All broadcast transmission chains have at least a rudimentary degree of broadcast audio processing to reduce the dynamic range of the program material. Now "reducing the dynamic range" may sound bad in a hi-fi sense, but do not let that put you off. In the real world most your listeners are not hi-fi buffs, and besides hardly any are listening in hi-fi environments. Most will be listening to you as they get on with their daily lives in the kitchen, in offices, factories, driving in the car etc. These environments are "hostile" in audio terms, so you need something to help your station cut-through the background noise to make it listenable.
At its simplest an audio processor is just an audio limiter that keeps your transfers legal by preventing them from being too loud (the technical term is "over-deviating"). This is OK to keep you legal, but does not sound nice when the limiter operates. You could just set your audio levels low enough so it never limits, but then your broadcasts would sound very quiet.
The answer is multi-band audio processing. Practically all public service and commercial stations use this. The primary advantage is that it boosts the loudness of the broadcast, while preventing over-deviation. There are also some additional benefits, even there can be pitfalls too – Broadcast audio processors have many, many adjustments available, and when set-up incorrectly they can make broadcasts tiring to listen to, or just plain "horrible"! That's where the "black-artist" comes in – when correctly adjusted broadcast audio processors can make the whole station sound better and more professional – we are not exaggerating!
Broadcast Audio Processors typically have three main stages:
- AGC – Automatic Gain Control – this section acts a bit like a technical operator sitting in the studio. If the programming (or guest) is a bit quiet, it slowly and gently boosts the audio level. If things are getting a bit loud it turns it down slowly. Just this function alone is incredibly useful in Community Radio where many presenters are inexperienced and have poor control of their audio levels
- Multi-band compressor or limiter – this section is the clever bit. It splits the audio into multiple frequency bands (often 5 or more) so that bass sounds, mids and trebles are processed separately. This results most / all of the problems of the basic single-band limiters mentioned above. Each band is compressed in dynamic range – ie: the quiet bits boosted and the loud bits reduced. This is similar to the AGC above, but done much more quickly. It is in this area where the "sound-signature" is created as it can drastically alter the tonal nature of the audio
- Final limiter – this is a very fast-acting limiter that cuts any remaining signal peaks to ensure that your transmissions stay legal and you do not over-deviate. Non-broadcast audio processors made by some of the cheaper makes do not have this stage.
So that's it in a nutshell. Broadcast audio processing is really important to your station. You can go for hardware broadcast audio processing units such as the Orban Optmod (in your dreams!), Omnia or the more affordable DSPX, or there are various software solutions as well. On our website we have an audio demonstration of the impact of broadcast audio processing. Check it out here: http://a-bc.co.uk/audio-processing/