IT'S A DIGITAL WORLD
A digital mixer has to provide the three basic operations provided by an analogue mixer
which are attenuating, mixing and selection of channels.
Attenuation
In an analogue system attenuation is achieved by simply using a potential divider connected to Vin and earth. This provides a complete range of values from 0V to Vin. The digital version works in a similar way. A potential divider is connected to a reference voltage and earth. The output of the potential divider is passed through an A/D converter. A multiplier then multiplies the two signals providing a range from 0V to Vin
Mixing
In an analogue system mixing is achieved using a summing Op-Amp. In a digital system initially a multiplexer interleaves the channels then a full adder is used to sum the channels.
Selection and routing
In an analogue system selection is achieved using a multipole switch. The digital equivalent is a De-multiplexer which is used to extract channels and an encoder which is used to select the desired channel.
At first sight it may seem that we are going to a lot of trouble just to overcomplicate an
existing system which works well. The digital system allows for greater manipulation of the digital
information providing access to many effects which were not possible under an analogue system.
Digital effects
In many digital systems there are still processes that are best implemented in an analogue fashion. I
n the recording process the use of compressors, which reduces the dynamic range of an incoming signal,
and gates, which expand the dynamic range of an incoming signal, are still generally implemented
using analogue circuitry. All effects involving time manipulation are implemented digitally.
Digital Delay
The digital signal is split up into two paths. One path is left untouched and is passed directly to a mixer.
The other path is known as the delay path. It is passed into RAM where it is stored for a short
period. A microprocessor is then used to manipulate this data. The microprocessor has two main functions.
Adjusting the delay of the signal by varying the sample clock rate
Providing varying modulation controlled by an oscillator. The user can set the depth and rate of modulation..
The signal is then fed-back to the input of the delay line so that multiple echoes can be set up and t
hen the two signals are recombined.
Effects
Using the Digital delay loop (DDL) a wide variety of effects can be realized.
Single Delay
Single delay creates a single echo and is often used to accentuate the rhythm of the music. It is achieved by setting the feedback, modulation depth and rate to 0. The delay of the echo can then be adjusted to repeat in time with the music. The next stage is to add a repeating echo. This is achieved by simply increasing the feedback.
Chorus
Chorus provides a wide dynamic sound and is often used on strings and electric guitar. It is achieved by setting the delay to approximately 10ms and then using low depth modulation at a frequency of about 3Hz. It is then mixed half and half with the original signal. To provide extra depth to the sound the chorus can be panned to either the left or right channel.
Vibrato
Vibrato is an effect used by most musicians to create a richer fuller tone. It is most visible when watching a violinist. The violinist moves the finger which is playing the note back and forwards causing a slight periodic change in pitch. Vibrato is achieved in a similar way to chorus. The delay used is only a few milliseconds and the modulation is about 3Hz but the original signal is not used.
Phasing
Phasing is an effect which is widely used. Two identical musical channels are allowed to drift in and out of phase creating a sense of movement. It is achieved in the same way as vibrato, but the original signal is also combined.
Flanging
Flanging is a stronger form of phasing generating a characteristic 'whoosh' which if panned across both channels creates a sense of movement. It is achieved in by using a modulation period of about 1 Hz, a delay of about 10ms and a subtle use of feedback.
Samples
The DDL can be used to play samples. A user defined delay is used. The sample is played when the user releases the hold. This is often used for repetitive spoken lyrics.
It can be seen that with only slight alterations in input to the DDL a large number of effects can be
created. In analogue systems phasing for example used to be achieved by playing two copies of
the track on separate tape players and slowing down one of the tapes using the hand.
Obviously this lacks the sophistication of the digital alternative.
Reverberation
Reverberation is a natural phenomenon which is caused by reflections of sound waves.
The delay of the reflections describes the size of the room and the time taken for the decay of the
signal describes the material that the waves are being reflected off. A small room with absorbent walls would create a signal with a fast decay and a fast build up of echo whereas a large room with reflective walls would create a signal which slowly decays and has a slow build up of echo. The effect can be quite subtle but the human brain can distinguish between a large and a small room by just hearing a sound. Before digital technology reverberation was generated by either using a room with the desired characteristics or using artificial reverb generated using a plate. The plate is suspended and then driven by a transducer. Two contact transducers are used to pick up the signal which is then recombined with the original signal at the mixing desk. The effect is quite convincing as the vibrations in the metal plate closely simulate natural reverberations. Creating electronic reverb is a very complex process. It has been shown that approximately 1000 to 3000
separate echoes are needed to create a convincing reproduction of natural reverb. The kind of reverb needs to be defined as well. Different materials have different frequency responses so different forms of digital filtering need to be implemented to allow the user choice over the kind of reverb used. There are many digital reverberation systems available most of which are implemented using software. Digital filtering is much easier to implement using software as the filter's characteristics can be stored in a lookup table. Most systems have predefined reverberation characteristics. This may seem at first to be very restrictive but the subtle differences that can be made using a fully programmable system would not be noticeable in a complete mix.
Monitoring
During the production of a piece of music monitoring is of vital importance because it provides the
sound engineer with many tools. It is possible to monitor just one track at a time so that the track
can be set up precisely. The channels can then be recombined so the engineer can see how the
mix is sounding. Monitoring requires the conversion of the digital signal back into analogue. A D/A
converter is used so that monitoring using either studio monitors (loudspeakers) or headphones can be achieved. Headphones provide the sound engineer with detailed reproduction of the track being recorded but they lack the feel associated with a good pair of loudspeakers.
Cueing
When using a recording studio cueing is very important. There are two main purposes. Cueing is used to relay messages between the sound engineer and the musician because the recording part of the studio is soundproofed from the mixing room. The other main use is when extra tracks are being laid on top of existing tracks. The musician who is being recorded has a pair of headphones which are given the backing track. The musician can then play along as if the full band was there.
Digital recording
The final part of the studio process is the actual production of the master copy which will be
used for replication. In modern digital studios this is usually implemented using a DAT machine,
which is a digital tape recording system, or a Compact disc writer.
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