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Equipping an Audio Digitisation System

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Summary

This document aims to provide information needed when equipping an audio digitisation project of analogue media to a digital hard drive. This document is written with the digitisation of stereo and mono files in mind. For information regarding digitising multi-track recordings, there is a section at the end of this document with some recommendations and considerations. This document is intended to provide a guide to staff who wish to undertake digitisation projects of any scale, be it small or large.

This document is intended to be read alongside Selecting an Audio Digitisation System, which provides a variety of system specifications for differing budgets.

Overview

The method of digitisation outlined here involves the use of a computer-based system and dedicated hardware to convert an analogue signal into a digital file with the maximum accuracy possible. This method is fast becoming the standard for digitising audio for archival purposes and when calibrated and setup correctly offers exceptionally high quality digital replication. Lets look further into the components that make up an audio digitisation system.

Workspace

One of the most crucial factors to consider when equipping your project is the workspace in which it will be taking place. The size, shape, and materials of the room and its furniture will have a huge effect on how the sound you hear is perceived. For further information on this subject please refer to the advice document Preparing your Audio Digitisation Workstation.

Analogue playback device

The first component in the digitisation chain is the analogue playback device. This needs to be carefully selected to correspond to the exact specific format of your media. Many of these machines have now become obsolete, even ones that are of relatively new technology. Analogue playback devices are these days often only available to purchase second hand from specialist companies and need to be serviced by an experienced technician. Although there are many variations still available to buy, it is recommended that professional machines, as opposed to domestic ones, are used for digitisation.Once serviced, many analogue playback devices need further maintenance and upkeep. Calibration of the machine should be done to the specifications of the manufacturer and will most likely require some form of calibration equipment, such as test tape for an open-reel machine. Calibration should be done regularly and it is advised that it should take place before each digitisation session (if this is daily, then at the beginning of every day).

Diagram 1

An open-reel tape machine

Alongside thorough maintenance, where applicable the playback heads of the device should be cleaned before every use with an alcohol based solution, such as Isopropyl alcohol.

Pre-amplifier

Most analogue playback equipment has been designed to be used alongside a dedicated pre-amplifier. The pre-amplifier amplifies the signal to what is known as line-level, suitable for audible playback and for routing into another device and in some cases is designed to compensate in frequency deficiencies of the analogue media. This is certainly the case in early vinyl records that were pressed with a frequency curve relative to the record company who produced them. In order for them to be listened to correctly a pre-amplifier with the corresponding equalisation settings to balance out the offset is needed.

An analogue signal is connected to left and right analogue audio input connections from stereo playback equipment and commonly through the left input for mono input signals.

Analogue to digital converter

An analogue to digital converter is the key component in the digitisation process. It takes an electrical analogue signal and converts it into digital data. Converters range in price from just under a hundred pounds, to a few thousand pounds. As a result the quality of conversion, and subsequently the accuracy of replicating the analogue signal differs considerably, and generally relative to cost.

Analogue to digital converters now offer wide interconnect support and bandwidth offered by USB and Firewire signals can transfer multiple channels of uncompressed high quality audio. More expensive systems use PCI (Peripheral Component Interconnect) card technology which plugs into an expansion slot of a desktop computer's motherboard. A converter will offer monitoring outputs and controls to allow accurate aural monitoring of the signal before and after conversion.

Key considerations when choosing an analogue to digital converter:

Are the type of inputs compatible with the types of outputs your analogue playback device has? If not, an adaptor can sometimes be used, for more information see the advice document Interconnects and Signal Types.
Does it have the right amount of inputs for your analogue playback devices? Are you planning on digitising from more than one playback device at once?
Does the converter digitise audio to a sufficient quality to meet your own quality assurance benchmarks? With several well supported and inexpensive converters now offering lossless compression or even uncompressed audio digitisation, you can expect results of a very high standard.

Diagram 2

An Analogue to digital converter

Audio interface

The difference between an audio interface and an analogue to digital converter becomes apparent in more high end and expensive systems, where the two can become separate.

Most A>D converters incorporate the ability to interface with external capture devices, in this method a computer. However there are instances where the digital data from an A>D converter requires a separate device to decode the data transfer the new digital information across to the computer.

Diagram 3

A standalone audio interface

Computer

The specific make and model of computer used in a digitisation project is largely irrelevant. The two main factors to consider are (a) the machines compatibility with the external hardware and software being used, and (b) the technical specifications of the computer's components.

Depending on the number of audio channels you wish to capture at one time, and proposing that there is no high post-production demands on the computer, a low cost PC can provide a good starting point for a digitisation system.

With regards to operating systems, the capture process of audio digitisation is neither hindered nor helped by any specific platform, be it Windows, Linux, or Mac OSX. You may find, however, that some of the more production orientated software and hardware packages are geared towards and only compatible with the Apple Mac OSX platform, as is production industry standard. It is important to note that it is common within institutions that the Windows platform is often the only one supported by central IT support teams.

Whichever operating system is decided upon it is important that the computer has a minimum of specifications to provide uncompressed stereo (and some multi-channel, to a point) audio capture. A computer for digitising audio will typically have base line specifications such as a 1.33GHz processor, 2GHz of RAM (Random Access Memory), and two internal hard drives; one for the operating system (about 100GB minimum) and one for recording directly to.

The size of this second hard drive will normally depend on the size of the project you are undertaking, however it does not need to be particularly large as you should be transferring your audio files to an external hard drive, discussed later on in this document.

The computer will also need some connections ports which are crucial to setting up your digitisation system. Depending on A>D converter and/or interface you have chosen, you will need either USB 2.0, Firewire or PCI connectivity (note that some versions of PCI cards may differ i.e. PCI-e or PCI-x, and these need to be made compatible with your computers card slots). Further external devices need to be taken into consideration when choosing the number of connection ports, such as an external hard drive, or a USB keyboard, and it is recommended to account for any possible future additions to your equipment.

A computer monitor will be required, and Wi-Fi capability, a DVD writer, and a decent graphics card are all recommended.

Diagram 4

A desktop computer system

External hard drive

For safety in backing up files away from the workstation an external hard drive is recommended, and these commonly use USB or Firewire connectivity to the computer. You will more than likely find it essential when working with large amounts of audio files. It is worth noting that although USB 2.0 has a faster rate of data transfer, Firewire 400 performs better when data is being transferred continuously, however Firewire 800 has a faster rate overall.

Software

There is a wealth of audio capture software available for digitisation, which varies in price from open standard and free to over a thousand pounds. A lot of capture software has a decent quality audio editor and a range of post-production features. The advice document Choosing your Digital Audio Software discusses considerations for choosing your software is greater detail.

Digital to analogue converter

To convert your digital audio back into the analogue realm so that it can be heard, digital to analogue conversion is needed. Again, this feature is included in most Audio Interface units along with A>D conversion and interfacing capability. However, in rare cases, generally with more expensive hardware systems, this unit may need to be purchased separately.

Monitors

For accurate measuring of the input and output signals of your system, a good pair of ‘studio' standard monitors with a flat frequency response (check the manufacturers specifications) are recommended. Speaker placement and environment can massively affect the audible output from your monitors.

These points are discussed in the advice document Preparing your Audio Digitisation Workstation.

Diagram 5

Monitors

Headphones

As well as monitors, a pair of headphones, of good quality, is a useful tool for analysing audio files and for referencing against your main monitoring source. Again, ‘studio' standard headphones are recommended over cheaper consumer products to provide extra quality.

Diagram 6

Headphones

Considerations for equipping a project for multi-track digitisation

There are a number of analogue tape formats that record audio on a number of separate tracks and it may be that you need to preserve the original tracks as opposed to digitising them as a stereo or mono audio file. If this is the case some changes will be need to the equipment described above.

Firstly the number of inputs to the A>D converter will need to match the number of outputs from the multi-track playback device. If you have an existing digitisation setup, a second or third A>D converter can sometimes be added to a system providing the equipment is compatible.

Secondly, if the multi-track media is intended for multi-channel playback, for example surround sound, then the number of outputs of the D>A converter need to match the number of channels. You will also require extra speakers to match the corresponding channels for accurate surround sound or multi-channel monitoring.

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