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Vienna University of Technology
Institute of Software Technology and Interactive Systems

Recovering Digital Objects from Analogue Audio Waveforms

Digital Preservation

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The System

Migration Tool

Use with Original System


Additional Information

Further Links



Specimens of early computer systems stop to work every day. It becomes necessary to prepare ourselves for the future situation of having storage media and no working systems to read data from these carriers. With storage media residing in archives for already obsolete systems it is necessary to extract the data from these media once it surfaces before it can be migrated for long term preservation.
One storage medium that was popular for home computers in the 1980s was the audio tape . The first home computer systems allowed the use of standard cassette players to record and replay data. Audio tapes are more durable than old home computers when properly stored. Devices playing this medium can be found in working condition or can be repaired as they are made out of standard components. This makes it easier to record the audio stream on a system today than to use the original system to read the audio stream.
The tool presented on this page allows us to extract data created on one of these early home computer system. The original system was re-engineered and an application was written to support the migration of data stored on tapes without using the original system thus eliminating the necessity of keeping an obsolete system alive for the preservation of data on storage media for this system in the future.


Philips Videopac+ G7400 & C7420 Basic Module

The Philips Videopac+ G7400 was a video game console system released in Europe in 1983. It was the successor to the Philips Videopac/Magnavox Odyssey2 which was sold in Europe and the US since 1978. With an add-on cartridge, the C7420 Home Computer Module that was also released in 1983 the Philips G7400 could be converted to a home computer programmable in Microsoft BASIC.
The BASIC-programming language for the C7420 allowed the storage of different kinds of data on standard audio tapes by providing connector cables to the headphone and microphone plugs of standard audio systems. Besides BASIC programs screenshots, arrays, strings, and memory dumps could be saved on tape using the BASIC command "CSAVE" in various forms.

Pictures below show a Philips G7400 (left), a C7420 Home Computer Module (middle), and a collection of original tapes (right).

Philips Videopac+ G7400 Philips Videopac C7420 Microsoft BASIC cartridge Collection of original tapes
(Click images to enlarge)


Migration Tool

During research on the longevity of audio tapes and preservation of software and video games it became clear, that data stored on audio tapes has to be migrated to a different media as soon as possible. Most of the home computer systems using this media became obsolete more than 20 years ago, which is also the expected life time of data stored on magnetic tapes (VanBogart, 1998).
By re-engineering the original system and making use of re-engineering work already done by René van den Enden, a very active member of a community still supporting the system, we developed a tool with the following major functions:

  • Opening an audio stream and loading the contained files (either from an audio file (WAV or FLAC) or directly from an audio-in device)
  • Opening files in the C7420-native file format (binary streams converted from WAV-file)
  • Saving the opened audio stream as a C7420-native file format (binary stream)
  • Saving data in a non-obsolete format (screenshots as PNG, basic-programs and arrays as text files, binary data as binary)
  • Saving data as an audio stream (either to an audio file (WAV or FLAC) or directly onto the standard audio-out device)
  • Opening and saving compressed Zip-archives containing a collection of migrated files
  • Creating new files of the different formats in the application (including syntax highlighting for BASIC-programs)
  • Editing images and conversion of PC-images to C7420 screenshot-format
The tool is written in JAVA to be operating system and hardware independent. It can be downloaded in the Files section below. After starting the tool File->Import can be used to extract data from audio files or other sources, File->Export is used to encode the data again to audio files or the default audio device. Extracted data can be migrated to non-obsolete formats (e.g. CSV, PNG) using File->Migrate selected Tab.

The screenshot below shows the migration tool with an image that has been extracted from the waveform and two other tabs containing the "Hello World" program and the "Hello World" string.

Screenshot of C7420 Migration Tool
(Click image to enlarge)


Using the migration tool with the original system

To use the migration tool with an original system, the following order of connecting/powering on the system components was advised by René van den Enden:

  • plug C7420 cartridge in G7400
  • plug audio connectors of C7420 to audio jacks of soundcard/notebook
  • plug video or RF cable of G7400 to TV
  • insert power cable of G7400 into power outlet
  • plug power supply of notebook to power outlet
BE AWARE THAT THIS IS NOT THE INTENDED USE OF THE C7420 AND DAMAGE TO THE CARTRIDGE AND/OR YOUR PC IS POSSIBLE!!! No garanty can be given that you will not destroy any of the involved parts, even if you follow the order of connecting the components.

After powering up and loading the application on your PC, any of the sample programs can be either imported from audio files or from native data (e.g. textfiles containing BASIC programs). Using File->Export->To audio playback the data is played back as waveform on the default audio device and can be loaded on the G7400 (e.g. by using "CLOAD" for a BASIC program).



The latest version of the migration tool can be downloaded either as a JAR file that can be executed using a JAVA 6 runtime using "java -jar basic.jar" or as JAVA source.

Example files in different formats are available for download below. The audio-files contain the data encoded in the waveform and have to be imported to the migration tool using File->Import->Audio File. BASIC-programs can be loaded into the tool using File->Open.

Audio Files:

BASIC programs:


Additional Information

A case study evaluating the methods used to extract data from the tool explaing the different file formats was presented at iPres 2009. The paper is available for download here.

The people behind this project are: Mihai Ghete, Chrisanth Lederer, Annu John, Mark Guttenbrunner.


Further Links