Streaming Media Bible - Streaming Magazine Column

Streaming Magazine Column - July 2002


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Audio Production - It's All About Level.

There is no reason why your audio streams should not sound as good as your local radio station. Audio codecs have progressed to a point where you can achieve FM radio quality at as low as 20Kbps. This means even folks on dial-up modems can listen to high quality streams. Another benefit to high quality audio, if you're producing streaming media presentations that include a visual component, is that people are more tolerant of questionable image quality if the audio quality is high.

Excellent audio quality is easy to achieve as long as you use good equipment (see last month's column) and employ standard audio production practice. In this regard, there is nothing more important than the audio signal level. If your audio signal level is too low, noise may creep into your signal, and streaming media encoders cannot distinguish between signal and noise. If your audio signal level is too high, you run the risk of signal distortion, which can be distinctly unpleasant to the audience.

Setting up a Gain Structure

Every audio production chain generally includes a number of different pieces of equipment. For the highest audio fidelity, you want to work within the optimal range of each piece of equipment, neither too quiet nor too loud. You do this by adjusting input and output levels for each piece of equipment. This process is known as setting up a gain structure.

To set up a gain structure, start with the first piece of equipment and set levels for each piece of equipment in the signal chain, ending with your soundcard input. Audio level is generally measured in decibels (dB). Most audio equipment gives a visual indication of the level via some sort of meters, be they old-style VU (volume units) meters or the more modern LED peak meters (see diagram). A good place to start with your levels is at 2/3 to 3/4 of the maximum operating range. Your audio will be loud enough so that noise should not be a problem but not so loud that it will distort.

Three types of meters: analog VU, LED peak, and software peak meters.

Analog equipment levels (such as compressors and mixing desks) should be set to approximately 0dB, whereas digital equipment levels (such as your soundcard input) should be set to somewhere between -10dB and -6dB. The settings differ because levels are measured differently in the analog and digital worlds.

Analog equipment has what is known as headroom to handle occasional peaks above 0dB. Digital equipment does not. If your signal goes above 0dB in the digital world, it will distort. If the peak is very short, it may be inaudible; if your signal is regularly above 0dB, you'll hear distortion.

For example, if you have a microphone plugged into a mixing desk that is connected to your sound card, you'd do the following:

Set the input level on the mixing desk so that the microphone input level is near 0dB
Set the output level of the mixing desk so it reads approximately 0dB
Set the input level of the soundcard so that it reads approximately -10 to -6dB. This gives you some headroom in case the level from the microphone suddenly increases (i.e. someone coughs or raises their voice)

Now you have a gain structure whereby each piece of equipment is running near its optimal level, without distortion. The best part is that once you've set your gain structure, you shouldn't have to change it too often. A little tweaking from time to time should suffice.

Consumer vs. Professional audio levels

One problem that often arises is that different pieces of audio gear operate at different signal voltages. Professional equipment operates at a higher voltage than consumer gear. What this means is that if you plug a consumer CD player into a professional mixing desk, the output of the CD player will seem very low. Conversely, plugging in this professional mixing desk into a consumer soundcard may distort the soundcard inputs if you're not careful. Mixing and matching different kinds of audio gear shouldn't be a problem as long as you check your levels at every point in your signal chain.

Balanced vs. unbalanced cables

In addition to different audio equipment having different operating levels, some equipment uses unbalanced cable; others use balanced cable. Balanced cables are noise resistant and far superior. Unbalanced cables, like the cables that interconnect your stereo system at home, are fine for short runs, but should be avoided if possible. You can mix and match your cables as long as you're careful with your levels.

Once your gain structure is set, you should be feeding your encoder a good signal level, free of noise and distortion. This results in professional-sounding streams. Of course, there are other tips & tricks you can employ, which will be covered next month.

Next Month: Audio Production - Fun with Signal Processing