aspects of this new service, I will not be addressing these issues.
The point of this series is not to convince anyone to change their opinions on the validity of this system one way or another, but rather to help point broadcasters in the right direction to get the best audio performance from what we have to work with today. Since my specialty is audio processing, naturally my focus is in that area.
I will pick apart what is going on (as best as anyone outside the iron gates of Ibiquity can) and with these tips my hope is that you will be able to get the best audio performance on your digital transmission system from understanding both the audio processing and the system in general.
NOW FOR SOMETHING COMPLETELY DIFFERENT
Broadcasting with HD Radio technology is an entirely different beast. From a technology point of view, it shares almost nothing in common with the legacy broadcasting technology with which we have become accustomed.
The biggest difference – and the hardest concept for many broadcasters to grasp – is that HD Radio is not a “linear” transmission system.
Analog broadcasting can be thought of as a linear process. That is, every sound that leaves the audio processor and enters the transmitter will be sent over the air with very little change.
On the other hand, HD Radio is not a linear process. Only a portion of the audio you feed into the HD Radio system actually makes it to your listeners. The art of deleting large amounts of audio data while preventing the human ear from “hearing” it – for the most part – is called “Perceptual Coding.”
In fact, most of the audio data is thrown away and, through some neat ear trickery and the proper use of technology, very few people will ever know!
What we are describing here is not a difference between digital audio and analog audio. Digital audio can be linear too.
Analog audio is given its name because the entire process by which it works is by literally electrically copying sound waveforms onto some medium, making a literal copy of the sound image onto the medium of choice.
For our discussion here, this medium is a radio wave. As the sounds from the mouths of your announcers strike a microphone, their voice is instantly turned into an electrical signal which travels through your audio chain to change the radio signal directly in proportion to the sound at the studio microphone.
In the case of digital audio, the sounds of your announcers are still picked up by a microphone and the electrical signals are turned into digital data. This is done in a device known as an Analog to Digital converter. (The reverse happens in a Digital to Analog converter.)
There is one major problem with the basic concept of such a linear digital transmission system: assuming the process is meant to be of “CD Quality” – whatever that is – we find the system takes an enormous amount of data to accomplish its task when compared to the analog system.
By way of comparison, the analog system can create the same sound quality of digital with only 20 thousand Hertz of electrical space. Digital, on the other hand, requires almost 1.5 million Hertz of electrical bandwidth per second to reproduce the same kind of audio. (While this watered down explanation is not entirely technically accurate, it is meant to get the point across to as many readers as possible.)
If the quality is the same, and digital is not as efficient as analog, why even bother with digital?
The advantage digital audio has over analog is that the process of converting audio into digital bits is inherently immune to noises present in any transmission or storage medium. In other words, for all its disadvantages, the main thing you gain is the ability to make endless copies of the data and still have it sound as good as the original.
Please note that this benefit assumes you are not changing the data in any way during the copying process. This is an important factor to remember for reasons that will become apparent very soon.
There is no way to broadcast full linear CD Quality audio to listeners with the transmission systems in use for the past 80 or 90 years. Remember: it takes about 1.5 million Hertz of electrical space to reproduce linear digital audio; the most electrical bandwidth any Digital Audio Broadcast (DAB) service in existence has to work with is about 256 thousand Hertz of space.
For broadcasters using IBOC (HD Radio), the space available is even less: about 96 thousand Hertz of space. And this is assuming there is only one digital program service; there is even less space available if the secondary (or tertiary) channels are used in “Multicasting.”
The digital audio needs to “fit” in about 1/16 its “normal” bandwidth
So, how do you squeeze 1.5 million Hertz of data into 96 thousand Hertz of space?
PUTTING 1.5 MILLION HERTZ IN A 96 THOUSAND HERTZ BAG
Digital Audio Broadcast services have to use methods to permanently, and destructively, discard most of the digital audio data in order to make it all “fit” within the tight spectrum constraints.
The method of throwing away this “excess” data is commonly called “bit reduction” – where varying amounts of digital bits of data are discarded to make what is left fit within signal bandwidth constraints.
Now, remember what I said before: Perfect copies of digital audio data contain no noise nor errors so long as there is no change in the digital data across many copies. Bit reduced audio involves major changes to the digital data right from the first copy and, as a result, the decoded audio has very little resemblance to the original source.
The trick is for the decoded bit reduced audio to be perceived to be a “good enough” (if not close to a perfect) copy of the original. Audio processing becomes extremely important in this area as having optimum audio performance can enhance what is left of the audio and can even make or break the entire process.
SPECIAL PROCESSING NEEDS
Over the past nine or so years, this is the area in which I have been working. How can an audio processor enhance this process for the better? What new processes can be developed specifically for this new technology?
As my wife and friends can tell you, I am obsessed with these questions. What I could not have realized back then is how much of what I learned over the years of doing this is paying off now in such a major way.
I got involved with mixing audio processing with bit-rate-reduced perceptual coding technology back in 1996 when a friend and I decided to start up a 24/7 Internet radio station. Of course, the big thing that stuck out at me was the quality of the coded audio.
It was not good, of course, and I set out to see just how far I could take improving audio quality. What started out sounding like a gravely telephone-grade programming rapidly evolved into something that sounded more like AM radio broadcasts within a month of intense audio processing work.
INTO THE CODEC JUNGLE
Along the way I became intrigued by these perceptual audio CODECs and how you can use audio processing to get the most out of their performance.
It also did not hurt that this interest took hold when I started to work for Telos Systems – one of the leaders in the handling of coded audio for broadcast applications. If I ever need to know why certain CODECS behaved the way they did, the answers were in a thick deep technical reference book somewhere in their library!
Since that time, with every new CODEC that is released, I anxiously jump on board to see what it can do – and then immediately after that what I could do with it audio performance-wise.
I had not done much research work with AM or FM audio processing in quite some time. With my normal day job and dealings with small non-commercial stations I still spend lots of time adjusting what I call “legacy broadcast audio processing” on a regular basis.
But, by far, most of my research fun comes from learning how I can make perceptual CODECs “play” at peak performance through the use of external audio processing. To make bitrate reduced perceptual CODECs work at their best level, I find it necessary to research as much as possible about the technology in question.
This is also the same sort of information the broadcast engineer in the field needs to understand to make HD Radio technology play at its best.
After all, just how good would your ability to adjust your legacy AM or FM station to sound its best if you did not understand certain fundamental things such as the internal design of the transmitter, the choice of the transmission line and antenna, and the way all of that can have an effect on your audio processing efforts?
HELP ON HAND
The major audio processing manufacturers have been doing a great job at staying ahead of the curve for you. Each of them have come up with decent presets that will work acceptably right out of the box, but you and I know that the best results come from hand-tailoring your processing to your facility and market.
Doing this with bit reduction CODECs requires some knowledge of what is going on under the digital radio transmitter hood. As this series progresses, my goal is to shed some light on this and point you in the right direction to learn more as you need it.
For example, while the exact nature of the CODEC used for HD Radio is a complete mystery to anyone outside Ibiquity corporate circles, a reasonable guess by many (including me) is that it is either the HE-AAC CODEC or some derivative closely related to it.
WHERE WE ARE HEADED
During my audio processing experimentation with both the HE-AAC / aacplus technology and HD Radio, I find the results to be extremely similar – close enough that I can test ideas at home in my workshop using aacplus and implement them the next day through the HDRadio system with virtually identical results.
With that correlation in mind, I plan to base our discussions around making HE-AAC / aacplus sound its best with audio processing. To start this series, we need to look at how the HE-AAC bit reducing CODEC operates.
In a previous article (Radio Guide, Septemer 2003), I have already touched upon the basics of perceptual coding, although it was somewhat outside the scope of that series of articles. However, if you want to go back into the archives, the article was titled “The Rock and The Pin” – it is a simplified discussion on perceptual audio coding, but will make a nice foundation as we start this series.