Welcome to this tutorial on direct injection.

Direct injection, using either a DI box or the DI input of a mic pre-amp or audio interface, is a process that allows analogue audio signals to be converted, most commonly from guitar pickup level to mic level, and from unbalanced to balanced.

Caption - The primary uses of direct injection

Direct injection has many uses, for example it can be used to ...

• provide a way for an electric guitar or electric bass guitar to connect directly to a mixing desk/mic pre-amp.
• provide a way for an electric guitar or electric bass guitar to simultaneously connect to a guitar amplifier and a mic pre-amp
• provide a way to route a signal to a variety of differing outputs
• convert an unbalanced mic or line level signal to a balanced one
• provide a way to connect the line level output (eg headphone jack) of a laptop to a mixing desk without buzzes and hum
• solve earth/ground-loop hum and buzz problems.
• to improve the electrical safety of connection between components by isolating them from one another

Caption - About impedance

Impedance (Z) is an electronic property relating to electrical signals, devices and cables. Differing devices (guitar pickups, microphones, mic pre-amps etc) have differing impedance requirements, some high, others low. Sometimes this causes problems when connecting one to another. Mismatching can result in problems such as loss of high or low frequency and overall level.

For example, the high impedance unbalanced instrument level output of an electric bass or guitar pickup needs to be converted to low impedance mic level if the signal is to be sent to the mic pre-amp of a mixer, voice channel, or audio interface.

Caption - Application - DI'ing an electric guitar

Most classic electric guitar tones are produced by the combination of instrument and amplifier. This is also true of bass guitars but less so. However, sometimes we want to capture the unaltered tone coming directly from the pickups, for the following reasons ..

1. to transparently capture the wide frequency response of an electric guitar or electric bass guitar pickup. Guitar amplifiers/speakers filter out hi-freq and introduce significant amounts of tonal colour into the signal which is not always wanted
2. to record a signal that will later be re-amped (sent back out to an amplifier) or processed with software emulations
3. in order to process it with some other device, such as an non-guitar hardware effects processor
4. to simultaneously record the unaltered tone coming directly from the pickups, and the signal from a microphone capturing the sound from a guitar cab or combo

Because the signal produced by pickups is different to conventional mic and line level signals, it must first be processed by a DI box, which will convert it to a conventional balanced mic level signal.

Caption - Application - Balancing signals

Sometimes it is necessary to convert unbalanced signals to balanced ones. The two primary reasons are ..

1. to protect a line level signal that must be sent over a log distance, such as from a stage to a live mixing desk at the back of a hall
2. to integrate an unbalanced device into a balanced system, perhaps in order to eliminate interference or ground loops

DI boxes are ideal devices for accomplishing this.

Caption - Types of DI boxes

Mic pre-amps and audio interfaces often feature a basic DI input for electric guitars and basses, but if you need advanced features you will need to use a dedicated DI box. There are 2 common types of DI box, passive and active.

Caption - 1. Passive DI boxes

The passive DI box has no powered electrical circuitry and comprises only passive components, a few resistors and an output transformer. When an electric guitar or bass is connected, the circuitry is driven solely by the signal from the pickup.

In cheaper designs, this can result in the so called "loading effect" which causes a deterioration in sound quality, often noticeable by a loss of the natural tonal balance and high frequency. In addition, the response of the pickup is altered leading to a change in the playability of an instrument. This effect can be large or minimal depending on the design/quality of the DI box and the design and type of pickup, which itself may be active or passive.

Never the less, many passive DI boxes have a character sound favoured for particular applications, and they are a convenient solution for live performance.

Caption - 2. Active DI boxes

An active DI box contains a buffering amplifier which isolates a guitars pickup from the loading effect, potentially helping a guitar pickup to operate as it should. Active designs allow for more features such as very hi-impedance input and line inputs.

A disadvantage is that they are more expensive and require power, usually in the form of 48v phantom power sent from a mic pre-amp. Many models have battery compartments which can be fitted with a 9v battery for situations such as live performance, where phantom power is unavailable. However, using the lower voltages results in a loss in signal to noise and high frequency performance. This may not be a problem or even audible with electric guitars and electric bass guitars.

Caption - Typical DI box features

A given DI box may have some or all of the following features ..

• Instrument input - this will accept the unbalanced low level signal (typically up to 1volt) from an electric guitar or electric bass guitar pickup.
• Line input - this will accept an unbalanced line level signal (typically up to 30volts), from a source such as a keyboard or synthesiser, and convert it to a balanced signal. Often used to transmit signals free from signal loss or interferences, long distance, such as from a stage to a mixer located at the back of a venue.
• Speaker input - this will accept a high level signal (typically up to 100volts) from an amplifiers speaker output, or a guitar cabinets speaker thru/output, which may then be sent to mixer.
• Balanced output - this normally carries a mic level signal to a mixer channel/mic pre-amp.
• Output transformer - DI boxes can be designed with or without a transformer on the output. The primary advantage of an output transformer is to isolate the input from the output in order to help prevent ground loops, and therefore hum, and noise such as buzzes from being passed on from noisy source devices. But an output transformer can also prevent injury if there are faults with instruments and equipment connected to the DI box. However, the presence of a transformer does not necessarily indicate higher quality audio as a cheap poorly specified transformer can introduce significant amounts of harmonic distortion into the signal, and cope badly with high levels and low frequencies. Good quality well designed and built transformers are expensive and add significantly to the price of a DI box. Also, many applications do not benefit from a device with a transformer.
• Link or Thru output -creates and sends out a copy of the input signal. This would allow the simultaneous connection of a guitar to a mixer and its amp/cabinet, or perhaps a guitar tuner.
• Earth Lift Switch - used to eliminate 50Hz mains earth hum caused by so called earth loops (a situation in which two interconnected pieces of equipment, both with their own earthed power supplies, cause an audible hum to be sent down the audio return or earth wire). An earth lift switch allows the input earth to be disconnected from the output earth.
• Pad switch - to reduce the input sensitivity of the DI box and thus prevent it overloading when a strong (line level) signal is input.
• Filters switches - low and/or high pass to reduce extremes of treble and bass.
• Phantom power switch - to allow an active DI box to receive and utilise phantom power should it be unable to use batteries.
• Impedance (Z) switch - which changes the impedance characteristics of the input. A high setting will be ideal for receiving the output of a guitar pickups, a low setting for the line level signals of electronic instruments.
• Phase reverse switch - to reverses the polarity of the signal.

Caption - Acoustic guitar piezo pickup requirements

It should be noted that acoustic guitar piezo type pickups need to be connected to a higher independence input than electric guitar and electric bass guitar pickups. Not all DI boxes provide suitable inputs.

Caption - Thanks for watching

The script for this video, with accompanying images, can be found at projectstudiohandbook.com 

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