Digital Signal Processing (DSP) What is it?
Essentially it is the ability to manipulate a signal (in our case audio) in the digital domain in near real time although every form of processing delays the signal slightly, but the low cost fast processing time of todays products can be considered instantaneous as far as the application in a well designed sound system.
This manipulation can be defined as any of the following main processes-
Equalisation – The advanced control of level at particular frequency points across the audio spectrum. These points can be user defined or fixed and can even be automatically varied as in the case of a feedback elimination filter system. This equalisation is typically used for three functions,
- Room compensation – reducing the amplification of frequencies already enhanced by natural room resonances or boosting frequencies absorbed by room features.
- System equalisation – correcting for non linearities in loudspeaker or other system components to create a flat response (adjusting for frequencies where the loudspeaker is deficient or to prominent in reproduction, this is the case for all loudspeakers in some measure as it is impossible to build the perfect speaker).
- ‘Enhancing’ the natural sound of a voice or instrument (this is subjective of course).
This last case is just applied to a single mixer channel where points 1&2 apply to the wider system.
Delay – where it is desirable to delay the signal from loudspeaker so that it reaches the listeners ear at the same time as the natural sound from the sound source eg a presenter at the front of a room. This serves to keep the sound focussed preserving inteligibility and also maintain the impression that the source of the sound is the presenter not the nearer loudspeaker. This is important in both cases reducing ‘brain strain’ by preserving the relationship between the eyes and the ears.
Filtering – This is usually in the form of high or low pass filters used to form a ‘crossover’.
This splits the audio signal range and routes the low frequencies only to a subwoofer and the mid and high frequencies to a loudspeaker designed to reproduce those.
Compression (Levelling) – This function takes the audio signal which normally has wide variations in level due to the dynamics of speech for example and lifts the quieter moments while squashing the louder ones. This gives a more consistent level appreciated by those with hearing difficulties and when speech is reproduced in an area of high background noise.
There are many other possible functions such as pitch shifting (autotune) possible but those listed above are the most commonly used. In the old analogue days each would have required racks full of hardware to achieve what can now be done on a single circuit board.
Why do we think it is important?
Well in our usual working environments such as churches and heritage buildings there are two main factors that generally mean there is a great deal of benefit from having a system with integrated DSP.
The first is that the environments are mostly quite lively acoustically, by this I mean that they are large spaces with little in them to absorb sound leading to undesirable reflections (reverberation) and in some cases distinct echo (where the reflected sound is delayed enough to hear your words back).
The second is that there are rarely trained audio professionals on hand to manage the sound system and reproduction day by day.
By installing and configuring a system with built in DSP some of these negative factors can be reduced by careful tuning of a system to a room, by dialing down frequencies that encourage room or loudspeaker resonance (this includes selecting loudspeakers with the best directivity to avoid reflections and mechanical alignment).
In the second case the use of DSP provides a preset system environment offering the best baseline level of system performance so that a user is left with just basic controls to operate the system, For example, careful system tuning can reduce the onset of acoustic feedback dramatically giving the user more available level on a microphone being used by a quiet reader. It can also allow more microphones to be ‘on’ at the same time or even provide automatic mixing removing the need for any user interaction.
Increasingly DSP is being incorporated into equipment in all sectors of the market however as with all things, cheap is often too cheap. You can be pretty sure though if you purchase from well known long established brands the DSP on board will offer a good level of performance. One thing to note is that there is always a limit to the number of threads of operation a DSP processor can handle at once. So in some cases certain DSP functions will reduce the availability of others, so its worth checking before purchase. For example a mixing desk may have 2 FX channels which can have a single function allocated to them from a list of many, but it may also have global equalisation and compression functions which have no such limit.
There will be additional costs involved in setting up the system as this takes time and potentially expensive measurement equipment to get right, although the human ear is still the best device available to confirm measured results.
For any new audio system there are tremendous advantages in installing equipment with integrated DSP or adding DSP to an existing system. There will always be improvements in inteligibility and probably listening comfort. However it must be remembered that the most important component in any sound system is the room itself, so there will be a limit to the improvements offered by DSP compared to the improvement potentially gained by acoustic treatment of the room itself. Having said that, room treatment may not be possible due to architectural consideration or practical due to the cost, so DSP will always offer the best value for money whilst not being a complete solution.
The APi Approach
Except for our most basic projects we will always recommend and include digital signal processing in our projects so that you get the best possible sound and least difficulty operating your system.
Take a look at our YouTube channel for videos of our latest projects