It is generally advisable to use a separate processor when setting
up an Internet stream for the purpose of re-broadcasting your station
over the Internet. With the recent AFTRA scare, many stations have
resorted to a split between the over-the-air feed and a source for the
Internet audience during commercial breaks.
Depending on the method used, stations may find themselves needing
some kind of separate audio processing for webcasting operations. Many
are faced with the question, "What should I do for Internet
There all kinds of options available to you ranging from just using
an Aphex Compellor to employing one of the full-featured digital audio
processors available from companies such as Omnia or Orban.
Audio processors designed for webcasting will produce the best sonic
What should you choose? It depends on what streaming system you're
using. It will also depend on your format.
Digital reduction basics
Bit-rate reduction algorithms, such as MPEG, Real Audio, Microsoft's
MSV2 codec and others used for webcasting generally take the form of
"lossy" data compression. Remember that a linear 44.1kHz stereo audio
stream is about 1.6Mb/s. The MPEG Layer 3 at 256kb/s compression ratio
is about 6:1. At 128kb/s the ratio is approximately 11:1.
Through data reduction, data is removed to make the audio
stream (or file) smaller. This data is gone forever. (A complete
description of how masking works is covered in "How it works," BE
Radio February 2002, page 64.)
The lower the bit-rate, the more artifacts are let through. This is
why a 28.8kb/s MP3 Internet audio stream sounds so unnatural when
compared to one at 128kb/s.
The critical area in any of these perceptual coding schemes is the
high-frequency area. Our ears are most sensitive to what goes on in
high-frequency areas, and it's pretty hard to manipulate data in this
area without the change being noticeable.
The key to minimizing some of these effects is to keep the codec's
input levels as close to zero as possible. It also helps to keep the
upper spectrum free from clipping distortion or excessive
Most perceptual codecs were designed without audio processing and
deliberate spectrum manipulation in mind, so in some ways, the use of
audio processing is not the best approach. But the alternative is
sloppy levels, which can be much worse.
Another option to consider is using a lower sample rate, as this
will reduce the amount of high-frequency components the codec sees.
This approach will usually result in better performance in suppressing
Practical solutions and caveats
A strong, quick solution to webcast processing is a general AGC such
as the Aphex Compellor. However, a big weakness in the use of such a
device is in a lack of consistent spectral balance over a wide range of
material. About 50 percent of the programming content (for current
music) will sound acceptable; the rest will fail due to the lack of
proper spectrum management. Multiband audio processing is a must for
higher degree of quality control.
One drawback to using a spare multiband processor is that it was
likely not designed for the Internet. Diode clippers and poor crossover
design can make the sound worse.
Crossovers that are not dynamically flat are also an enemy of
codecs. Many processors are designed using textbook crossover filters.
These filters operate fine as long as the gain state is static, like a
speaker crossover. However, when you change the gain relationship of
the output of the crossover, many textbook filters will exhibit
problems with peaks or notches forming in some parts of their passband
due to the absolute phase relationship of the same frequencies
appearing in the lowpass/highpass skirts of neighboring filters.
A high-quality sound card, preferably with digital inputs, is the best
choice for the good streaming audio quality.
In such a case, the phase angles of audio within the
highpass/lowpass skirts will rotate and either be out of phase with or
in phase with frequencies in the passband of neighboring filters. When
this mess is all summed together after feeding through compressors, you
have random peaks and notches floating around the audio spectrum.
Old analog multiband processors can suffer from another problem. Due
to parts tolerances and changes in temperature and humidity, the left
and right crossovers will most likely not match each other. The roaming
notches will be different for the left and right channels.
These random, narrow-bandwidth peaks and notches roaming around the
audio spectrum drive low-bitrate codecs crazy, contributing to the
strange phased-out sound of many Internet channels that attempt to use
multiband processing to some degree. This is especially true for any
low-bitrate stereo encoding.
The DSP units produced by the leading audio processor manufacturers
address all of these issues. But if you cannot afford one of these,
knowing what you're up against can help you make sound judgement calls
and nifty modifications to some old box lying in the corner of the
As good as the weakest link
The first rule of sound cards is that not all sound cards are
created equal. The biggest quality issue you will face is how well the
sound card will function at the desired sample rate. A $3000 Net
processor connected to a $20 sound card will yield unimpressive
Of concern here is how well the sound card filters frequencies above
the Nyquist frequency of the sample rate in use for streaming.
The Nyquist frequency is highest analog frequency that can be converted
to digital without severe problems. This point is exactly half of the
sample being used. As an example, a 44.1kHz audio sample rate is
capable of reproducing audio frequencies up to 22.05kHz.
Exceeding the Nyquist frequency generates digital aliasing
distortion. Steep filtering of the analog signal, called anti-aliasing
filters, are used to ensure that no data beyond the Nyquist is present
at the input of the analog to digital converter.
This aliasing distortion accounts for most of the artifacts observed
in the majority webcasts. These aliasing products produce scratchy
and/or metallic audio, high-frequency birdies or jingling noises heard
on top of white noise sources (such as tape hiss, or crowd cheers).
Choosing a sound card with support for your desired bit-rate and
with exceptional anti-aliasing filters is a must. Look to professional
soundcards that support the sample rates you wish to use.
Another problem with most budget sound cards is poor design. The
layouts on budget cards can introduce computer noise into the
Use a sound card with a digital input. This will allow you to take
the digital output of a DSP-based processor and feed your codec via a
direct digital link. Alter the output sample rate of the DSP processor
to match what is used online. A sample rate converter between the DSP
box and the sound card can also be used.
Thanks to Rolf Taylor, a patient second set of ears as the author
experimented with hundreds of different configurations to try to get
the best webcast audio performance.
Gould is senior staff engineer for Infinity Broadcasting
Corporation, Cleveland, OH.